Propagation of RF Signals

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					                          Chapter 20
                    Propagation of
                    RF Signals
   Radio waves, like light waves and all         Fig 20.1. Unlike sound waves or ocean          waves travel more slowly through any other
other forms of electromagnetic radiation,        waves, electromagnetic waves need no           medium. The decrease in speed through the
normally travel in straight lines. Obviously     propagating medium, such as air or water.      atmosphere is so slight that it is usually
this does not happen all the time, because       This property enables electromagnetic          ignored, but sometimes even this small dif­
long-distance communication depends on           waves to travel through the vacuum of space.   ference is significant. The speed of a radio
radio waves traveling beyond the horizon.                                                       wave in a piece of wire, by contrast, is about
How radio waves propagate in other than          Velocity                                       95% that of free space, and the speed can be
straight-line paths is a complicated subject,      Radio waves, like all other electromag­      even slower in other media.
but one that need not be a mystery. This         netic radiation, travel nearly 300,000 km         The speed of a radio wave is always the
chapter, by Emil Pocock, W3EP, provides          (186,400 mi) per second in a vacuum. Radio     product of wavelength and frequency,
basic understanding of the principles of
electromagnetic radiation, the structure of
the Earth’s atmosphere and solar-terrestrial     Table 20.1
interactions necessary for a working knowl­      The Electromagnetic Spectrum
edge of radio propagation. More detailed         Radiation         Frequency                       Wavelength
discussions and the underlying mathemat­         X-ray             3 × 105 THz and higher          10 Å and shorter
ics of radio propagation physics can be          Ultraviolet       800 THz - 3 × 105 THz           4000 - 10 Å
                                                 Visible light     400 THz - 800 THz               8000 - 4000 Å
found in the references listed at the end of     Infrared          300 GHz - 400 THz               1 mm - 0.0008 mm
this chapter.                                    Radio             10 kHz - 300 GHz                30,000 km - 1 mm

                                                                                     Z                                   HBK05_20-001
   Radio belongs to a family of electro­
magnetic radiation that includes infrared
(radiation heat), visible light, ultraviolet,
X-rays and the even shorter-wavelength
gamma and cosmic rays. Radio has the
longest wavelength and thus the lowest                                                                          Y
frequency of this group. See Table 20.1.                                                   H
Electromagnetic waves result from the
interaction of an electric and a magnetic                                                            E
field. An oscillating electric charge in a
piece of wire, for example, creates an elec­
tric field and a corresponding magnetic
field. The magnetic field in turn creates an
electric field, which creates another mag­                                                                       H
netic field, and so on.
   These two fields sustain themselves as a                                               E                                  E
composite electromagnetic wave, which
propagates itself into space. The electric and                                                       H
magnetic components are oriented at right
angles to each other and 90° to the direction                                                                                    X
of travel. The polarization of a radio wave is
usually designated the same as its electric      Fig 20.1—Electric and magnetic field components of the electromagnetic wave. The
field. This relationship can be visualized in    polarization of a radio wave is the same direction as the plane of its electric field.

                                                                                                  Propagation of RF Signals             20.1
whatever the medium. That relationship         the frequency. Attenuation in the atmo­        the refractive indices of two media is great
can be stated simply as:                       sphere is minor from 10 MHz to 3 GHz,          enough, radio waves can be reflected, just
                                               but at higher frequencies, absorption due      like light waves striking a mirror. The
                                               to water vapor and oxygen can be high.         Earth is a rather lossy reflector, but a metal
where                                             Radio energy is also lost during refrac­    surface works well if it is several wave­
   c = speed in m/s                            tion, diffraction and reflection — the very    lengths in diameter.
   f = frequency in hertz                      phenomena that allow long-distance
                                               propagation. Indeed, any form of useful        Scattering
   λ = wavelength in m
                                               propagation is accompanied by attenua­            The direction of radio waves can also be
  The wavelength (λ) of any radio fre­         tion. This may vary from the slight losses     altered through scattering. The effect seen
quency can be determined from this simple      encountered by refraction from sporadic-       by a beam of light attempting to penetrate
formula. In free space, where the speed is     E clouds near the maximum usable fre­          fog is a good example of light-wave scat­
3 × 108 m/s, the wavelength of a 30-MHz        quency, to the more considerable losses        tering. Even on a clear night, a highly
radio signal is thus 10 m. Wavelength          involved with tropospheric forward scat­       directional searchlight is visible due to a
decreases in other media because the propa­    ter or D-Layer absorption in the lower HF      small amount of atmospheric scattering
gating speed is slower. In a piece of wire,    bands. These topics will be covered later.     perpendicular to the beam. Radio waves are
the wavelength of a 30-MHz signal short­       In many circumstances, total losses can        similarly scattered when they encounter
ens to about 9.5 m. This factor must be        become so great that radio signals become      randomly arranged objects of wavelength
taken into consideration in antenna designs    too weak for communication.                    size or smaller, such as masses of electrons
and other applications.                                                                       or water droplets. When the density of scat­
                                               Refraction                                     tering objects becomes great enough, they
Wave Attenuation and Absorption
                                                  Electromagnetic waves travel in             behave more like a propagating medium
   Radio waves weaken as they travel,          straight lines until they are deflected by     with a characteristic refractive index.
whether in the near vacuum of cosmic           something. Radio waves are refracted, or          If the scattering objects are arranged in
space or within the Earth’s atmosphere.        bent, slightly when traveling from one         some alignment or order, scattering takes
Free-space attenuation results from the        medium to another. Radio waves behave          place only at certain angles. A rainbow
dispersal of radio energy from its source.     no differently from other familiar forms       provides a good analogy for field-aligned
See Fig 20.2. Attenuation grows rapidly        of electromagnetic radiation in this regard.   scattering of light waves. The arc of a rain­
with distance because signals weaken with      The apparent bending of a pencil partially     bow can be seen only at a precise angle
the square of the distance traveled. If the    immersed in a glass of water demonstrates      away from the sun, while the colors result
distance between transmitter and receiver      this principle quite dramatically.             from the variance in scattering across the
is increased from 1 km to 10 km (0.6 to           Refraction is caused by a change in         light-wave frequency range. Ionospheric
6 mi), the signal will be only one-hun­        the velocity of a wave when it crosses         electrons can be field-aligned by magnetic
dredth as strong. Free-space attenuation is    the boundary between one propagating           forces in auroras and under other unusual
a major factor governing signal strength,      medium and another. If this transition is      circumstances. Scattering in such cases is
but radio signals undergo a variety of other   made at an angle, one portion of the           best perpendicular to the Earth’s magnetic
losses as well.                                wavefront slows down (or speeds up)            field lines.
   Energy is lost to absorption when radio     before the other, thus bending the wave
waves travel through media other than a        slightly. This is shown schematically in       Reflection
vacuum. Radio waves propagate through          Fig 20.3.                                         At amateur frequencies above 30 MHz,
the atmosphere or solid material (like a          The amount of bending increases with        reflections from a variety of large objects,
wire) by exciting electrons, which then        the ratio of the refractive indices of the     such as water towers, buildings, airplanes,
reradiate energy at the same frequency.        two media. Refractive index is simply the      mountains and the like can provide a use­
This process is not perfectly efficient, so    velocity of a radio wave in free space         ful means of extending over-the-horizon
some radio energy is transformed into heat     divided by its velocity in the medium.         paths several hundred km. Two stations
and retained by the medium. The amount         Radio waves are commonly refracted             need only beam toward a common reflec­
of radio energy lost in this way depends on    when they travel through different layers      tor, whether stationary or moving. Con­
the characteristics of the medium and on       of the atmosphere, whether the highly          trary to common sense notions, the best
                                               charged ionospheric layers 100 km (60 mi)      position for a reflector is not midway
                                               and higher, or the weather-sensitive area      between two stations. Signal strength
                                               near the Earth’s surface. When the ratio of    increases as the reflector approaches one

                                                                                              Fig 20.3—Radio waves are refracted as
                                                                                              they pass at an angle between dissimilar
                                                                                              media. The lines represent the crests of
                                                                                              a moving wave front and the distance
                                                                                              between them is the wavelength. The
                                                                                              direction of the wave changes because
                                                                                              one end of the wave slows down before
                                                                                              the other as it crosses the boundary
Fig 20.2—Radio energy disperses as the                                                        between the two media. The wavelength
square of the distance from its source.                                                       is simultaneously shortened, but the
For the change of one distance unit                                                           wave frequency (number of crests that
shown the signal is only one quarter as                                                       pass a certain point in a given unit of
strong. Each spherical section has the                                                        time) remains constant.
same surface area.

20.2    Chapter 20
end of the path, so the most effective
reflectors are those closest to one station                                                                              HBK05_20-004
or the other.
   Maximum range is limited by the radio
line-of-sight distance of both stations to
the reflector and by reflector size and
shape. The reflectors must be many wave­
lengths in size and ideally have flat sur­
faces. Large airplanes make fair reflectors
and may provide the best opportunity for
long-distance contacts. The calculated
limit for airplane reflections is 900 km
(560 mi), assuming the largest jets fly no
higher than 12,000 m (40,000 ft), but
actual airplane reflection contacts are
likely to be considerably shorter.

Knife-Edge Diffraction
   Radio waves can also pass behind solid
objects with sharp upper edges, such as a
mountain range, by knife-edge diffraction.
This is a common natural phenomenon
that affects light, sound, radio and other
coherent waves, but it is difficult to com­
prehend. Fig 20.4 depicts radio signals
approaching an idealized knife-edge. The       Fig 20.4—Radio, light and other waves are diffracted around the sharp edge of a solid
portion of the radio waves that strike the     object that is large in terms of wavelengths. Diffraction results from interference
base of the knife-edge is entirely blocked,    between waves right at the knife-edge and those that are passing above it. Some
while that portion passing several wave­       signals appear behind the knife-edge as a consequence of the interference pattern.
lengths above the edge travel on relatively    Hills or mountains can serve as natural knife-edges at radio frequencies.
unaffected. It might seem at first glance
that a knife-edge as large as a mountain,
for example, would completely prevent
radio signals from appearing on the other      even rounded hills may serve as a diffract­   lower part of the wave front loses energy
side but that is not quite true. Something     ing edge. Alternating bands of strong and     due to currents induced in the ground. This
quite unexpected happens to radio signals      weak signals, corresponding to the inter­     slows down the lower part of the wave,
that pass just over a knife-edge.              ference pattern, will appear on the surface   causing the entire wave to tilt forward
   Normally, radio signals along a wave        of the Earth behind the mountain, known       slightly. This tilting follows the curvature
front interfere with each other continu­       as the shadow zone. The phenomenon is         of the Earth, thus allowing low- and me­
ously as they propagate through unob­          generally reciprocal, so that two-way         dium-wave radio signals to propagate over
structed space, but the overall result is a    communication can be established under        distances well beyond line of sight.
uniformly expanding wave. When a por­          optimal conditions. Knife-edge diffrac­          Ground wave is most useful during the
tion of the wave front is blocked by a         tion can make it possible to complete paths   day at 1.8 and 3.5 MHz, when D-layer
knife-edge, the resulting interference pat­    of 100 km or more that might otherwise be     absorption makes skywave propagation
tern is no longer uniform. This can be         entirely obstructed by mountains or seem­     more difficult. Vertically polarized anten­
understood by visualizing the radio sig­       ingly impossible terrain.                     nas with excellent ground systems provide
nals right at the knife-edge as if they con­                                                 the best results. Ground-wave losses are
stituted a new and separate transmitting       Ground Waves                                  reduced considerably over saltwater and
point, but in-phase with the source wave         A ground wave is the result of a special    are worst over dry and rocky land.
at that point. The signals adjacent to the     form of diffraction that primarily affects
knife-edge still interact with signals pass­   longer-wavelength vertically polarized        SKY-WAVE PROPAGATION AND
ing above the edge, but they cannot inter­     radio waves. It is most apparent in the 80­   THE SUN
act with signals that have been obstructed     and 160-m amateur bands, where practi­           The Earth’s atmosphere is composed
below the edge. The resulting interference     cal ground-wave distances may extend          primarily of nitrogen (78%), oxygen
pattern no longer creates a uniformly          beyond 200 km (120 mi). The term ground       (21%) and argon (1%), with smaller
expanding wave front, but rather appears       wave is often mistakenly applied to any       amounts of a dozen other gases. Water
as a pattern of alternating strong and weak    short-distance communication, but the         vapor can account for as much as 5% of
bands of waves that spread in a nearly 180°    actual mechanism is unique to the longer­     the atmosphere under certain conditions.
arc behind the knife-edge.                     wave bands.                                   This ratio of gases is maintained until an
   The crest of a range of hills or moun­        Radio waves are bent slightly as they       altitude of about 80 km (50 mi), when the
tains 50 to 100 wavelengths long can serve     pass over a sharp edge, but the effect        mix begins to change. At the highest lev­
as a reasonable knife-edge diffractor at       extends to edges that are considerably        els, helium and hydrogen predominate.
radio frequencies. Hillcrests that are         rounded. At medium and long wavelengths,         Solar radiation acts directly or indi­
clearly defined and free of trees, buildings   the curvature of the Earth looks like a       rectly on all levels of the atmosphere.
and other clutter make the best edges, but     rounded edge. Bending results when the        Adjacent to the surface of the Earth, solar

