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```					                       HF Radio Wave Propagation

I.      Introduction

Radio wave propagation is an important issue for amateurs. Different frequencies
propagate through different modes, and each mode has its own special characteristics.
Choosing the proper mode for a particular frequency can mean the difference between
making and missing a contact to a particular part of the world.

spans 3 to 30 MHz. This includes the 80, 40, 30, 20, 17, 15, 12, and 10 meter bands. The
only MF amateur band, 160m, will not be discussed, nor will bands above 10 meters.

II.     Overview of HF Propagation
HF radio propagation exhibits certain distinguishing characteristics. The first is that
propagation is possible over thousands of miles. Early experimenters were quite surprised
by this, because laboratory experiments had confirmed that radio waves, like light waves,
travel in a straight line.

The second characteristic of HF propagation is that is highly variable. It has daily and
seasonal variation, as well as a much longer 11 year cycle. This stands in contrast to line
of sight propagation, which is generally quite constant and predictable (a good example
of this is reception of commercial FM broadcasts).

HF radio waves may travel by any of the following modes:

1. Ground Wave
2. Direct Wave (line-of-sight)
3. Sky Wave

In the HF region, the ground is a poor conductor and the ground wave is quickly
attenuated by ground losses. Some ground wave communication is possible on 80m, but
at frequencies above 5 MHz, the ground wave is irrelevant.

Direct waves follow the line-of-sight path between transmitter and receiver. In order for
direct wave communication to occur, antennas at both ends of the path have to have low
angles of radiation (so they can “see” each other). This is difficult to do on the lower
bands, and as a result, direct wave communication is normally restricted to bands above
20m. Its range is determined by the height of both antennas and generally less than 20
miles.

Sky waves are waves that leave the transmitting antenna in a straight line and are
returned to the earth at a considerable distance by an electrically charged layer known as
the ionosphere. Communication is possible throughout much of the day to almost
anywhere in the world via sky wave.

Throughout the remainder of the article, the attention will be on the sky wave and the
phenomena that produce it

III.    The Ionosphere

At high altitudes, high energy solar radiation can ionize the atmosphere. This region,
known as the ionosphere, is electrically active as a result of the ionization. It can bend
and attenuate radio waves that travel through it, causing some to be returned to earth and
others simply to disappear. At frequencies above 200 MHz, the ionosphere becomes
completely transparent to radio waves and has little effect on them. Below 30 MHz the
ionosphere exerts a profound effect on radio waves, creating many of the propagation
phenomena observed at HF, MF, LF and VLF frequencies.

The ionosphere generally consists of 4 highly ionized regions, separated by regions of
lower ionization:

1.   The D layer at a height of 38 – 55 mi
2.   The E layer at a height of 62 – 75 mi
3.   The F1 layer at a height of 125 –150 mi (winter) and 160 – 180 mi (summer)
4.   The F2 layer at a height of 150 – 180 mi (winter) and 240 – 260 mi (summer)

The density of ionization is greatest in the F layers and least in the D layer.

One might expect that since the ionosphere is created by solar radiation, it would
disappear shortly after sunset. The ionosphere changes after dark, but does not
completely disappear. The D and E layers are located at lower altitudes, where the
pressure is higher. The recombination rate for ions is very high at these pressures so the
D and E layers disappear very quickly after sunset. The F layers are located in a low
pressure region where recombination is very slow. The F1 and F2 layers do not
disappear, but merge into a single F layer residing at a distance of 150 – 250 mi above the
earth.

The D layer plays only a negative role in HF communications. It acts as an attenuator,
absorbing the radio signals, rather than returning them to earth. This absorption is
inversely proportional to the square of the frequency, severely restricting
communications on the lower HF bands during daylight.

The E layer can return lower HF frequencies to the Earth, resulting in daytime short skip
on the lower HF bands. It has very little effect on higher frequency HF radio waves, other
than to change slightly their direction of travel.

The F layers are primarily responsible for long-haul HF communications. Because there
is only F layer ionization throughout the hours of darkness, it is carries almost all
nighttime communications over intercontinental distances.

IV.        The Critical Frequency (fc) and Maximum Usable Frequency (MUF)

It has already been noted that the ionosphere can bend radio waves sufficently to send
them back in the direction of the earth. The geometry of this “hop” is shown in the figure
below.

When radio waves are transmitted straight up towards the ionosphere (vertical incidence),
the radio wave will be returned to earth at all frequencies below the critical frequency,
(fc) . The critical frequency depends on the degree of ionization of the F layer, as shown
in the following equation:
Ne
f cr 
1.24 * 10 10

where fcr is the critical frequency and Ne is the electron concentration in the F layer.
Daytime electron concentration in the F layer range from 5*1011 m-3 to 25*1011 m-3 . At
night, the values are about 10 times lower.

In most real world communications, the radio waves leave the transmitter more or less in
the direction of the horizon (0 – 30 degrees takeoff angle). In this case, the radio wave
hits the ionosphere obliquely and less bending is required to return the wave to earth.
This means that frequencies above the critical frequency can be returned. For a takeoff
angle of 0 degrees, the maximum frequency returned by the ionosphere is called the
maximum usable frequency (MUF). The critical frequency and the MUF are related by
the following equation:

f cr
MUF 
2
 R 
1       
 R  h
where R is the earth’s radius and h is the height of the F (F2) layer. The daytime MUF
can range from 15 to over 40 MHz. At night, the MUF drops to 3 to 14 MHz.

