"Propagation of RF Signals"
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 FUNDAMENTALS OF RADIO WAVES 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 c=fλ 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 (MUFF) 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 (www.arrl.org/qst/propcharts/), 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 dx.qsl.net/ 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 propagation. 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 www.taborsoft.com/. 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: elbert.its.bldrdoc.gov/hf.html. 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 www.qsl.net/w6elprop. further menu entries. Available from IPS Radio and Space Services. See: www.ips.gov.au/index.php. 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: www.spacew.com/www/ 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: www.ips.gov.au/index.php 2VOACAP available at: elbert.its.bldrdoc.gov/hf.html 3W6EL Prop, see: www.qsl.net/w6elprop 4CAPMan and WinCAP Wizard 2, see: www.taborsoft.com/ 5PropLab Pro, see: www.spacew.com/www/proplab.html 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 Ducts 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