                                                                                               Propagation of RF Signals            20.3
 Propagation Summary, by Band
 Medium Frequencies (300 kHz-3 MHz)
    The only amateur medium-frequency band is situated        absorption is not a significant factor. Communication
 just above the domestic AM broadcast band. Ground            up to 3000 km (1900 mi) is typical during the daytime,
 wave provides reliable communication out to 150 km           and this extends halfway around the world via all­
 (90 mi) during the day, when no other form of propaga­       darkness paths. The band is generally open via F2 on
 tion is available. Long-distance paths are made at night     a 24-hour basis, but during a solar minimum, the MUF
 via the F2 layer.                                            on some DX paths may drop below 10 MHz at night.
                                                              Under these conditions, 30 m adopts the characteris­
 1.8-2.0 MHz (160 m)                                          tics of the daytime bands at 14 MHz and higher. The
   The top band, as it is sometimes called, suffers from      30-m band shows the least variation in conditions over
 extreme daytime D-layer absorption. Even at high             the 11-year solar cycle, thus making it generally useful
 radiation angles, virtually no signal can pass through to    for long-distance communication anytime.
 the F layer, so daytime communication is limited to
 ground-wave coverage. At night, the D layer quickly          14.0-14.35 MHz (20 m)
 disappears and worldwide 160-m communication                    The 20-m band is traditionally regarded as the
 becomes possible via F2-layer skip. Atmospheric and          amateurs’ primary long-haul DX favorite. Regardless
 man-made noise limit propagation. Tropical and               of the 11-year solar cycle, 20 m can be depended on
 midlatitude thunderstorms cause high levels of static in     for at least a few hours of worldwide F2 propagation
 summer, making winter evenings the best time to work         during the day. During solar-maximum periods, 20 m
 DX at 1.8 MHz. A proper choice of receiving antenna          will often stay open to distant locations throughout the
 can often significantly reduce the amount of received        night. Skip distance is usually appreciable and is
 noise while enhancing desired signals.                       always present to some degree. Daytime E-layer
                                                              propagation may be detected along very short paths.
 High Frequencies (3-30 MHz)                                  Atmospheric noise is not a serious consideration, even
   A wide variety of propagation modes are useful on the      in the summer. Because of its popularity, 20 m tends
 HF bands. The lowest two bands in this range share           to be very congested during the daylight hours.
 many daytime characteristics with 160 m. The transition
 between bands primarily useful at night or during the        18.068-18.168 MHz (17 m)
 day appears around 10 MHz. Most long-distance                  The 17-m band is similar to the 20-m band in many
 contacts are made via F2-layer skip. Above 21 MHz,           respects, but the effects of fluctuating solar activity on
 more exotic propagation, including TE, sporadic E,           F2 propagation are more pronounced. During the
 aurora and meteor scatter, begin to be practical.            years of high solar activity, 17 m is reliable for daytime
                                                              and early-evening long-range communication, often
 3.5-4.0 MHz (80 m)                                           lasting well after sunset. During moderate years, the
    The lowest HF band is similar to 160 m in many            band may open only during sunlight hours and close
 respects. Daytime absorption is significant, but not quite   shortly after sunset. At solar minimum, 17 m will open
 as extreme as at 1.8 MHz. High-angle signals may             to middle and equatorial latitudes, but only for short
 penetrate to the E and F layers. Daytime communication       periods during midday on north-south paths.
 range is typically limited to 400 km (250 mi) by ground­
 wave and skywave propagation. At night, signals are          21.0-21.45 MHz (15 m)
 often propagated halfway around the world. As at                The 15-m band has long been considered a prime
 1.8 MHz, atmospheric noise is a nuisance, making winter      DX band during solar cycle maxima, but it is sensitive
 the most attractive season for the 80-m DXer.                to changing solar activity. During peak years, 15 m is
                                                              reliable for daytime F2-layer DXing and will often stay
 7.0-7.3 MHz (40 m)                                           open well into the night. During periods of moderate
    The popular 40-m band has a clearly defined skip          solar activity, 15 m is basically a daytime-only band,
 zone during the day. D-layer absorption is not as            closing shortly after sunset. During solar minimum
 severe as on the lower bands, so short-distance skip         periods, 15 m may not open at all except for infre­
 via the E and F layers is possible. During the day, a        quent north-south transequatorial circuits. Sporadic E
 typical station can cover a radius of approximately          is observed occasionally in early summer and mid­
 800 km (500 mi). Ground-wave propagation is not              winter, although this is not common and the effects
 important. At night, reliable worldwide communication        are not as pronounced as on the higher frequencies.
 via F2 is common on the 40-m band.
    Atmospheric noise is less troublesome than on 160         24.89-24.99 MHz (12 m)
 and 80 m, and 40-m DX signals are often of sufficient          This band offers propagation that combines the best
 strength to override even high-level summer static. For      of the 10- and 15-m bands. Although 12 m is primarily
 these reasons, 40 m is the lowest-frequency amateur          a daytime band during low and moderate sunspot
 band considered reliable for DX communication in all         years, it may stay open well after sunset during the
 seasons. Even during the lowest point in the solar           solar maximum. During years of moderate solar
 cycle, 40 m may be open for worldwide DX throughout          activity, 12 m opens to the low and middle latitudes
 the night.                                                   during the daytime hours, but it seldom remains open
                                                              after sunset. Periods of low solar activity seldom
 10.1-10.15 MHz (30 m)                                        cause this band to go completely dead, except at
   The 30-m band is unique because it shares charac­          higher latitudes. Occasional daytime openings,
 teristics of both daytime and nighttime bands. D-layer       especially in the lower latitudes, are likely over north­

20.4   Chapter 20
south paths. The main sporadic-E season on 24 MHz             tainly the most popular form of propagation on the
lasts from late spring through summer and short               6-m band. Single-hop E-skip openings may last many
openings may be observed in mid-winter.                       hours for contacts from 600 to 2300-km (370 to
                                                              1400 mi), primarily during the spring and early sum­
28.0-29.7 MHz (10 m)                                          mer. Multiple-hop Es provides transcontinental contacts
   The 10-m band is well known for extreme variations in      several times a year, and contacts between the US and
characteristics and variety of propagation modes.             South America, Europe and Japan via multiple-hop
During solar maxima, long-distance F2 propagation is so       E-skip occur nearly every summer.
efficient that very low power can produce loud signals           Other types of E-layer ionospheric propagation make
halfway around the globe. DX is abundant with modest          6 m an exciting band. Maximum distances of about
equipment. Under these conditions, the band is usually        2300 km (1400 mi) are typical for all types of E-layer
open from sunrise to a few hours past sunset. During          modes. Propagation via FAI often provides additional
periods of moderate solar activity, 10 m usually opens        hours of contacts immediately following sporadic E
only to low and transequatorial latitudes around noon.        events. Auroral propagation often makes its appear­
During the solar minimum, there may be no F2 propaga­         ance in late afternoon when the geomagnetic field is
tion at any time during the day or night.                     disturbed. Closely related auroral-E propagation may
   Sporadic E is fairly common on 10 m, especially May        extend the 6-m range to 4000 km (2500 mi) and
through August, although it may appear at any time.           sometimes farther across the northern states and
Short skip, as sporadic E is sometimes called on the HF       Canada, usually after midnight. Meteor scatter pro­
bands, has little relation to the solar cycle and occurs      vides brief contacts during the early morning hours,
regardless of F-layer conditions. It provides single-hop      especially during one of the dozen or so prominent
communication from 300 to 2300 km (190 to 1400 mi)            annual meteor showers.
and multiple-hop opportunities of 4500 km (2800 mi)
and farther.                                                  144-148 MHz (2 m)
   Ten meters is a transitional band in that it also shares      Ionospheric effects are significantly reduced at
some of the propagation modes more characteristic of          144 MHz, but they are far from absent. F-layer propa­
VHF. Meteor scatter, aurora, auroral E and transequa­         gation is unknown except for TE, which is responsible
torial spread-F provide the means of making contacts          for the current 144-MHz terrestrial DX record of nearly
out to 2300 km (1400 mi) and farther, but these modes         8000 km (5000 mi). Sporadic E occurs as high as
often go unnoticed at 28 MHz. Techniques similar to           144 MHz less than a tenth as often as at 50 MHz, but
those used at VHF can be very effective on 10 m, as           the usual maximum single-hop distance is the same,
signals are usually stronger and more persistent. These       about 2300 km (1400 mi). Multiple-hop sporadic-E
exotic modes can be more fully exploited, especially          contacts greater than 3000 km (1900 mi) have
during the solar minimum when F2 DXing has waned.             occurred from time to time across the continental US,
                                                              as well as across Southern Europe.
Very High Frequencies (30-300 MHz)                               Auroral propagation is quite similar to that found at
   A wide variety of propagation modes are useful in the      50 MHz, except that signals are weaker and more
VHF range. F-layer skip appears on 50 MHz during              Doppler-distorted. Auroral-E contacts are rare. Meteor­
solar cycle peaks. Sporadic E and several other E-layer       scatter contacts are limited primarily to the periods of
phenomena are most effective in the VHF range. Still          the great annual meteor showers and require much
other forms of VHF ionospheric propagation, such as           patience and operating skill. Contacts have been made
field-aligned irregularities (FAI) and transequatorial        via FAI on 144 MHz, but its potential has not been fully
spread F (TE), are rarely observed at HF. Tropospheric        explored.
propagation, which is not a factor at HF, becomes                Tropospheric effects improve with increasing
increasingly important above 50 MHz.                          frequency, and 144 MHz is the lowest VHF band at
                                                              which weather plays an important propagation role.
50-54 MHz (6 m)                                               Weather-induced enhancements may extend the
   The lowest amateur VHF band shares many of the             normal 300- to 600-km (190- to 370-mi) range of well­
characteristics of both lower and higher frequencies. In      equipped stations to 800 km (500 mi) and more,
the absence of any favorable ionospheric propagation          especially during the summer and early fall. Tropo­
conditions, well-equipped 50-MHz stations work regu­          spheric ducting extends this range to 2000 km
larly over a radius of 300 km (190 mi) via tropospheric       (1200 mi) and farther over the continent and at least to
scatter, depending on terrain, power, receiver capabili­      4000 km (2500 mi) over some well-known all-water
ties and antenna. Weak-signal troposcatter allows the         paths, such as that between California and Hawaii.
best stations to make 500-km (310-mi) contacts nearly
any time. Weather effects may extend the normal range         222-225 MHz (135 cm)
by a few hundred km, especially during the summer               The 135-cm band shares many characteristics with
months, but true tropospheric ducting is rare.                the 2-m band. The normal working range of 222-MHz
   During the peak of the 11-year sunspot cycle, world­       stations is nearly as far as comparably equipped
wide 50-MHz DX is possible via the F2 layer during            144-MHz stations. The 135-cm band is slightly more
daylight hours. F2 backscatter provides an additional         sensitive to tropospheric effects, but ionospheric modes
propagation mode for contacts as far as 4000 km               are more difficult to use. Auroral and meteor-scatter
(2500 mi) when the MUF is just below 50 MHz. TE               signals are somewhat weaker than at 144 MHz, and
paths as long as 8000 km (5000 mi) across the mag­            sporadic-E contacts on 222 MHz are extremely rare.
netic equator are common around the spring and fall           FAI and TE may also be well within the possibilities of
equinoxes of peak solar cycle years.
   Sporadic E is probably the most common and cer­            (continued on next page)

                                                                                   Propagation of RF Signals         20.5
   222 MHz, but reports of these modes on the 135-cm                     902-928 MHz (33-cm) and Higher
   band are uncommon. Increased activity on 222-MHz                         Ionospheric modes of propagation are nearly
   will eventually reveal the extent of the propagation                  unknown in the bands above 902 MHz. Auroral scatter
   modes on the highest of the amateur VHF bands.                        may be just within amateur capabilities at 902 MHz,
   Ultra-High Frequencies (300-3000 MHz) and Higher                      but signal levels will be well below those at 432 MHz.
                                                                         Doppler shift and distortion will be considerable, and
     Tropospheric propagation dominates the bands at                     the signal bandwidth may be quite wide. No other
   UHF and higher, although some forms of E-layer                        ionospheric propagation modes are likely, although
   propagation are still useful at 432 MHz. Above                        high-powered research radars have received echoes
   10 GHz, atmospheric attenuation increasingly be-                      from auroras and meteors as high as 3 GHz.
   comes the limiting factor over long-distance paths.                      Almost all extended-distance work in the UHF and
   Reflections from airplanes, mountains and other                       microwave bands is accomplished with the aid of
   stationary objects may be useful adjuncts to propa-                   tropospheric enhancement. The frequencies above
   gation at 432 MHz and higher.                                         902 MHz are very sensitive to changes in the weather.
   420-450 MHz (70 cm)                                                   Tropospheric ducting occurs more frequently than in
                                                                         the VHF bands and the potential range is similar. At
      The lowest amateur UHF band marks the highest                      1296 MHz, 2000-km (1200-mi) continental paths and
   frequency on which ionospheric propagation is com-                    4000-km (2500-mi) paths between California and
   monly observed. Auroral signals are weaker and more                   Hawaii have been spanned many times. Contacts
   Doppler distorted; the range is usually less than at 144              of 1000 km (620 mi) have been made on all bands
   or 222 MHz. Meteor scatter is much more difficult than                through 10 GHz in the US and over 1600 km
   on the lower bands, because bursts are significantly                  (1000 mi) across the Mediterranean Sea. Well­
   weaker and of much shorter duration. Although spo-                    equipped 903- and 1296-MHz stations can work
   radic E and FAI are unknown as high as 432 MHz and                    reliably up to 300 km (190 mi), but normal working
   probably impossible, TE may be possible.                              ranges generally shorten with increasing frequency.
      Well-equipped 432-MHz stations can expect to work                     Other tropospheric effects become evident in the
   over a radius of at least 300 km (190 mi) in the                      GHz bands. Evaporation inversions, which form over
   absence of any propagation enhancement. Tropo-                        very warm bodies of water, are usable at 3.3 GHz and
   spheric refraction is more pronounced at 432 MHz and                  higher. It is also possible to complete paths by
   provides the most frequent and useful means of                        scattering from rain, snow and hail in the lower GHz
   extended-range contacts. Tropospheric ducting                         bands. Above 10 GHz, attenuation caused by atmo­
   supports contacts of 1500 km (930 mi) and farther                     spheric water vapor and oxygen become the most
   over land. The current 432-MHz terrestrial DX record                  significant limiting factors in long-distance communi­
   of more than 4000 km (2500 mi) was accomplished by                    cation.
   ducting over water.

warming controls all aspects of the             (6 and 30 mi) are the stratosphere and the      the Van Allen belts. These have only a minor
weather, powering wind, rain and other          imbedded ozonosphere, where ultraviolet         effect on terrestrial radio propagation.
familiar phenomena. Solar ultraviolet           absorbing ozone reaches its highest con­
(UV) radiation creates small concentra­         centrations. About 99% of atmospheric           The Ionosphere
tions of ozone (O3) molecules between 10        gases are contained within these two low­          The ionosphere plays a basic role in long­
and 50 km (6 and 30 mi). Most UV radia­         est regions.                                    distance communication in all the amateur
tion is absorbed by this process and never         Above 50 km to about 600 km (370 mi)         bands from 1.8 MHz to 30 MHz. Iono­
reaches the Earth.                              is the ionosphere, notable for its effects on   spheric effects are less apparent in the very
   At even higher altitudes, UV and X-ray       radio propagation. At these altitudes,          high frequencies (30-300 MHz), but they
radiation partially ionize atmospheric          atomic oxygen and nitrogen predominate          persist at least through 432 MHz. As early
gases. Electrons freed from gas atoms           under very low pressure. High-energy            as 1902, Oliver Heaviside and Arthur E.
eventually recombine with positive ions         solar UV and X-ray radiation ionize these       Kennelly independently suggested the
to recreate neutral gas atoms, but this takes   gases, creating a broad region where ions       existence of a layer in the upper atmosphere
some time. In the low-pressure environ­         are created in relative abundance. The          that could account for the long-distance
ment at the highest altitudes, atoms are        ionosphere is subdivided into distinctive       radio transmissions made the previous year
spaced far apart and the gases may remain       D, E and F regions.                             by Guglielmo Marconi and others. Edward
ionized for many hours. At lower altitudes,        The magnetosphere begins around              Appleton confirmed the existence of the
recombination happens rather quickly, and       600 km (370 mi) and extends as far as           Kennelly-Heaviside layer during the early
only constant radiation can keep any            160,000 km (100,000 mi) into space. The         1920s and used the letter E on his diagrams
appreciable portion of the gas ionized.         predominant component of atmospheric            to designate the electric waves that were
                                                gases gradually shifts from atomic oxygen,      apparently reflected from it.
Structure of the Earth’s Atmosphere             to helium and finally to hydrogen at the           In 1924, Appleton discovered two addi­
   The atmosphere, which reaches to more        highest levels. The lighter gases may reach     tional layers in the ionosphere, as he and
than 600 km (370 mi) altitude, is divided       escape velocity or be swept off the atmo­       Robert Watson-Watt named this atmo­
into a number of regions, shown in Fig 20.5.    sphere by the solar wind. At about 3,200        spheric region, and noted them with the
The weather-producing troposphere lies          and 16,000 km (2000 and 9900 mi), the           letters D and F. Appleton was reluctant to
between the surface and an average alti­        Earth’s magnetic field traps energetic elec­    alter this arbitrary nomenclature for fear
tude of 10 km (6 mi). Between 10 and 50 km      trons and protons in two bands, known as        of discovering yet other layers, so it has

20.6    Chapter 20
stuck to the present day. The basic physics   ing were among additional ionospheric       of the F layer, that density can reach a tril­
of ionospheric propagation was largely        phenomena that required explanation.        lion electrons per cubic meter (1012 e/m3).
worked out by the 1920s, yet both amateur                                                 Even at this high level, radio waves are
and professional experimenters made fur-      Ionospheric Refraction                      refracted gradually over a considerable
ther discoveries through the 1930s and          The refractive index of an ionospheric    vertical distance, usually amounting to
1940s. Sporadic E, aurora, meteor scatter     layer increases with the density of free­   tens of km. Radio waves become useful
and several types of field-aligned scatter-   moving electrons. In the densest regions    for terrestrial propagation only when they
                                                                                          are refracted enough to bring them back to
                                                                                          Earth. See Fig 20.6.
                                                                                             Although refraction is the primary
                                                                                          mechanism of ionospheric propagation, it
                                                                                          is usually more convenient to think of the
                                                                                          process as a reflection. The virtual height
                                                                                          of an ionospheric layer is the equivalent
                                                                                          altitude of a reflection that would produce
                                                                                          the same effect as the actual refraction.
                                                                                          The virtual height of any ionospheric layer
                                                                                          can be determined using an ionospheric
                                                                                          sounder, or ionosonde, a sort of vertically
                                                                                          oriented radar. The ionosonde sends
                                                                                          pulses that sweep over a wide frequency
                                                                                          range, generally from 2 MHz to 6 MHz or
                                                                                          higher, straight up into the ionosphere.
                                                                                          The frequencies of any echoes are
                                                                                          recorded against time and then plotted as
                                                                                          distance on an ionogram. Fig 20.7 depicts
                                                                                          a simple ionogram.
                                                                                             The highest frequency that returns
                                                                                          echoes at vertical incidence is known as
                                                                                          the vertical incidence or critical fre­
                                                                                          quency. The critical frequency is almost
                                                                                          totally a function of ion density. The
                                                                                          higher the ionization at a particular alti­
                                                                                          tude, the higher becomes the critical fre­
                                                                                          quency. Physicists are more apt to call this
                                                                                          the plasma frequency, because technically
                                                                                          gases in the ionosphere are in a plasma, or
                                                                                          partially ionized state. F-layer critical fre­
                                                                                          quencies commonly range from about
                                                                                          1 MHz to as high as 15 MHz.

Fig 20.5—Regions of the ionosphere.

                                                                                          Fig 20.7—Simplified vertical incidence
Fig 20.6—Gradual refraction in the ionosphere allows radio signals to be propagated       ionogram showing echoes returned
long distances. It is often convenient to imagine the process as a reflection with an     from the E, F1 and F2 layers. The critical
imaginary reflection point at some virtual height above the actual refracting region.     frequencies of each layer (4.1, 4.8 and
The other figures in this chapter show ray paths as equivalent reflections, but you       6.8 MHz) can be read directly from the
should keep in mind that the actual process is a gradual refraction.                      ionogram scale.

                                                                                            Propagation of RF Signals              20.7
Maximum and Lowest Usable                        lar frequency because the ionosphere is        this case, the frequency is said to be below
Frequencies                                      unable to refract the signal from one to the   the lowest usable frequency (LUF). This
   When the frequency of a vertically            other through the required angle — that is,    occurs most frequently below 10 MHz,
incident signal is raised above the critical     the frequency is below the MUF — the           where atmospheric and man-made noises
frequency of an ionospheric layer, that          stations are said to be in the skip zone for   are most troublesome. The LUF can be low­
portion of the ionosphere is unable to           that frequency. Stations within the skip       ered somewhat by the use of high power
refract the signal back to Earth. However,       zone may be able to work each other at a       and directive antennas, or through the use
a signal above the critical frequency may        lower frequency, or by ground wave if they     of communications modes that permit
be returned to Earth if it enters the layer at   are close enough. There is no skip zone at     reduced receiver bandwidth or are less
an oblique angle, rather than at vertical        frequencies below the critical frequency.      demanding of SNR — CW instead of SSB,
incidence. This is fortunate because it             The MUF at any time on a particular path    for example. This is not true of the MUF,
permits two widely separated stations to         is just that — the maximum usable fre­         which is limited by the physics of iono­
communicate on significantly higher fre­         quency. Frequencies below the MUF will         spheric refraction, no matter how high your
quencies than the critical frequency. See        also propagate along the path, but iono­       transmitter power or how narrow your
Fig 20.8.                                        spheric absorption and noise at the receiv­    receiver bandwidth. The LUF can be higher
   The highest frequency supported by the        ing location (perhaps due to thunderstorms,    than the MUF, in which case there is no
ionosphere between two stations is the           local or distant) may make the received sig­   frequency that supports communication on
maximum usable frequency (MUF) for that          nal-to-noise ratio too low to be usable. In    the particular path at that time.
path. If the separation between the stations
is increased, a still higher frequency can
be supported at lower launch angles. The
MUF for this longer path is higher than the
MUF for the shorter path. When the dis­
tance is increased to the maximum one­
hop distance, the launch angle of the
signals between the two stations is zero
(that is, the ray path is tangential to the
Earth at the two stations) and the MUF for
this path is the highest that can be sup­
ported by that layer of the ionosphere at
that location. This maximum distance is
about 4000 km (2500 mi) for the F 2 layer
and about 2300 km (1400 mi) for the E
layer. See Fig 20.9.
   The MUF is a function of path, time of        Fig 20.8—The relationships between critical frequency, maximum usable frequency
day, season, location, solar UV and X-ray        (MUF) and skip zone can be visualized in this simplified, hypothetical case. The
radiation levels and ionospheric distur­         critical frequency is 7 MHz, allowing frequencies below this to be used for short­
bances. For vertically incident waves, the       distance ionospheric communication by stations in the vicinity of point M. These
                                                 stations cannot communicate by the ionosphere at 14 MHz. Stations at points B
MUF is the same as the critical frequency.       and E (and beyond) can communicate because signals at this frequency are
For path lengths at the limit of one-hop         refracted back to Earth because they encounter the ionosphere at an oblique angle
propagation, the MUF can be several times        of incidence. At greater distances, higher frequencies can be used because the
the critical frequency. See Table 20.2. The      MUF is higher at the larger angles of incidence (low launch angles). In this figure,
ratio between the MUF and the critical fre­      the MUF for the path between points A and F, with a small launch angle, is shown
quency is known as the maximum usable            to be 28 MHz. Each pair of stations can communicate at frequencies at or below the
                                                 MUF of the path between them, but not below the LUF—see text.
frequency factor (MUFF).
   The term skip zone is closely related to
MUF. When two stations are unable to
communicate with each other on a particu-

Table 20.2
Maximum Usable Frequency Factors
      Maximum                  Useful
      Critical                 Operating
Layer Frequency        MUFF Frequencies
      (MHz)                    (MHz)
F2    15.0             3.3-4.0  1-60
F1*    5.5             4.0     10-20
E*     4.0             4.8      5-20
Es    30.0             5.3     20-160
D*    Not observed      —      None
                                                 Fig 20.9—Signals at the MUF propagated at a low angle to the horizon provide the
* Daylight only	                                 longest possible one-hop distances. In this example, 28-MHz signals entering the
                                                 ionosphere at higher angles are not refracted enough to bring them back to Earth.

20.8     Chapter 20
Ionospheric Fading
   HF signal strengths typically rise and
fall over periods of a few seconds to sev­
eral minutes, and rarely hold at a constant
level for very long. Fading is generally
caused by the interaction of several radio
waves from the same source arriving along
different propagation paths. Waves that
arrive in-phase combine to produce a
stronger signal, while those out-of-phase      Fig 20.10—Average monthly sunspot numbers for Solar Cycles 19 to 22.
cause destructive interference and a lower
net signal strength. Short-term variations
in ionospheric conditions may change
individual path lengths or signal strengths    lites, UV and X-ray radiation could not be
enough to cause fading. Even signals that      measured directly, because they were
arrive primarily over a single path may        almost entirely absorbed in the upper
vary as the propagating medium changes.        atmosphere. The sunspot number provided
Fading may be most notable at sunrise and      the most convenient approximation of gen­
sunset, especially near the MUF, when the      eral solar activity. The sunspot number is
ionosphere undergoes dramatic transfor­        not a simple count of the number of visual
mations. Other ionospheric traumas, such       spots, but rather the result of a complicated
as auroras and geomagnetic storms, also        formula that takes into consideration size,
produce severe forms of HF fading.             number and grouping. The sunspot number
                                               varies from near zero during the solar-cycle
The 11-Year Solar Cycle                        minimum to over 200.
   The density of ionospheric layers              Another method of gauging solar activ­
depends on the amount of solar radiation       ity is the solar flux, which is a measure of
reaching the Earth, but solar radiation is     the intensity of 2800-MHz (10.7-cm)
not constant. Variations result from daily     radio noise coming from the sun. The            Fig 20.11—Approximate conversion
and seasonal motions of the Earth, the         2800-MHz radio flux correlates well with        between solar flux and sunspot number.
sun’s own 27-day rotation and the 11-year      the intensity of ionizing UV and X-ray
cycle of solar activity. One visual indica­    radiation and provides a convenient alter­
tor of both the sun’s rotation and the solar   native to sunspot numbers. It commonly
cycle is the periodic appearance of dark       varies on a scale of 60-300 and can be          posed of extremely hot gases and does not
spots on the sun, which have been              related to sunspot numbers, as shown in         turn uniformly. At the equator, the period
observed continuously since the mid-18th       Fig 20.11. The Dominion Radio Astro­            is just over 25 days, but it approaches 35
century. On average, the number of sun­        physical Observatory, Penticton, British        days at the poles. Sunspots that affect the
spots reaches a maximum every 10.7             Columbia, measures the 2800-MHz solar           Earth’s ionosphere, which appear almost
years, but the period has varied between       flux daily at local noon. (Prior to June        entirely within 35° of the sun’s equator,
7 and 17 years. Cycle 19 peaked in 1958,       1991, the Algonquin Radio Observatory,          take about 26 days for one rotation. After
with an average sunspot number of              Ontario, made the measurements.) Radio          taking into account the Earth’s movement
over 200, the highest recorded to date.        station WWV broadcasts the latest solar­        around the sun, the apparent period of solar
Fig 20.10 shows average monthly sunspot        flux index at 18 minutes after each hour;       rotation is about 27 days.
numbers for the past four cycles.              WWVH does the same at 45 minutes after             Active regions must face the Earth in
   Sunspots are cooler areas on the sun’s      the hour. The Penticton solar flux is           the proper orientation to have an impact
surface associated with high magnetic          employed in a wide variety of other appli­      on the ionosphere. They may face the
activity. Active regions adjacent to sun­      cations. Daily, weekly, monthly and even        Earth only once before rotating out of
spot groups, called plages, are capable        13-month smoothed average solar flux            view, but they often persist for several
of producing great flares and sustained        readings are commonly used in propaga­          solar rotations. The net effect is that solar
bursts of radiation in the radio through       tion predictions.                               activity often appears in 27-day cycles
X-ray spectrum. During the peak of the            High flux values generally result in         corresponding to the sun’s rotation, even
11-year solar cycle, average solar radia­      higher MUFs, but the actual procedures          though the active regions themselves may
tion increases along with the number of        for predicting the MUF at any given hour        last for several solar rotations.
flares and sunspots. The ionosphere            and path are quite complicated. Solar flux
becomes more intensely ionized as a con­       is not the sole determinant, as the angle of    Solar-Ionospheric Disturbances
sequence, resulting in higher critical fre­    the sun to the Earth, season, time of day,         Like a campfire that occasionally spits
quencies, particularly in the F2 layer. The    exact location of the radio path and other      out a flaming ember, our sun sometimes
possibilities for long-distance commu­         factors must all be taken into account.         erupts spasmodically — but on a much
nications are considerably improved            MUF forecasting a few days or months            grander scale than a summer campfire here
during solar maxima, especially in the         ahead involves additional variables and         on Earth. After all, any event that violently
higher-frequency bands.                        even more uncertainties.                        releases as much as 10 billion tons of solar
   One key to forecasting F-layer critical                                                     material traveling up to four and a half
frequencies, and thus long-distance propa­     The Sun’s 27-Day Rotation                       million miles per hour has to be consid­
gation, is the intensity of ionizing UV and      Sunspot observations also reveal that the     ered pretty impressive!
X-ray radiation. Until the advent of satel-    sun rotates on its own axis. The sun is com-       There are two main types of solar erup-

                                                                                                 Propagation of RF Signals             20.9
tions, distinguished partly by where they                                                        netic storms and associated auroras, espe­
originate on the sun: solar flares and coro­                                                     cially at HF. Radio emissions from solar
nal mass ejections. A solar flare erupts                                                         flares may be heard as sudden increases in
from the sun’s surface, and its main effect                                                      noise on the VHF bands.
is to launch out into space a wide spectrum                                                         Effects on ionospheric storms (another
of electromagnetic energy, although a big                                                        name for geomagnetic storms) at HF vary
flare can also release matter into space,                                                        considerably. Communications may be
mainly in the form of energetic protons.                                                         temporarily blacked out during an SID, but
Since electromagnetic energy travels at the                                                      ionospheric paths may be generally noisy,
speed of light, the first indication of a solar                                                  weakened or disrupted for several days.
flare reaches the Earth in about eight min­                                                      Transpolar signals at 14 MHz and higher
utes. A large flare shows up as an increase                                                      may be considerably attenuated and take
in visible brightness near a sunspot group,                                                      on a hollow multipath sound. The number
accompanied by increases in UV and X-ray          Fig 20.12—Geomagnetic activity                 of geomagnetic storms varies consider­
radiation and high levels of noise in the         (measured as the A-index) also follows         ably from year to year, with peak geomag­
VHF radio bands.                                  an 11-year cycle. Average values over          netic activity following the peak of solar
                                                  the past few cycles show that geo­
   A coronal mass ejection (CME) origi­                                                          activity. See Fig 20.12.
                                                  magnetic activity peaks before and
nates in the sun’s outer atmosphere, its          after the peak of solar flux.                     Devices known as magnetometers moni­
corona. With several sophisticated satel­                                                        tor geomagnetic activity. These may be as
lites launched in the mid 1990s, we have                                                         simple as a magnetic compass rigged to
gained powerful new tools to monitor the          Table 20.3
                                                                                                 record its movements. Small variations in
intricacies of solar activity. The reality of                                                    the geomagnetic field are scaled to two
how the sun operates is far more complex          Geomagnetic Storms                             measures known as the K and A indexes.
than initially expected. Using the latest sat­    Typical      Description      Days per         The K index provides an indication of mag­
ellite technology (and also some re-engi­         Kp                            Solar Cycle      netic activity on a finite scale of 0-9. Very
neered earthbound instruments), scientists        9            Extreme            4              quiet conditions are reported as 0 or 1, while
                                                  8            Severe             0
have observed many CMEs, greatly                  7            Strong           130
                                                                                                 geomagnetic storm levels begin at 4. See
expanding our knowledge about them.               6            Moderate         360              Table 20.3 for the latest NOAA descrip­
Previously, the only direct observations we       5            Minor            900              tions of geomagnetic storms.
had of coronal activity were during solar                                                           A worldwide network of magnetometers
eclipses — and eclipses don’t occur very                                                         constantly monitors the Earth’s magnetic
often.                                                                                           field, because the Earth’s magnetic field
   One surprise has been that a large CME         cases, nearly all background noise will be     varies with location. K indices that indi­
can involve as much as half of the entire         gone as well. SIDs may last up to an hour,     cate average planetary conditions are indi­
solar coronal region. Flares are far more         after which ionospheric conditions tempo­      cated as Kp. Daily geomagnetic conditions
limited spatially — they are launched from        rarily return to normal.                       are also summarized by the open-ended A
the area around active sunspot regions. At           Very energetic protons ejected during a     index, which corresponds roughly to the
one time, scientists believed that flares         large flare, and arriving in the vicinity of   cumulative K index values. The A index
and CMEs were causally related, but now           the Earth from several minutes to several      commonly varies between 0 and 30 during
they recognize that many CMEs occur               hours after the flare, can penetrate deep      quiet to active conditions, and up to 100
without an accompanying flare. And while          into the ionosphere at the Earth’s poles.      and higher during geomagnetic storms.
many flares do result in an ejection of           This can produce intense ionization and           At 18 minutes past the hour, radio sta­
some solar material, many do not. It now          consequent absorption of HF signals known      tions WWV and WWVH broadcast the
seems clear that flares don’t cause CMEs          as a polar cap absorption (PCA) event. A       latest solar flux number, the average plane­
and vice versa.                                   PCA event may last for days, dramatically      tary A-Index and the latest Boulder
   While large flares can wreak disastrous        affecting transpolar HF propagation.           K-Index. In addition, they broadcast a
effects on HF propagation, discussed fur­            When a CME occurs, whether or not it        descriptive account of the condition of the
ther below, CMEs are the main causes of           accompanies a solar flare, most of the time    geomagnetic field and a forecast for the
long-lasting magnetic storms here on Earth.       the electrons and protons ejected from the     next three hours. You should keep in mind
Such storms can dramatically affect HF            sun do not reach the Earth. This is because    that the A-Index is a description of what
radio propagation — unfortunately, almost         their trajectory takes them in another di­     happened yesterday. Strictly speaking, the
always in a negative fashion.                     rection. If they do reach Earth, however,      K-Index is valid only for Boulder, Colo­
   This is not to minimize the effects that a     they do so 20 to 40 hours after the CME.       rado. However, the trend of the K-Index is
major solar flare can have on ionospheric         As these charged particles sweep past, if      very important for propagation analysis
propagation. After all, NASA rightly calls        their magnetic orientation is just right,      and forecasting. A rising K foretells wors­
solar flares “the biggest explosions in the       they can distort the Earth’s geomagnetic       ening HF propagation conditions, particu­
solar system.” X-ray radiation from a large       field, causing a geomagnetic storm. This       larly for transpolar paths. At the same
flare aimed towards Earth can cause an            results in acceleration of the particles to    time, a rising K alerts VHF operators to
immediate increase in D- and E-layer ion­         energy levels that permit them to penetrate    the possibility of enhanced auroral activ­
ization known as a sudden ionospheric dis­        into the ionosphere at the poles. This tre­    ity, particularly when the K-Index rises
turbance (SID). Severe D-layer absorption         mendous energy influx causes auroral dis­      above 3.
may cause a short-term blackout of all HF         plays at mid-latitudes and can disrupt HF
communications on the sun-facing side of          communications for several hours or even       D-Layer Propagation
the Earth. Signals in the 2 to 30-MHz range       much longer. Extraordinary radio noise           The D layer is the lowest region of the
may completely disappear. In extreme              and interference can accompany geomag­         ionosphere, situated between 55 and 90 km

20.10     Chapter 20
(30 and 60 mi). See Fig 20.13. It is ionized                                                    Daytime E Layer
primarily by the strong ultraviolet emission                                                       The E layer plays a small role in propa­
of solar hydrogen and short X-rays, both of                                                     gating HF signals but can be a major factor
which penetrate through the upper atmo­                                                         limiting propagation during daytime hours.
sphere. The D layer exists only during day­                                                     Its usual critical frequency of 3 to 4 MHz,
light, because constant radiation is needed                                                     with a maximum MUF factor of about 4.8,
to replenish ions that quickly recombine                                                        suggests that single-hop E-layer skip might
into neutral molecules. The D layer abrupt­                                                     be useful between 5 and 20 MHz at dis­
ly disappears at night so far as amateur MF                                                     tances up to 2300 km (1400 mi). In practice
and HF signals are concerned. D-layer ion­                                                      this is not the case, because the potential
ization varies a small amount over the
                                                                                                for E-layer skip is severely limited by D­
solar cycle. It is unsuitable as a refracting
                                                                                                layer absorption. Signals radiated at low
medium for any radio signals.
                                                                                                angles at 7 and 10 MHz, which might be
Daytime D-Layer Absorption                                                                      useful for the longest-distance contacts,
                                                                                                are largely absorbed by the D layer. Only
   Nevertheless, the D layer plays an impor­
                                                                                                high-angle signals pass through the D layer
tant role in HF communications. During
                                                                                                at these frequencies, but high-angle E­
daylight hours, radio energy as high as
5 MHz is effectively absorbed by the D                                                          layer skip is typically limited to 1200 km
layer, severely limiting the range of day­                                                      (750 mi) or so. Signals at 14 MHz penetrate
time 1.8- and 3.5-MHz signals. Signals at                                                       the D layer at lower angles at the cost of
                                                Fig 20.13—Typical electron densities for        some absorption, but the casual operator
7 MHz and 10 MHz pass through the D             the various ionospheric regions.
layer and on to the E and F layers only at                                                      may not be able to distinguish between sig­
relatively high angles. Low-angle waves,                                                        nals propagated by the E layer or higher­
which must travel a much longer distance                                                        angle F-layer propagation.
through the D layer, are subject to greater                                                        An astonishing variety of other propaga­
absorption. As the frequency increases          path length of about 2000 km (1200 mi) is       tion modes finds their home in the E layer,
above 10 MHz, radio waves pass through          limited by the height of the scattering         and this perhaps more than makes up for its
the D layer with increasing ease.               region, which is centered about 70 km           ordinary limitations. Each of these other
                                                (40 mi). Ionospheric scatter signals are        modes — sporadic E, field-aligned irregu­
Nighttime D Layer                               typically weak, fluttery and near the noise     larities, aurora, auroral E and meteor scat­
   D-layer ionization falls 100-fold as soon    level. Ionization from meteors sometimes        ter — are aberrant forms of propagation
as the sun sets and the source of ionizing      temporarily raises signals well out of the      with unique characteristics. They are pri­
radiation is removed. Low-band HF signals       noise for up to a few seconds at a time.        marily useful only on the highest HF and
are then free to pass through to the E layer       This mode may find its greatest use when     lower VHF bands.
(also greatly diminished at night) and on to    all other forms of propagation are absent,
the F layer, where the MUF is almost            primarily because ionospheric scatter sig­      Sporadic E
always high enough to propagate 1.8- and        nals are so weak. For best results at 28 and       Short skip, long familiar on the 10-m
3.5-MHz signals half way around the             50 MHz, a 3-element Yagi or larger, sev­        band during the summer months, affects the
world. Long-distance propagation at 7 and       eral hundred watts of power and a sensitive     VHF bands as high as 222 MHz. Sporadic
10 MHz generally improves at night as           receiver are required. The paths are direct.    E (E s), as this phenomenon is properly
well, because absorption is less and low­       CW is preferred, although, under optimal        called, commonly propagates 28, 50 and
angle waves are able to reach the F layer.      conditions, ionospheric scatter signals may     144-MHz radio signals between 500 and
                                                be consistent enough to support SSB com­        2300 km (300 and 1400 mi). Signals are apt
D-Layer Ionospheric Forward Scatter             munications. Scattering is not efficient be­    to be exceedingly strong, allowing even
   Radio signals in the 25-100 MHz range        low 25 MHz. The very best-equipped pairs        modest stations to make E s contacts. At
can be scattered by ionospheric irregulari­     of 144-MHz stations may also be able to         21 MHz, the skip distance may only be a
ties, turbulence and stratification in the D    complete ionospheric scatter contacts.          few hundred km. During the most intense
and lower reaches of the E layers. Signals                                                      Es events, skip may shorten to less than
propagated by ionospheric forward scat­         E-Layer Propagation                             200 km (120 mi) on the 10-m band and
ter undergo very high losses, so signals           The E layer lies between 90 and 150 km       disappear entirely on 15 m. Unusual
are apt to be very weak. Typical scatter        (60 and 90 mi) altitude, but a narrower         multiple-hop Es has supported contacts up
distances at 50 MHz are 800-1500 km             region centered at 95 to 120 km (60 to          to 10,000 km (6200 mi) on 28 and
(500-930 mi). This is not a common mode         70 mi) is more important for radio propa­       50 MHz and more than 3,000 km (1900 mi)
of propagation, but under certain condi­        gation. E-layer nitrogen and oxygen atoms       on 144 MHz. The first confirmed 220-MHz
tions, ionospheric forward scatter can be       are ionized by short UV and long X-ray          Es contact was made in June 1987, but such
very useful.                                    radiation. The normal E layer exists pri­       contacts are likely to remain very rare.
   Ionospheric forward scatter is best dur­     marily during daylight hours, because like         Sporadic E at midlatitudes (roughly 15°
ing daylight hours from 10 AM to 2 PM           the D layer, it requires a constant source of   to 45°) may occur at any time, but it is
local time, when the sun is highest in the      ionizing radiation. Recombination is not        most common in the Northern Hemisphere
sky and D-layer ionization peaks. It is worst   as fast as in the denser D layer and absorp­    during May, June and July, with a less­
at night. Scattering may be marginally more     tion is much less. The E layer has a day­       intense season at the end of December and
effective during the summer and during the      time critical frequency that varies between     early January. Its appearance is indepen­
solar cycle maximum due to somewhat             3 and 4 MHz with the solar cycle. At night,     dent of the solar cycle. Sporadic E is most
higher D-layer ionization. The maximum          the normal E layer all but disappears.          likely to occur from 9 AM to noon local

                                                                                                Propagation of RF Signals            20.11
time and again early in the evening
between 5 PM and 8 PM. Midlatitude Es
events may last only a few minutes to
many hours. In contrast, sporadic E is an
almost constant feature of the polar
regions at night and the equatorial belt
during the day.
   Efforts to predict midlatitude Es have not
been successful, probably because its causes
are complex and not well understood. Stud­
ies have demonstrated that thin and unusu­
ally dense patches of ionization in the E
layer, between 100 and 110 km (60 and
70 mi) altitude and 10 to 100 km (6 to 60 mi)
in extent, are responsible for most Es reflec­
tions. Sporadic-E clouds may form sud­
denly, move quickly from their birthplace,
and dissipate within a few hours. Profes­
sional studies have recently focused on the
role of heavy metal ions, probably of mete­
oric origin, and wind shears as two key fac­
tors in creating the dense patchy regions of
E-layer ionization.
   Sporadic-E clouds exhibit an MUF that
can rise from 28 MHz through the 50-MHz
band and higher in just a few minutes.
When the skip distance on 28 MHz is as
short as 400 or 500 km (250 or 310 mi), it
is an indication that the MUF has reached
50 MHz for longer paths at low launch
                                                 Fig 20.14—50 MHz sporadic-E contacts of 700 km (435 mi) or shorter (such as
angles. Contacts at the maximum one­             between Peoria and Little Rock) indicate that the MUF on longer paths is above
hop sporadic-E distance, about 2300 km           144 MHz. Using the same sporadic-E region reflecting point, 144-MHz contacts of
(1400 mi), should then be possible at            2200 km (1400 mi), such as between Pierre and Tallahassee, should be possible.
50 MHz. E-skip contacts as short as 700
km (435 mi) on 50 MHz, in turn, may in­
dicate that 144-MHz contacts in the 2300­        magnetic field, in something like moving          other forms of E-layer propagation, or
km (1400 mi) range can be completed. See         vertical rods. A similar process of elec­         about 2300 km (1400 mi).
Fig 20.14. Sporadic-E openings occur             tron field-alignment takes place during
about a tenth as often at 144 MHz in com­        radio aurora, making the two phenomena            Aurora
parison to 50 MHz and for much shorter           quite similar.                                       Radar signals as high as 3000 MHz have
periods.                                            Most reports suggest that 8 PM to mid­         been scattered by the aurora borealis or
   Sporadic E can also have a detrimental        night may be the most productive time for         northern lights (aurora australis in the
effect on HF propagation by masking the          FAI. Stations attempting FAI contacts             Southern Hemisphere), but amateur aurora
F2 layer from below. HF signals may be           point their antennas toward a common scat­        contacts are common only from 28 through
prevented from reaching the higher levels        tering region that corresponds to an active       432 MHz. By pointing directional antennas
of the ionosphere and the possibilities of       or recent E s reflection point. The best          generally north toward the center of aurora
long F2 skip. Reflections from the tops of       direction must be probed experimentally,          activity, oblique paths between stations up
sporadic-E clouds can also have a mask­          for the result is rarely along the great-circle   to 2300 km (1400 mi) apart can be com­
ing effect, but they may also lengthen the       path. Stations in south Florida, for example,     pleted. See Fig 20.15. High power and large
F2 propagation path with a top-side inter­       have completed 144-MHz FAI contacts               antennas are not necessary. Stations with
mediate hop that never reaches the Earth.        with north Texas when participating sta­          small Yagis and as little as 10 W output
                                                 tions were beamed toward a common scat­           have used auroras on frequencies as high as
E-Layer Field-Aligned Irregularities             tering region over northern Alabama.              432 MHz, but contacts at 902 MHz and
   Amateurs have experimented with a                FAI-propagated signals are weak and            higher are exceedingly rare. Auroral propa­
little-known scattering mode known as            fluttery, reminiscent of aurora signals.          gation works just as well in the Southern
field-aligned irregularities (FAI) at 50 and     Doppler shifts of as much as 3 kHz have           Hemisphere, in which case antennas must
144 MHz since 1978. FAI commonly                 been observed in some tests. Stations run­        be pointed south.
appear directly after sporadic-E events and      ning as little as 100 W and a single Yagi            The appearance of auroras is closely
may persist for several hours. Oblique­          should be able to complete FAI contacts           linked to solar activity. During massive
angle scattering becomes possible when           during the most favorable times, but              geomagnetic storms, high-energy par­
electrons are compressed together due to         higher power and larger antennas may              ticles flow into the ionosphere near the
the action of high-velocity ionospheric          yield better results. Contacts have been          polar regions, where they ionize the gases
acoustic (sound) waves. The resulting            made on 50 and 144 MHz and 222-MHz                of the E layer and higher. This unusual
irregularities in the distribution of free       FAI seems probable as well. Expected              ionization produces spectacular visual
electrons are aligned parallel to the Earth’s    maximum distances should be similar to            auroral displays, which often spread

20.12     Chapter 20
southward into the midlatitudes. Auroral
ionization in the E layer scatters radio sig­
nals in the VHF and UHF ranges.
   In addition to scattering radio signals,
auroras have other effects on worldwide
radio propagation. Communication below
20 MHz is disrupted in high latitudes, pri­
marily by absorption, and is especially
noticeable over polar and near-polar
paths. Signals on the AM broadcast band
through the 40-m band late in the after­
noon may become weak and watery. The
20-m band may close down altogether.
Satellite operators have also noticed that
144-MHz downlink signals are often weak
and distorted when satellites pass near the
polar regions. At the same time, the MUF
in equatorial regions may temporarily rise
dramatically, providing transequatorial
paths at frequencies as high as 50 MHz.
   Auroras occur most often around the           Fig 20.15—Point antennas generally north to make oblique long-distance contacts
spring and fall equinoxes (March-April           on 28 through 432 MHz via aurora scattering. Optimal antenna headings may shift
and September-October), but auroras may          considerably to the east or west depending on the location of the aurora.
appear in any month. Aurora activity gen­
erally peaks about two years before and
after solar cycle maximum. Radio aurora          east and west to peak signals, because          able from sporadic E. Auroral-E paths are
activity is usually heard first in late after­   auroral ionization is field aligned. This       almost always east-west oriented, perhaps
noon and may reappear later in the               means that for any pair of stations, there is   because there are few stations at very
evening. Auroras may be anticipated by           an optimal direction for aurora scatter.        northern latitudes to take advantage of this
following the A- and K-index reports on          Offsets from north are usually greatest         propagation.
WWV. A K index of five or greater and            when the aurora is closest and often pro­          Auroral E may also appear while espe­
an A index of at least 30 are indications        vide the longest contacts. There may be         cially intense auroras are still in progress,
that a geomagnetic storm is in progress          some advantage to antennas that can be          as happened during the great aurora of
and an aurora likely. The probability, in­       elevated, especially when auroras are high      March 1989. On that occasion, 50-MHz
tensity and southerly extent of auroras          in the sky.                                     propagation shifted from Doppler-distorted
increase as the two index numbers rise.                                                          aurora paths to clear-sounding auroral E
Stations north of 42° latitude in North          Auroral E                                       over a period of a few minutes. Many 6-m
America experience many auroral open­               Radio auroras may evolve into a propa­       operators as far south as Florida and South­
ings each year, while those in the Gulf          gation mode known as auroral E at 28, 50        ern California made single- and double-hop
Coast states may hear auroral signals no         and rarely 144 MHz. Doppler distortion          auroral-E contacts across the country. At
more than once a year, if that often.            disappears and signals take on the charac­      about the same time, the MUF reached
   Aurora-scattered signals are easy to          teristics of sporadic E. The most effective     144 MHz for stations west of the Great
identify. On 28- and 50-MHz SSB, signals         antenna headings shift dramatically away        Lakes to the Northeast, the first time au­
sound very distorted and somewhat wider          from oblique aurora paths to direct great­      roral E had been reported so high in fre­
than normal; at 144 MHz and above, the           circle bearings. The usual maximum dis­         quency. At least two other rare instances of
distortion may be so severe that only CW is      tance is 2300 km (1400 mi), typical for         2-m auroral E have been reported.
useful. Auroral CW signals have a distinc­       E-layer modes, but 28- and 50-MHz
tive note variously described as a buzz, hiss    auroral-E contacts of 5000 km (3100 mi)         Meteor Scatter
or mushy sound. This characteristic auroral      are sometimes made across Canada and               Contacts between 800 and 2300 km (500
signal is due to Doppler broadening, caused      the northern US, apparently using two           and 1400 mi) can be made at 28 through
by the movement of electrons within the          hops. Contacts at 50 MHz between Alaska         432 MHz via reflections from the ionized
aurora. An additional Doppler shift of           and the east coasts of Canada and the           trails left by meteors as they travel through
1 kHz or more may be evident at 144 MHz          northern US have been completed this            the ionosphere. The kinetic energy of
and several kilohertz at 432 MHz. This           way. Transatlantic 50-MHz auroral-E             meteors no larger than grains of rice are
second Doppler shift is the result of mas­       paths are also likely, although only one        sufficient to ionize a column of air 20 km
sive electrical currents that sweep electrons    such contact has been reported.                 (12 mi) long in the E layer. The particle
toward the sun side of the Earth during             Typically, 28- and 50-MHz auroral E          itself evaporates and never reaches the
magnetic storms. Doppler shift and distor­       appears across the northern third of the        ground, but the ionized column may per­
tion increase with higher frequencies, while     US and southern Canada when aurora              sist for a few seconds to a minute or more
signal strength dramatically decreases.          activity is diminishing. This usually hap­      before it dissipates. This is enough time to
   It is not necessary to see an aurora to       pens after midnight on the eastern end of       make very brief contacts by reflections
make auroral contacts. Useful auroras may        the path. Auroral-E signals sometimes           from the ionized trails. Millions of meteors
be 500-1000 km (310-620 mi) away and             have a slightly hollow sound to them and        enter the Earth’s atmosphere every day, but
below the visual horizon. Antennas should        build slowly in strength over an hour or        few have the required size, speed and ori­
be pointed generally north and then probed       two, but otherwise they are indistinguish­      entation to the Earth to make them useful

                                                                                                 Propagation of RF Signals             20.13
for meteor-scatter propagation.                that provides 30 seconds of communica­           is by far the most important for long­
   Radio signals in the 30- to 100-MHz         tion at 50 MHz will last only a few sec­         distance HF communications. F-region
range are reflected best by meteor trails,     onds at 144 MHz, and less than a second at       oxygen atoms are ionized primarily by
making the 50-MHz band prime for meteor­       432 MHz.                                         ultraviolet radiation. During the day, ion­
scatter work. The early morning hours             Meteor scatter opportunities are some­        ization reaches maxima in two distinct
around dawn are usually the most produc­       what better during July and August because       layers. The F1 layer forms between 150
tive, because the morning side of the Earth    the average number of meteors entering the       and 250 km (90 and 160 mi) and disap­
faces in the direction of the planet’s orbit   Earth’s atmosphere peaks during those            pears at night. The F2 layer extends above
around the Sun. The relative velocity of       months. The best times are during one of         250 km (160 mi), with a peak of ionization
meteors that head toward the Earth’s morn­     the great annual meteor showers, when the        around 300 km (190 mi). At night,
ing side are thus increased by up to 30 km/    number of useful meteors may increase ten­       F-region ionization collapses into one
sec, the average rotational speed of the       fold over the normal rate of five to ten per     broad layer at 300-400 km (190-250 mi)
Earth in orbit. See Fig 20.16. The maximum     hour. See Table 20.4. A meteor shower            altitude. Ions recombine very slowly at
velocity of meteors in orbit around the Sun    occurs when the Earth passes through a           these altitudes, because molecular density
is 42 km/sec. Thus when the relative veloc­    relatively dense stream of particles,            is relatively low. Maximum ionization
ity of the Earth is considered, most meteors   thought to be the remnants of a comet, that      levels change significantly with time of
must enter the Earth’s atmosphere some­        are also in orbit around the sun. The most­      day, season and year of the solar cycle.
where between 12 and 72 km/sec.                productive showers are relatively consis­
   Meteor contacts ranging from a second       tent from year to year, although several can     F1 Layer
or two to more than a minute can be made       produce great storms periodically.                  The daytime F1 layer is not important to
nearly any morning at 28 or 50 MHz.               Because meteors provide only fleeting         HF communication. It exists only during
Meteor-scatter contacts at 144 MHz and         moments of communication even during one         daylight hours and is largely absent in win­
higher are more difficult because reflected    of the great meteor showers, special operat­     ter. Radio signals below 10 MHz are not
signal strength and duration drop sharply      ing techniques are often used to increase the    likely to reach the F1 layer, because they are
with increasing frequency. A meteor trail      chances of completing a contact. Prear­          either absorbed by the D layer or refracted
                                               ranged schedules between two stations            by the E layer. Signals higher than 20 MHz
                                               establish times, frequencies and precise         that pass through both of the lower iono­
                                               operating standards. Usually, each station       spheric regions are likely to pass through the
                                               transmits on alternate 15-second periods         F1 layer as well, because the F1 MUF rarely
                                               until enough information is pieced together      rises above 20 MHz. Absorption diminishes
                                               a bit at a time to confirm contact. High-speed   the strength of any signals that continue
                                               Morse code of several hundred words per          through to the F2 layer during the day. Some
                                               minute, generated and slowed down by spe­        useful F1-layer refraction may take place
                                               cial computer programs, can make effective       between 10 and 20 MHz during summer
                                               use of very short meteor bursts. Nonsched­       days, yielding paths as long as 3000 km
                                               uled random meteor contacts are common           (1900 mi), but these would be practically
                                               on 50 MHz and 144 MHz, but short trans­          indistinguishable from F2 skip.
                                               missions and alert operating habits are
                                               required.                                        F2 and Nighttime F Layers
                                                  It is helpful to run several hundred watts       The F 2 layer forms between 250 and
                                               to a single Yagi, but meteor-scatter can be      400 km (160 and 250 mi) during the day­
                                               used by modest stations under optimal            time and persists throughout the night as a
                                               conditions. During the best showers, a few       single consolidated F region 50 km (30 mi)
                                               watts and a small directional antenna are        higher in altitude. Typical ion densities are
                                               sufficient at 28 or 50 MHz. At 144 MHz,          the highest of any ionospheric layer, with
                                               at least 100 W output and a long Yagi are        the possible exception of some unusual
                                               needed for consistent results. Proportion­       E-layer phenomenon. In contrast to the
                                               ately higher power is required for 222 and       other ionospheric layers, F2 ionization var­
                                               432 MHz even under the best conditions.          ies considerably with time of day, season
                                                                                                and position in the solar cycle, but it is
                                               F-Layer Propagation                              never altogether absent. These two charac­
                                                 The region of the F layers, from 150 km        teristics make the F2 layer the most impor­
                                               (90 mi) to over 400 km (250 mi) altitude,        tant for long-distance HF communications.
                                                                                                   The F2-layer MUF is nearly a direct func­
                                                                                                tion of UV solar radiation, which in turn
                                               Table 20.4                                       follows closely the solar cycle. During the
                                               Major Annual Meteor Showers                      lowest years of the cycle, the daytime MUF
Fig 20.16—The relative velocity of                                           Approximate        may climb above 14 MHz for only a few
meteors that meet the Earth head-on is                                       Rate               hours a day. In contrast, the MUF may rise
increased by the rotational velocity of        Name           Peak Dates     (meteors/hour)     beyond 50 MHz during peak years and
the Earth in orbit. Fast meteors strike        Quadrantids    Jan 3             50              stay above 14 MHz throughout the night.
the morning side of the Earth because          Arietids       Jun 7-8           60              The virtual height of F 2 averages 330 km
their velocity adds to the Earth’s             Perseids       Aug 11-13         80              (210 mi), but varies between 200 and
rotational velocity, while the relative        Orionids       Oct 20-22         20
velocity of meteors that “catch up from                                                         400 km (120 and 250 mi). Maximum one­
                                               Geminids       Dec 12-13         60
behind” is reduced.                                                                             hop distance is about 4000 km (2500 mi).

20.14     Chapter 20
Near-vertical incidence skywave propaga­         sporadic E) and F-layer hops may be            F-Layer Long Path
tion just below the critical frequency pro­      mixed. In practice, multihop signals ar­          Most HF communication takes place
vides reliable coverage out to 200-300 km        rive via many different paths, which often     along the shortest great-circle path between
(120-190 mi) with no skip zone. It is most       increases the problems of fading. Analyz­      two stations. Short-path propagation is
often observed on 7 MHz during the day.          ing multihop paths is complicated by the       always less than 20,000 km (12,000 mi) —
   The extraordinary high-angle Pedersen         effects of D- and E-layer absorption, pos­     halfway around the Earth. Nevertheless, it
Ray can create effective single-hop paths of     sible reflections from the tops of sporadic-   may be possible at times to make the same
5,000 to 12,000 km under certain conditions,     E layers, disruptions in the auroral zone      contact in exactly the opposite direction via
but most operators will not be able to distin­   and other phenomena.                           the long path. The long-path distance will
guish Pedersen-Ray paths from normal                                                            be 40,000 km (25,000 mi) minus the short­
F-layer propagation. Pedersen-Ray paths are                                                     path length. Signal strength via the long
most evident over high-latitude east-west                                                       path is usually considerably less than the
paths at frequencies near the MUF. They                                                         more direct short-path. When both paths
appear most often about noon local time at                                                      are open simultaneously, there may be a
mid-path when the geomagnetic field is very                                                     distinctive sort of echo on received signals.
quiet. Pedersen-Ray propagation may be                                                          The time interval of the echo represents the
responsible for 50 MHz paths between the                                                        difference between the short-path and long­
US Northeast and Western Europe, for                                                            path distances.
example, when ordinary MUF analysis could                                                          Sometimes there is a great advantage
not explain the 5,000-km contacts. See                                                          to using the long path when it is open,
Fig 20.17E.                                                                                     because signals can be stronger and fading
   In general, both F2-layer ionization and                                                     less troublesome. There are times when
MUF build rapidly at sunrise, usually                                                           the short path may be closed or disrupted
reach a maximum in the afternoon, and                                                           by E-layer blanketing, D-layer absorption
then decrease to a minimum prior to sun­                                                        or F-layer gaps, especially when operating
rise. Depending on the season, the MUF is                                                       just below the MUF. Long paths that
generally highest within 20° of the equator                                                     predominantly cross the night side of the
and lower toward the poles. For this rea­                                                       Earth, for example, are sometimes useful
son, transequatorial paths may be open at                                                       because they generally avoid blanketing
a particular frequency when all other paths                                                     and absorption problems. Daylight-side
are closed.                                                                                     long paths may take advantage of higher
   In contrast to all the other ionospheric                                                     F-layer MUFs that occur over the sunlit
layers, daytime ionization in the winter F2                                                     portions of the Earth.
layer averages four times the level of the
summer at the same period in the solar                                                          F-Layer Gray-Line
cycle, doubling the MUF. This so-called                                                            Gray-line paths can be considered a
winter anomaly is caused by the Earth                                                           special form of long-path propagation that
moving closer to the Sun and tilting. Win­                                                      take into account the unusual ionospheric
tertime F2 conditions are much superior to                                                      configuration along the twilight region
those in summer, because the MUF is much                                                        between night and day. The gray line, as
higher.                                                                                         the twilight region is sometimes called,
                                                                                                extends completely around the world. It is
Multihop F-Layer Propagation                                                                    not precisely a line, for the distinction
   Most HF communication beyond 4000 km                                                         between daylight and darkness is a gradual
(2500 mi) takes place via multiple iono­                                                        transition due to atmospheric scattering.
spheric hops. Radio signals are reflected                                                       On one side, the gray line heralds sunrise
from the Earth back toward space for addi­       Fig 20.17—Multihop paths can take many         and the beginning of a new day; on the
tional ionospheric refractions. A series of      different configurations, including a          opposite side, it marks the end of the day
ionospheric refractions and terrestrial          mixture of E- and F-layer hops. (A) Two        and sunset.
reflections commonly create paths half­          F-layer hops. Five or more consecutive            The ionosphere undergoes a significant
                                                 F-layer hops are possible. (B) An E-layer
way around the Earth. Each hop involves                                                         transformation between night and day. As
                                                 hookup to the F layer. (C) A top-side
additional attenuation and absorption, so        E-layer reflection can shorten the dis­        day begins, the highly absorbent D and E
the longest-distance signals tend to be the      tance of two F-layer hops. (D) Refraction      layers are recreated, while the F-layer
weakest. Even so, it is possible for signals     in the E layer above the MUF is insuffi­       MUF rises from its pre-dawn minimum.
to be propagated completely around the           cient to return the signal to Earth, but it    At the end of the day, the D and E layers
world and arrive back at their originating       can go on to be refracted in the F layer.
                                                                                                quickly disappear, while the F-layer MUF
                                                 (E) The Pedersen Ray, which originates
point. Multiple reflections within the F         from a signal launched at a relatively high    continues its slow decline from late after­
layer may bypass ground reflections alto­        angle above the horizon into the E or F        noon. For a brief period just along the
gether, creating what are known as chordal       region, may result in a single-hop path,       gray-line transition, the D and E layers are
hops, with lower total attenuation. It takes     5000 km (3100 mi) or more. This is             not well formed, yet the F2 MUF usually
a radio signal about 0.15 second to make a       considerably further than the normal           remains higher than 5 MHz. This provides
round-the-world trip.                            4000-km (2500 mi) maximum F-region
                                                 single-hop distance, where the signal is
                                                                                                a special opportunity for stations at 1.8
   Multihop paths can take on many dif­          launched at a very low takeoff angle. The      and 3.5 MHz.
ferent configurations, as shown in the           Pedersen Ray can easily be disrupted by           Normally, long-distance communica­
examples of Fig 20.17. E-layer (especially       any sort of ionospheric gradient.              tion on the lowest two amateur bands can

                                                                                                Propagation of RF Signals             20.15
take place only via all-darkness paths           create unusual paths within the skip zone.        is oblique to the normal great-circle path.
because of daytime D-layer absorption.           Backscatter and sidescatter signals are usu­      Two stations can make use of a common
The gray-line propagation path, in con­          ally observed just below the MUF for the          side-scattering region well off the direct
trast, extends completely around the             direct path and allow communications not          path, often toward the south. European and
world. See Fig 20.18. This unusual situa­        normally possible by other means. Stations        North American stations sometimes com­
tion lasts less than an hour at sunrise and      using backscatter point their antennas            plete 28-MHz contacts via a scattering
sunset when the D-layer is largely absent,       toward a common scattering region at the          region over Africa. US and Finnish 50-MHz
and may support contacts that are difficult      one-hop distance, rather than toward each         operators observed a similar effect early
or impossible at other times.                    other. Backscattered signals are generally        one morning in November 1989 when they
   The gray line generally runs north-south,     weak and have a characteristic hollow             made contact by beaming off the coast of
but it varies by 23° either side of true north   sound. Useful communication distances             West Africa.
as measured at the equator over the course of    range from 100 km (60 mi) to the normal              When backscattered signals cross an
the year. This variation is caused by the tilt   one-hop distance of 4000 km (2500 mi).            area where there is a sharp gradient in
in the Earth’s axis. The gray line is exactly       Backscatter and sidescatter are closely        ionospheric density, such as between night
north-south through the poles at the equi­       related and the terminology does not pre­         and day, the path may take on a different
noxes (March 21 and September 21) and is         cisely distinguish between the two. Back­         geometry, as shown in Fig 20.20. In this
at its 23° extremes on June 21 and December      scatter usually refers to single-hop signals      case, stations can communicate because
20. Over a one-year period, the gray line        that have been scattered by the Earth or the      backscattered signals return via the day
crosses a 46° sector of the Earth north and      ocean at some distant point back toward the       side ionosphere on a shorter hop than the
south of the equator, providing optimum          transmitting station. Two stations spaced a       night side. This is possible because the
paths to slightly different parts of the world   few hundred km apart can often communi­           dayside MUF is higher and thus the skip
each day. Many commonly available com­           cate via a backscatter path near the MUF.         distance shorter. The net effect is to create
puter programs plot the gray line on a flat      See Fig 20.19.                                    a backscatter path between two stations
map or globe. The ARRL Operating Manual             Sidescatter usually refers to a circuit that   within the normal skip zone.
provides sunrise and sunset times over the
entire year for several hundred worldwide
locations. The position of the gray line on
any date can also be plotted manually on a
globe from these data.

F-Layer Backscatter and Sidescatter
  Special forms of F-layer scattering can

                                                 Fig 20.19—Schematic of a simple backscatter path. Stations A and B are too close
                                                 to make contact via normal F-layer ionospheric refraction. Signals scattered back
                                                 from a distant point on the Earth’s surface (S), often the ocean, may be accessible
                                                 to both and create a backscatter circuit.

Fig 20.18—The gray line encircles the
Earth, but the tilt at the equator to the
poles varies over 46° with the seasons.
Long-distance contacts can often be
made halfway around the Earth along
the gray line, even as low as 1.8 and
3.5 MHz. The strength of the signals,
characteristic of gray-line propagation,
indicates that multiple Earth-ionosphere
hops are not the only mode of                    Fig 20.20—Backscatter path across the gray line. Stations A and B are too close to
propagation, since losses in many such           make contact via normal ionospheric refraction, but may hear each other’s signals
hops would be very great. Chordal                scattered from point S. Station A makes use of a high-angle refraction on the day
hops, where the signals are confined to          side of the gray line, where the MUF is high. Station B makes use of a night-time
the ionosphere for at least part of the          refraction, with a lower MUF and lower angle of propagation. Note that station A
journey, are involved.                           points away from B to complete the circuit.

20.16     Chapter 20
Fig 20.21—Transequatorial spread-F propagation takes place between stations equidistant across the geomagnetic equator.

Distances up to 8000 km (5000 mi) are possible on 28 through 432 MHz. Note the geomagnetic equator is considerably south

of the geographic equator in the Western Hemisphere.

Transequatorial Spread-F                        suggests that the F2 layer near the equator      up to 8000 km (5000 mi).
   Discovered in 1947, transequatorial          bulges and intensifies slightly, particu­           Spread-F propagation also occurs over
spread-F (TE) supports propagation              larly during solar maxima. Irregular field­      the polar regions, but because of low popu­
between 5000 and 8000 km (3100 and              aligned ionization forms shortly after           lation densities, amateurs have rarely
5000 mi) across the equator from 28 MHz         sunset in an area 100-200 km (60-120 mi)         reported making use of it. Near the north­
to as high as 432 MHz. Stations attempt­        north and south of the geomagnetic equator       ern magnetic pole (located in extreme
ing TE contacts must be nearly equidis­         and 500-3000 km (310-1900 mi) wide. For          northeastern Canada), spread-F is a nearly
tant from the geomagnetic equator. Many         this reason, the mode is sometimes called        permanent feature of winter. During sum­
contacts have been made at 50 and 144           transequatorial field-aligned irregulari­        mer, it appears most summer nights and at
MHz between Europe and South Africa,            ties. It moves west with the setting sun. The    least half the time during the day. There is
Japan and Australia and the Caribbean           MUF may increase to twice its normal level       a greater probability of polar spread-F
region and South America. Fewer contacts        15° either side of the geomagnetic equator.      appearing during the equinox periods and
have been made on the 222-MHz band. TE             Field alignment of ionospheric irregulari­    during the solar cycle maximum. Field­
signals have been heard at 432 MHz, but         ties favors refraction along magnetic field      alignment in the polar regions suggests
so far, no two-way contacts have resulted.      lines, that is north-south. VHF and UHF sig­     that some form of backscatter signals,
   Unfortunately for most continental US        nals are refracted twice over the geomag­        similar to aurora, would be most likely.
stations, the geomagnetic equator dips south    netic equator at angles that normally would
                                                be insufficient to bring the signals back to­    MUF PREDICTION
of the geographic equator in the Western
Hemisphere, as shown in Fig 20.21, making       ward Earth. See Fig 20.22. The geometry is          F-layer MUF prediction is key to fore­
only the most southerly portions of Florida     such that two shallow reflections in the F2      casting HF communications paths at par­
and Texas within TE range. TE contacts          layer can create north-south terrestrial paths   ticular frequencies, dates and times, but
from the southeastern part of the country
may be possible with Argentina, Chile and
even South Africa.
   Transequatorial spread-F peaks between
5 PM and 10 PM during the spring and fall
equinoxes, especially during the peak years
of the solar cycle. The lowest probability is
during the summer. Quiet geomagnetic con­
ditions are required for TE to form. Signals
have a rough aurora-like note, sometimes
termed flutter fading. High power and large
antennas are not required to work TE, as
VHF stations with 100 W and single long
Yagis have been successful.                     Fig 20.22—Cross-section of a transequatorial spread-F signal path, showing the

   The best explanation of TE propagation       effects of ionospheric bulging and a double refraction above the normal MUF.

                                                                                                 Propagation of RF Signals            20.17
forecasting is complicated by several vari­    propagated on at least 10% of the days in
ables. Solar radiation varies over the         the month. The given values might be ex­       Table 20.5
course of the day, season, year and solar      ceeded considerably on a few rare days.        Shortwave Broadcasting Bands
cycle. These regular intervals provide the     On at least half the days, propagation         Frequency          Band
main basis for prediction, yet recurrence      should be possible on frequencies as high      (MHz)              (m)
is far from reliable. In addition, forecasts   as the middle curve. Propagation will           2.300-2.495        120
are predicated on a quiet geomagnetic          exceed the lowest curve on at least 90% of      3.200-3.400         90
field, but the condition of the Earth’s mag­   the days. The exact MUF on any particular       3.900-4.000         75
                                                                                               4.750-5.060         60
netic field is most difficult to predict       day cannot be determined from these sta­        5.959-6.200         49
weeks or months ahead. For professional        tistical charts, but the calculated times       7.100-7.300         41
users of HF communications, uncertainty        when a band will open and close is reli­        9.500-9.900         31
is a nuisance for maintaining reliable com­    able. You would use a long-range forecast      11.650-12.050        25
munications paths, while for many ama­         to determine when you should start moni­       13.600-13.800        22
                                                                                              15.100-15.600        19
teurs it provides an aura of mystery and       toring a band to see if propagation actu­
                                                                                              17.550-17.900        16
chance that adds to the fun of DXing. Nev­     ally does occur that day, particularly at      21.450-21.850        13
ertheless, many amateurs want to know          frequencies above 30 MHz.                      25.600-26.100        11
what to expect on the HF bands to make            Short-range forecasts of a few days
best use of available on-the-air time, plan    ahead are marginally more reliable than
contest strategy, ensure successful net        long-range forecasts, because underlying
operations or engage in other activities.      solar indices and geomagnetic conditions       country. A Radio Moscow or BBC pro­
                                               can be anticipated with greater confi­         gram, for example, may be relayed to a
MUF Forecasts                                  dence. The tendency for solar disturbances     transmitter outside Russia or England for
   Long-range forecasts several months         to recur at 27-day intervals also enhance      retransmission. An excellent guide to
ahead, such as those formerly published in     short-term forecasts. Daily forecasts are      shortwave broadcast stations is the World
QST and other journals, provide only the       even more reliable, because they are based     Radio TV Handbook, available through the
most general form of prediction. A series of   on current solar and geophysical data, as      ARRL.
48 charts on the members-only ARRLWeb          well as warnings provided by observations
site (, simi­     of the sun in the visual to X-ray range.       WWV and WWVH
lar to Fig 20.23, forecast average propa­         The CD-ROM bundled with the 20th               The standard time stations WWV (Ft
gation for a one-month period over specific    Edition of The ARRL Antenna Book con­          Collins, Colorado) and WWVH (Kauai,
paths. The charts assume a single aver­        tains even more detailed propagation-pre­      Hawaii), which transmit on 2.5, 5, 10, 15
age solar flux value for the entire month      diction tables from 150+ QTHs around the       and 20 MHz, are also popular for propaga­
and they assume that the geomagnetic           world for six levels of solar activity, for    tion monitoring. They transmit 24 hours a
field is undisturbed.                          the 12 months of the year. Again, keep in      day. Daily monitoring of these stations for
   The uppermost curve in Fig 20.23            mind that these long-range forecasts           signal strength and quality can quickly
shows the highest frequency that will be       assume quiet geomagnetic conditions.           provide a good basic indication of propa­
                                               Real-time MUF forecasts are also avail­        gation conditions. In addition, each hour
                                               able in a variety of text and graphical        they broadcast the geomagnetic A and K
                                               forms on the WWW. Forecasts can also be        indices, the 2800-MHz (10.7-cm) solar
                                               made at home using one of several popu­        flux, and a short forecast of conditions for
                                               lar programs for personal computers,           the next day. These are heard on WWV at
                                               including ASAPS, CAPMan, VOACAP,               18 minutes past each hour and on WWVH
                                               W6ELProp and WinCAP Wizard 2.                  at 45 minutes after the hour. The same
                                                                                              information is also available by telephon­
                                               Direct Observation                             ing the recorded message at 303-497-3235
                                                  Propagation conditions can be deter­        or various Web sites, such as
                                               mined directly by listening to the HF bands.   propagation/index.html. The K index is
                                               The simplest method is to tune higher in       updated every three hours, while the A
                                               frequency until no more long-distance sta­     index and solar flux are updated after 2100
                                               tions are heard. This point is roughly just    UTC. These data are useful for making
                                               above the MUF to anywhere in the world at      predictions on home computers, espe­
                                               that moment. The highest usable amateur        cially when averaged over several days of
Fig 20.23—Propagation prediction               band would be the next lowest one. If HF       solar flux observations.
chart for West Coast to Western Europe         stations seem to disappear around 23 MHz,
from the ARRLWeb members-only site             for example, the 15-m band at 21 MHz           Beacons
for April 2001. An average 2800-MHz            might make a good choice for DXing. By           Automated beacons in the higher ama­
(10.7-cm) solar flux of 159 was assumed        carefully noting station locations as well,    teur bands can also be useful adjuncts to
for the month. On 10% of the days, the
                                               the MUF in various directions can also be      propagation watching. Beacons are ideal
highest frequency propagated is
predicted to be at least as high as the        determined quickly.                            for this purpose because most are designed
uppermost curve (the Highest Possible             The shortwave broadcast bands (see          to transmit 24 hours a day. One of the
Frequency, or HPF, approximately               Table 20.5) are most convenient for MUF        best organized beacon systems is designed
33 MHz), and for 50% of the days as            browsing, because there are many high­         by the Northern California DX Foundation,
high as the middle curve, the MUF. The         powered stations on regular schedules.         operating at 14.100, 18.110, 21.150, 24.930
lowest curve shows the Lowest Usable
Frequency (LUF) for a 1500-W CW
                                               Take care to ensure that programming is        and 28.200 MHz. Eleven beacons on five
transmitter.                                   actually transmitted from the originating      continents transmit in eighteen successive

20.18     Chapter 20
                                                                                               a 1000-m (3280-ft) mountain has a radio
Table 20.6                                                                                     horizon of 130 km (80 mi).
Popular Beacon Frequencies
Frequencies                                                                                    Atmospheric Absorption
(MHz)                        Comments                                                             Atmospheric gases, most notably oxygen
14.100, 18.110,              Northern California DX Foundation beacons                         and water vapor, absorb radio signals, but
21.150, 24.930, 28.200                                                                         neither is a significant factor below 10 GHz.
28.2-28.3                    Several dozen beacons worldwide
50.0-50.1                    Most US beacons are within 50.06-50.08 MHz                        Attenuation from rain becomes important
70.03-70.13                  Beacons in England, Ireland, Gibraltar and Cyprus                 at 3.3 GHz, where signals passing through
                                                                                               20 km (12 mi) of heavy showers incur an
                                                                                               additional 0.2 dB loss. That same rain
                                                                                               would impose 12 dB additional loss at
one-minute intervals. More on this system,      cause refraction. Under average condi­         10 GHz and losses continue to increase with
along with a longer list of HF, VHF and         tions, radio waves are refracted toward        frequency. Heavy fog is similarly a prob­
UHF beacons, can be found in The ARRL           Earth enough to make the horizon appear        lem only at 5.6 GHz and above. More
Operating Manual. Other interested groups       1.15 times farther away than the visual        detailed information about atmospheric
publish updated lists of beacons with call      horizon. Under unusual conditions, tropo­      absorption in the microwave bands can
sign, frequency, location, transmitter mode,    spheric refraction may extend this range       be found in the ARRL UHF/Microwave
power, and antenna. Beacons often include       significantly.                                 Experimenter’s Manual.
location as part of their automated message,       A simple formula can be used to esti­
and many can be located from their call         mate the distance to the radio horizon         Tropospheric Scatter
sign. Thus, even casual scanning of beacon      under average conditions:                         Contacts beyond the radio horizon out to
subbands can be useful. Table 20.6 pro­                                                        a working distance of 100 to 500 km (60 to
vides the frequencies where beacons useful      d = 2h                                         310 mi), depending on frequency, equip­
to HF propagation are most commonly             where                                          ment and local geography, are made every
placed.                                                                                        day without the aid of obvious propagation
                                                 d = distance to the radio horizon,
                                                                                               enhancement. At 1.8 and 3.5 MHz, local
PROPAGATION IN THE                                    miles
                                                                                               communication is due mostly to ground
TROPOSPHERE                                      h = height above average terrain, ft
                                                                                               wave. At higher frequencies, especially in
   All radio communication involves                                                            the VHF range and above, the primary
propagation through the troposphere for         d = 17h                                        mechanism is scattering in the troposphere,
at least part of the signal path. Radio waves   where                                          or troposcatter.
traveling through the lowest part of the         d = distance to the radio horizon, km            Most amateurs are unaware that they use
atmosphere are subject to refraction, scat­      h = height above average terrain, m           troposcatter even though it plays an essen­
tering and other phenomena, much like                                                          tial role in most local communication.
ionospheric effects. Tropospheric condi­           The distance to the radio horizon for an    Radio signals through the VHF range are
tions are rarely significant below 30 MHz,      antenna 30 m (98 ft) above average terrain     scattered primarily by wave-length sized
but they are very important at 50 MHz and       is thus 22.6 km (14 mi), a station on top of   gradients in the index of refraction of the
higher. Much of the long-distance work
on the VHF, UHF and microwave bands
depends on some form of tropospheric
propagation. Instead of watching solar
activity and geomagnetic indices, those
who use tropospheric propagation are
much more concerned about the weather.

Line of Sight
   At one time it was thought that commu­
nications in the VHF range and higher
would be restricted to line-of-sight paths.
Although this has not proven to be the case
even in the microwave region, the concept
of line of sight is still useful in under­
standing tropospheric propagation. In the
vacuum of space or in a completely homo­
geneous medium, radio waves do travel
essentially in straight lines, but these con­
ditions are almost never met in terrestrial
   Radio waves traveling through the tro­
posphere are ordinarily refracted slightly
earthward. The normal drop in tempera­          Fig 20.24—Tropospheric-scatter path geometry. The lower boundary of the
ture, pressure and water-vapor content          common scattering volume is limited by the take-off angle of both stations. The
with increasing altitude change the index       upper boundary of 10 km (6 mi) altitude is the limit of efficient scattering in the
of refraction of the atmosphere enough to       troposphere. Signal strength increases with the scattering volume.

                                                                                               Propagation of RF Signals             20.19
lower atmosphere due to turbulence, along      troposcatter paths, but typical maxima are        antenna height, because that lowers the
with changes in temperature. Radio signals     more like half that. Tropospheric scatter         take-off angle to the horizon. Working
in the microwave region can also be scat­      varies little with season or time of day, but     range increases less quickly with antenna
tered by rain, snow, fog, clouds and dust.     it is difficult to assess the effect of weather   gain and transmitter power. For this rea­
That tiny part that is scattered forward and   on troposcatter alone. Variations in tropo­       son, a mountaintop is the choice location
toward the Earth creates the over-the­         spheric refraction, which is very sensitive       for extending ordinary troposcatter work­
horizon paths. Troposcatter path losses are    to the weather, probably account for most         ing distances.
considerable and increase with frequency.      of the observed day-to-day differences in
   The maximum distance that can be            troposcatter signal strength.                     Rain Scatter in the Troposphere
linked via troposcatter is limited by the          Troposcatter does not require special           Scatter from raindrops is a special case
height of a scattering volume common to        operating techniques or equipment, as it is       of troposcatter practical in the 3.3- to
two stations, shown schematically in           used unwittingly all the time. In the ab­         24-GHz range. Stations simply point their
Fig 20.24. The highest altitude for which      sence of all other forms of propagation,          antennas toward a common area of rain. A
scattering is efficient at amateur power       especially at VHF and above, the usual            certain portion of radio energy is scattered
levels is about 10 km (6 mi). An application   working range is essentially the maximum          by the raindrops, making possible over­
of the distance-to-the-horizon formula         troposcatter distance. Ordinary working           the-horizon or obstructed-path contacts,
yields 800 km (500 mi) as the limit for        range increases most dramatically with            even with low power. The theoretical

  MUF Prediction on the Home Computer
      Like predicting the weather, predicting HF propaga­                for about 30 years in one form or another. The IONCAP
   tion — even with the best computer software available                 program has a well-deserved reputation for being difficult
   — is not an exact science. The processes occurring as                 to use, since it came from the world of Fortran punch cards
   a signal is propagated from one point on the Earth to                 and mainframe computers.
   another are enormously complicated and subject to an                    CAPMan is a DOS-based version of IONCAP that is
   incredible number of variables. Experience and a                      considerably more “user friendly” than the core program.
   knowledge of propagation conditions (as related to                    CAPMan produces excellent graphs, some calibrated in S
   solar activity, especially unusual solar activity, such as            units if the user wishes. It incorporates amateur call signs
   flares or Coronal Mass Ejections) are needed when you                 to specify locations, making it comfortable for amateurs to
   actually get on the air to check out the bands. Keep in               use. CAPMan also allows the user to specify multiple
   mind, too, that ordinary computer programs are written                antenna types for both transmitting and receiving. See:
   mainly to calculate propagation for great-circle paths      
   via the F layer. Scatter, skew-path, auroral and other                  VOACAP is another version of IONCAP, but this one
   such propagation modes may provide contacts when                      includes a sophisticated Windows interface. The Voice of
   computer predictions indicate no contacts are possible.               America (VOA) started work on VOACAP in the early
      It used to be possible to classify propagation­                    1990s and continued for several years before funding ran
   prediction programs by whether they were used                         out. The program is now maintained by a single, dedicated
   primarily for heavy-duty, long-term forecasting — for                 computer scientist, Greg Hand, at NTIA/ITS (Institute for
   planning a high-power shortwave broadcast station, for                Telecommunication Sciences), an agency of the US
   example — or for making a short-term forecast,                        Department of Commerce in Boulder, CO. Although
   perhaps to check out whether a band might be open                     VOACAP is not specifically designed for amateurs (and
   today for a particular DXpedition. But with the increas­              thus doesn’t include some features that amateurs are fond
   ing amount of computing power available nowadays,                     of, such as entry of locations by ham-radio call signs and
   that distinction has blurred. What follows is some brief              multiple receiving antennas), it is available for free by
   information about commercially available propagation­                 downloading from:
   prediction programs for the IBM PC and compatible
   computers. See Table 20.A.                                            W6ELProp, Version 1.0
                                                                            In 2001, W6EL ported his well known DOS-based
   ASAPS Version 5                                                       MINIPROP PLUS program into the Windows world. It uses
      An agency of the Australian government has devel­                  the same Fricker-based computation engine as its predeces­
   oped the ASAPS program, which stands for Advanced                     sor. W6ELProp has a highly intuitive, ham-friendly user
   Stand-Alone Prediction System. It rivals IONCAP (see                  interface. It produces the same detailed output tables as its
   below) in its analysis capability and in its prediction               DOS counterpart, along with a number of useful charts and
   accuracy. It is a Windows program that interacts                      maps, including the unique and useful “frequency map,”
   reasonably well with the user, once you become                        which shows the global MUFs from a given transmitting
   accustomed to the acronyms used. If you change                        location for a particular month/day/time and solar-activity
   transmit power levels, antennas and other parameters,                 level. W6ELProp is available for free by downloading from:
   you can see the new results almost instantly without        
   further menu entries. Available from IPS Radio and
   Space Services. See:                        WinCAP Wizard 2
                                                                           Kangaroo Tabor Software sells the CAPMan program
   IONCAP, CAPMan and VOACAP                                             and is also the creator of the Active Beacon Wizard
     IONCAP, short for Ionospheric Communications                        program included with the 19th and 20th Editions of The
   Analysis and Prediction, was written by an agency of                  ARRL Antenna Book. They also sell a Windows-based
   the US government and has been under development                      “mini” version of CAPMan, called WinCAP Wizard 2. This

20.20     Chapter 20
range for rain scatter is as great as 600 km   tion under standard atmospheric condi­       wide geographic area, signals may remain
(370 mi), but the experience of amateurs       tions extends the radio horizon some­        very strong over distances of 1500 km
in the microwave bands suggests that           what beyond the visual line of sight.        (930 mi) or more. Ducting results from
expected distances are less than 200 km        Favorable weather conditions further         the gradient created by a sharp increase in
(120 mi). Snow and hail make less effi­        enhance normal tropospheric refraction,      temperature with altitude, quite the oppo­
cient scattering media unless the ice par­     lengthening the useful VHF and UHF           site of normal atmospheric conditions. A
ticles are partially melted. Smoke and dust    range by several hundred kilometers and      simultaneous drop in humidity contrib­
particles are too small for extraordinary      increasing signal strength. Higher fre­      utes to increased refractivity. Useful tem­
scattering, even in the microwave bands.       quencies are more sensitive to refraction,   perature inversions form between 250 and
                                               so its effects may be observed in the        2000 m (800-6500 ft) above ground. The
Refraction and Ducting in the                  microwave bands before they are appar­       elevated inversion and the Earth’s surface
Troposphere                                    ent at lower frequencies.                    act something like the boundaries of a
   Radio waves are refracted by natural           Ducting takes place when refraction is    natural open-ended waveguide. Radio
gradients in the index of refraction of        so great that radio waves are bent back to   waves of the right frequency range caught
air with altitude, due to changes in tem­      the surface of the Earth. When tropo­        inside the duct will be propagated for
perature, humidity and pressure. Refrac­       spheric ducting conditions exist over a      long distances with relatively low losses.

   uses the CAPMan computing engine but limits the                   that can do complete 3D ray tracing through the iono­
   number of input parameters to those most commonly                 sphere, even taking complex geomagnetic effects into
   used by amateurs. The outputs are customizable and                account. The number of computations is huge, especially
   include dynamic summary tables, sunrise/sunset                    in the full-blown 3D mode and operation can be slow and
   tables and propagation maps.                                      tedious. The user interface is also very complex and
                                                                     demanding, with a steep user-learning curve. However, it
   PropLab Pro, Version 2                                            is fascinating to see exactly how a signal can bend off­
      PropLab Pro by Solar Terrestrial Dispatch repre­               azimuth or how it can split into the ordinary and extra­
   sents the high end of propagation-prediction programs.            ordinary waves. See:
   It is the only commercial program presently available             proplab.html.

   Table 20.A
   Features and Attributes of Propagation Prediction Programs
                                ASAPS       VOACAP      W6ELProp                  CAPMan       WinCAP           PropLab
                                V. 5        Windows     V. 1.00                                Wizard 2         Pro
   User Friendliness                  Good            Good          Good          Good         Good             Poor
   Operating System                   Windows         Windows       Windows       DOS          Windows          DOS
   Uses k index                       No              No            Yes           Yes          Yes              Yes
   User library of QTHs               Yes/Map         Yes           Yes           Yes          Yes              No
   Bearings, distances                Yes             Yes           Yes           Yes          Yes              Yes
   MUF calculation                    Yes             Yes           Yes           Yes          Yes              Yes
   LUF calculation                    Yes             Yes           No            Yes          Yes              Yes
   Wave angle calculation             Yes             Yes           Yes           Yes          Yes              Yes
   Vary minimum wave angle            Yes             Yes           Yes           Yes          Yes              Yes
   Path regions and hops              Yes             Yes           Yes           Yes          Yes              Yes
   Multipath effects                  Yes             Yes           No            Yes          Yes              Yes
   Path probability                   Yes             Yes           Yes           Yes          Yes              Yes
   Signal strengths                   Yes             Yes           Yes           Yes          Yes              Yes
   S/N ratios                         Yes             Yes           Yes           Yes          Yes              Yes
   Long path calculation              Yes             Yes           Yes           Yes          Yes              Yes
   Antenna selection                  Yes             Yes           Indirectly    Yes          Isotropic        Yes
   Vary antenna height                Yes             Yes           Indirectly    Yes          No               Yes
   Vary ground characteristics        Yes             Yes           No            Yes          No               No
   Vary transmit power                Yes             Yes           Indirectly    Yes          Yes              Yes
   Graphic displays                   Yes             Yes           Yes           Yes          Yes              2D/3D
   UT-day graphs                      Yes             Yes           Yes           Yes          Yes              Yes
   Area Mapping                       Yes             Yes           Yes           Yes          No               Yes
   Documentation                      Yes             On-line       Yes           Yes          Yes              Yes
   Price class                        $275 1          free 2        free 3        $89 4        $29.95 4         $150 5
   Prices are for early 2004 and are subject to change.
   1ASAPS: shipping and handling extra. See:
   2VOACAP available at:
   3W6EL Prop, see:
   4CAPMan and WinCAP Wizard 2, see:
   5PropLab Pro, see:

                                                                                            Propagation of RF Signals           20.21
Several common weather conditions can          pressed and heated. Layers of warmer        the south of high-pressure centers. See
create temperature inversions.                 air — temperature inversions — often form   Fig 20.27.
                                               between 500 and 3000 m (1500-10,000 ft)        Sluggish high-pressure systems likely
Radiation Inversions in the                    altitude, as shown in Fig 20.26. Ducts      to contain strong temperature inversions
Troposphere                                    usually intensify during the evening and    are common in late summer over the east­
   Radiation inversions are probably the       early morning hours, when surface tem­      ern half of the US. They generally move
most common and widespread of the vari­        peratures drop and suppress the tendency    southeastward out of Canada and linger
ous weather conditions that affect propa­      for daytime ground-warmed air to rise.      for days over the Midwest, providing
gation. Radiation inversions form only over    In the Northern Hemisphere, the longest     many hours of extended propagation. The
land after sunset as a result of progressive   and strongest radio paths usually lie to    southeastern part of the country and the
cooling of the air near the Earth’s surface.                                               lower Midwest experience the most high­
As the Earth cools by radiating heat into                                                  pressure openings; the upper Midwest and
space, the air just above the ground is                                                    East Coast somewhat less frequently; the
cooled in turn. At higher altitudes, the air                                               western mountain regions rarely.
remains relatively warmer, thus creating                                                      Semipermanent high-pressure systems,
the inversion. A typical radiation-inversion                                               which are nearly constant climatic fea­
temperature profile is shown in Fig 20.25.                                                 tures in certain parts of the world, sustain
   The cooling process may continue                                                        the longest and most exciting ducting
through the evening and predawn hours,                                                     paths. The Eastern Pacific High, which
creating inversions that extend as high as                                                 migrates northward off the coast of
500 m (1500 ft). Deep radiation inversions                                                 California during the summer, has been
are most common during clear, calm, sum­                                                   responsible for the longest ducting paths
mer evenings. They are more distinct in dry                                                reported to date. Countless contacts in the
climates, in valleys and over open ground.                                                 4000-km (2500 mi) range have been made
Their formation is inhibited by wind, wet                                                  from 144 MHz through 5.6 GHz between
ground and cloud cover. Although radia­                                                    California and Hawaii. The Bermuda High
tion inversions are common and wide­                                                       is a nearly permanent feature of the Carib­
spread, they are rarely strong enough to                                                   bean area, but during the summer it moves
cause true ducting. The enhanced condi­                                                    north and often covers the southeastern
tions so often observed after sunset during                                                US. It has supported contacts in excess of
the summer are usually a result of this mild                                               2800 km (1700 mi) from Florida and the
kind of inversion.                             Fig 20.26—Temperature and humidity          Carolinas to the West Indies, but its full
                                               profile across an elevated duct at
                                                                                           potential has not been exploited. Other
High-Pressure Weather Systems                  1000-m altitude. Such inversions
                                               typically form in summertime high­          semipermanent highs lie in the Indian
  Large, sluggish, high-pressure systems       pressure systems. Note the air is very      Ocean, the western Pacific and off the
(or anticyclones) create the most dramatic     dry in the inversion.                       coast of western Africa.
and widespread tropospheric ducts due
to subsidence. Subsidence inversions in
high-pressure systems are created by air
that is sinking. As air descends, it is com-

Fig 20.25—Temperature and dewpoint
profile of an early-morning radiation          Fig 20.27—Surface weather map for September 13, 1993, shows that the eastern
inversion. Fog may form near the               US was dominated by a sprawling high-pressure system. The shaded portion
ground. The midday surface                     shows the area in which ducting conditions existed on 144 through 1286 MHz
temperature would be at least 30°C.            and higher.

20.22     Chapter 20
Wave Cyclone                                         The land breeze is a light, steady, cool     that may remain for hours. Land-breeze
   The wave cyclone is a more dynamic             wind that commonly blows up to 50 km            inversions often bring enhanced conditions
weather system that usually appears dur­          (30 mi) inland from the oceans, although        and occasionally allow contacts in excess
ing the spring over the middle part of the        the distance may be greater in some circum­     of 800 km (500 mi) along coastal areas.
American continent. The wave begins as a          stances. Land breezes develop after sunset         In southern Europe, a hot, dry wind
disturbance along a boundary between              on clear summer evenings. The land cools        known as the sirocco sometimes blows
cooler northern and warmer southern air           more quickly than the adjacent ocean. Air       northward from the Sahara Desert over rela­
masses. Southwest of the disturbance, a           cooled over the land flows near the surface     tively cooler and moister Mediterranean
cold front forms and moves rapidly east­          of the Earth toward the ocean to displace       air. Sirocco inversions can be very strong
ward, while a warm front moves slowly             relatively warmer air that is rising. See       and extend from Israel and Lebanon west­
northward on the eastward side. When              Fig 20.29. The warmer ocean air, in turn,       ward past the Straits of Gibraltar. Sirocco­
the wave is in its open position, as shown        travels at 200-300 m (600-1000 ft) altitude     type inversions are probably responsible for
in Fig 20.28, north-south radio paths             to replace the cool surface air. The land-sea   record-breaking microwave contacts in
1500 km (930 mi) and longer may be pos­           circulation of cool air near the ground and     excess of 1500 km (930 mi) across the
sible in the area to the east of the cold front   warm air aloft creates a mild inversion         Mediterranean.
and south of the warm front, known as the
warm sector. East-west paths nearly as
long may also open in the southerly parts
of the warm sector.
   Wave cyclones are rarely productive for
more than a day in any given place, because
the eastward-moving cold front eventually
closes off the warm sector. Wave-cyclone
temperature inversions are created by a
southwesterly flow of warm, dry air above
1000 m (3200 ft) that covers relatively
cooler and moister gulf air flowing north­
ward near the Earth’s surface. Successive
waves spaced two or three days apart may
form along the same frontal boundary.

Warm Fronts and Cold Fronts
   Warm fronts and cold fronts sometimes
bring enhanced tropospheric conditions,
but rarely true ducting. A warm front
marks the surface boundary between a
mass of warm air flowing over an area of
relatively cooler and more stationary air.
Inversion conditions may be stable enough
several hundred kilometers ahead of the
warm front to create extraordinary paths.         Fig 20.28—Surface weather map for June 2, 1980, with a typical spring wave
   A cold front marks the surface bound­          cyclone over the southeastern quarter of the US. The shaded portion shows
ary between a mass of cool air that is wedg­      where ducting conditions existed.
ing itself under more stationary warm air.
The warmer air is pushed aloft in a narrow
band behind the cold front, creating a
strong but highly unstable temperature in­
version. The best chance for enhancement
occurs parallel to and behind the passing
cold front.
Other Conditions Associated With
   Certain kinds of wind may also create
useful inversions. The Chinook wind that
blows off the eastern slopes of the Rockies
can flood the Great Plains with warm and
very dry air, primarily in the springtime. If
the ground is cool or snow-covered, a
strong inversion can extend as far as
Canada to Texas and east to the Missis­
sippi River. Similar kinds of foehn winds,
as these mountain breezes are called, can
be found in the Alps, Caucasus Mountains          Fig 20.29—Land-breeze convection along a coast after sunset creates a
and other places.                                 temperature inversion over the land.

                                                                                                  Propagation of RF Signals            20.23
Marine Boundary Layer Effects                   from the moon may not arrive with the           VHF and especially UHF are largely
   Over warm water, such as the Caribbean       same polarization. An additional 20 dB of       immune from these effects, but free-space
and other tropical seas, evaporation inver­     path loss is incurred when polarization is      path losses are greater. Problems related
sions may create ducts that are useful          shifted by 90°, an intolerable amount           to polarization, including Faraday rota­
in the microwave region between 3.3 and         when signals are marginal.                      tion, intentional or accidental satellite
24 GHz. This inversion depends on a sharp          Faraday rotation is difficult to predict     tumbling and the orientation of a satel­
drop in water-vapor content rather than         and its effects change over time and with       lite’s antenna in relation to terrestrial
on an increase in temperature to create         operating frequency. At 144 MHz, the            antennas, are largely overcome by using
ducting conditions. Air just above the sur­     polarization of space waves may shift back      circularly polarized antennas. More on
face of water at least 30°C is saturated        into alignment with the antenna within a        using satellites can be found in the chapter
because of evaporation. Humidity drops          few minutes, so often just waiting can          on Space Communications.
significantly within 3 to 10 m (10 to 30 ft)    solve the Faraday problem. At 432 MHz,
                                                it may take half an hour or longer for the      NOISE AND PROPAGATION
altitude, creating a very shallow but stable
duct. Losses due to water vapor absorp­         polarization to become realigned. Use of           Noise simply consists of unwanted
tion may be intolerable at the highest duct­    circular polarization completely elimi­         radio signals that interfere with desired
ing frequencies, but breezes may raise          nates this problem, but creates a new one       communications. In some instances, noise
the effective height of the inversion and       for EME paths. The sense of circularly          imposes the practical limit on the lowest
open the duct to longer wavelengths. Sta­       polarized signals is reversed with reflec­      usable frequencies. Noise may be classi­
tions must be set up right on the beaches       tion, so two complete antenna systems are       fied by its sources: man-made, terrestrial
to ensure being inside an evaporation           normally required, one with left-hand and       and cosmic. Interference from other trans­
inversion.                                      one with right-hand polarization.               mitting stations on adjacent frequencies
                                                                                                is not usually considered noise and may
Tropospheric Fading                             Earth-Moon-Earth                                be controlled, to a some degree anyway,
   Tropospheric turbulence and small               Amateurs have used the moon as a             by careful station design.
changes in the weather are responsible for      reflector on the VHF and UHF bands since
most fading at VHF and higher. Local            1960. Maximum allowable power and               Man-Made Noise
weather conditions, such as precipitation,      large antennas, along with the best receiv­        Many unintentional radio emissions
warm air rising over cities and the effects     ers, are normally required to overcome the      result from man-made sources. Broadband
of lakes and rivers, can all contribute to      extreme free-space and reflection losses        radio signals are produced whenever there
tropospheric instabilities that affect radio    involved in Earth-Moon-Earth (EME)              is a spark, such as in contact switches,
propagation. Fast-flutter fading at 28 MHz      paths. More modest stations make EME            electric motors, gasoline engine spark
and above is often the result of an airplane    contacts by scheduling operating times          plugs and faulty electrical connections.
that temporarily creates a second propaga­      when the Moon is at perigee on the hori­        Household appliances, such as fluorescent
tion path. Flutter results as the phase rela­   zon. The Moon, which presents a target          lamps, microwave ovens, lamp dimmers
tionship between the ordinary tropospheric      only one-half degree wide, reflects only        and anything containing an electric motor
signal and that reflected by the airplane       7% of the radio signals that reach it. Tech­    may all produce undesirable broadband
change with the airplane’s movement.            niques have to be designed to cope with         radio energy. Devices of all sorts, espe­
                                                Faraday rotation, cosmic noise, Doppler         cially computers and anything controlled
EXTRATERRESTRIAL                                shift (due to the Moon’s movements) and         by microprocessors, television receivers
PROPAGATION                                     other difficulties. In spite of the problems    and many other electronics also emit radio
   Communication of all sorts into space        involved, hundreds of stations have made        signals that may be perceived as noise
has become increasingly important. Ama­         contacts via the Moon on all bands from         well into the UHF range. In many cases,
teurs confront extraterrestrial propagation     50 MHz to 10 GHz. The techniques of             these sources are local and can be con­
when accessing satellite repeaters or using     EME communication are discussed in the          trolled with proper measures. See the
the moon as a reflector. Special propaga­       chapter on Space Communications.                EMI/DFing chapter.
tion problems arise from signals that travel                                                       High-voltage transmission lines and
from the Earth through the ionosphere (or       Satellites                                      associated equipment, including trans­
a substantial portion of it) and back again.      Accessing amateur satellites generally        formers, switches and lightning arresters,
Tropospheric and ionospheric phenomena,         does not involve huge investments in            can generate high-level radio signals over
so useful for terrestrial paths, are unwanted   antennas and equipment, yet station             a wide area, especially if they are corroded
and serve only as a nuisance for space com­     design does have to take into account spe­      or improperly maintained. Transmission
munication. A phenomenon known as               cial challenges of space propagation.           lines may act as efficient antennas at some
Faraday rotation may change the polariza­       Free-space loss is a primary consideration,     frequencies, adding to the noise problem.
tion of radio waves traveling through the       but it is manageable when satellites are        Certain kinds of street lighting, neon signs
ionosphere, presenting special problems to      only a few hundred kilometers distant.          and industrial equipment also contribute
receiving weak signals. Cosmic noise also       Free-space path losses to satellites in high-   their share of noise.
becomes an important factor when anten­         Earth orbits are considerably greater, and
nas are intentionally pointed into space.       appropriately larger antennas and higher        Lightning
                                                powers are needed.                                 Static is a common term given to the
Faraday Rotation                                  Satellite frequencies below 30 MHz can        ear-splitting crashes of noise commonly
  Magnetic and electrical forces rotate the     be troublesome. Ionospheric absorption          heard on nearly all radio frequencies,
polarization of radio waves passing             and refraction may prevent signals from         although it is most severe on the lowest
through the ionosphere. For example, sig­       reaching space, especially to satellites at     frequency bands. Atmospheric static is
nals that leave the Earth as horizontally       very low elevations. In addition, man­          primarily caused by lightning and other
polarized, and return after a reflection        made and natural sources of noise are high.     natural electrical discharges. Static may

20.24     Chapter 20
result from close-by thunderstorms, but         the atmosphere with an audible pop before         Nov 1983, pp 7-14.
most static originates with tropical storms.    recharging. Precipitation static and corona     B. R. Bean and E. J. Dutton, Radio Meteo­
Like any radio signals, lightning-pro­          discharge can be a nuisance from LF to well       rology (New York: Dover, 1968).
duced static may be propagated over long        into the VHF range.                                         I
                                                                                                K. Davies, 	 onospheric Radio (London:
distances by the ionosphere. Thus static is                                                       Peter Peregrinus, 1989). Excellent,
generally higher during the summer, when        Cosmic Sources                                    though highly technical text on propa­
there are more nearby thunderstorms, and           The sun, distant stars, galaxies and other     gation.
at night, when radio propagation gener­         cosmic features all contribute radio noise      G. Grayer, “VHF/UHF Propagation,”
ally improves. Static is often the limiting     well into the gigahertz range. These cos­         Ch 2 of The VHF/UHF DX Book
factor on 1.8 and 3.5 MHz, making winter        mic sources are perceived primarily as a          (Buckingham, England: DIR, 1992).
a more favorable time for using these           more-or-less constant background noise at       G. Jacobs, T. Cohen, R. Rose, The NEW
frequencies.                                    HF. In the VHF range and higher, specific         Shortwave Propagation Handbook, CQ
                                                sources of cosmic noise can be identified         Communications, Inc. (Hicksville, NY:
Precipitation Static and Corona                 and may be a limiting factor in terrestrial       1995).
Discharge                                       and space communications. The sun is by         L. F. McNamara, Radio Amateur’s Guide
   Precipitation static is an almost continu­   far the greatest source of radio noise, but       to the Ionosphere (Malabar, Florida:
ous hash-type noise that often accompanies      its effects are largely absent at night. The      Krieger Publishing Company, 1994).
various kinds of precipitation, including       center of our own galaxy is nearly as noisy       Excellent, quite-readable text on HF
snowfall. Precipitation static is caused by     as the sun. Galactic noise is especially          propagation.
raindrops, snowflakes or even wind-blown        noticeable when high-gain VHF and UHF                        R
                                                                                                C. Newton, 	 adio Auroras (Potters Bar,
dust, transferring a small electrical charge    antennas, such as may be used for satellite       England: Radio Society of Great Brit­
on contact with an antenna. Electrical fields   or EME communications, are pointed                ain, 1991).
under thunderstorms are sufficient to place     toward the center of the Milky Way. Other       E. Pocock, “UHF and Microwave
many objects such as trees, hair and anten­     star clusters and galaxies are also radio         Propagation,” Ch 3 of The ARRL UHF/
nas, into corona discharge. Corona noise        hot-spots in the sky. Finally, there is a         Microwave Experimenter’s Manual
may sound like a harsh crackling in the         much lower cosmic background noise that           (Newington, Connecticut: ARRL,
radio — building in intensity, abruptly end­    seems to cover the entire sky.                    1990).
ing, and then building again, in cycles of                                                      E. Pocock, Ed., Beyond Line of Sight: A
a few seconds to as long as a minute. A         FURTHER READING                                   History of VHF Propagation from the
corona charge on an antenna may build to        J. E. Anderson, “MINIMUF for the Ham              Pages of QST (Newington, Connecticut:
some critical level and then discharge in          and the IBM Personal Computer,” QEX,           ARRL, 1992).

                                                                                                Propagation of RF Signals          20.25