The longest hop possible on the HF bands is approximately 2500 miles, for a takeoff
angle of 0 degrees. Higher takeoff angles yield shorter hops. Longer distances are
covered by multiple hop propagation. When the refracted radio wave returns to earth, it is
reflected back up towards the ionosphere, which begins another hop.

V.      Daily Propagation Effects

Earlier, it was noted that HF propagation was extremely variable. The simplest and
shortest propagation cycle is the daily cycle.

Shortly after sunrise, the D and E layers are formed and the F layer splits into two parts.
The D layer acts as a selective absorber, attenuating low frequency signals by the greatest
amount. D layer attenuation at 3 MHz can easily be 20 dB more than attenuation at 30
MHz. This makes frequencies below 5 or 6 MHz useless during the day for DX work.

The E and F1 layers increase steadily in intensity from sunrise to noon and then decreases
thereafter. It is possible to have some short skip propagation via the E or F1 layers when
the local time at the ionospheric refraction point is approximately noon. The MUF’s for
the E and F1 layers are about 5 and 10 MHz respectively.

The F2 layer is sufficiently ionized to radically bend (refract) the HF radio waves and
return them to earth. As long as the MUF is above 5 - 6 MHz, long distance
communications are possible. When the MUF falls below 5 MHz, the frequencies that
can be returned by the F layer are completely attenuated by the D layer.

During the daylight hours, the best bands to use are 15, 12, and 10m. There is usually
some short skip on 17, 20, 30 and 40m, but there will be no sky wave propagation on
80m. Once darkness falls, bands above 17m become quiet as the MUF drops. Worldwide
propagation is possible on the 80m, 40m, 30m and 20m bands at night, although high
noise levels on 80m can make working across continents very difficult.

VI.     Seasonal Propagation Effects

HF propagation varies throughout the seasons. During the winter months, the atmosphere
is colder and denser. This causes the ionospheric layers to be closer to the earth (see layer
data presented in III) and to have a higher electron density. During the brief daytime of
the Northern Hemisphere winter, the earth makes its closest approach to the sun, which
increases the intensity of the UV radiation striking the ionosphere and the electron
density as well. Electron density during the northern hemisphere winter can be 5 times
greater than summer’s. This leads to winter MUF’s that are about double the summer
daytime value.
At night, the denser F layer undergoes recombination more quickly, causing the MUF to
decrease more over the course of the winter evening than in summer. Additionally, the
winter evenings are longer, so the F layer has more time to lose ions. Shortly before dawn
in the dead of winter, the MUF can fall to extremely low values, perhaps to 2 MHz or
lower.

The D, E and F1 layers are relatively unaffected by the seasons. The D layer is a strong
absorber throughout the year, and the E and F1 layers can be used for short skip single
hop communications when the time halfway between the two stations in about 12 noon.

During the winter daylight hours, the best bands are 20, 17, 15, 12, and 10m, with some
short skip on 40m. At night, 80 and 40m are the best bets. Should the MUF after dark fall
below 3.5 MHz, no HF propagation via sky wave will be possible.

VII.    Geographical Variation

The sun’s ionizing radiation is most intense in the equatorial regions and least intense in
the polar regions. As a result, the daytime MUF of the E and F1 layers is highest in the
tropics. Polar region MUF’s for these layers can be three times lower. The F2 layer
shows a more complex geographical MUF variation. While equatorial F2 MUF’s are
generally higher that polar F2 MUF’s, the highest F2 MUF often occurs somewhere near
Japan and the lowest over Scandinavia.

VIII. Effects of Sunspots

The final major source of variation in HF propagation is sunspots. A sunspot is a cool
region on the sun’s surface that resembles a dark blemish on the sun. The number of
sunspots observed on the sun’s surface follows an 11 year cycle. The cycle starts when
the sun is free of spots. From that time about 4 – 5 years are required for the number of
sunspots to reach a maximum. Then the sunspot count begins a slow decline over the next
6 – 7 years. The cycle is complete after 11 years.

Sunspots carry with them intense magnetic fields. These fields energize a region of the
sun known as the chromosphere, which lies just above the sun’s surface. As more
sunspots appear on the solar disk, the chromosphere becomes more active, emitting more
ultraviolet radiation, which increases the electron density in the earth’s atmosphere.

The additional radiation affects primarily the F2 layer. During periods of peak sunspot
activity, such as December 2001 or February 1958 the F2 MUF can rise to more than 50
MHz.

During a sunspot maximum, the highly ionized F2 layer acts like a mirror, refracting the
higher HF frequencies (above 20 MHz) with almost no loss. This allows amateurs to
make contacts on the 15, 12 and 10m bands in excess of 10,000 miles using 10 watts or
less. During short summer evenings, the MUF can stay above 14 MHz. The 20 m band
stays open to some point in the world around the clock.
During periods of high activity it is also possible to have backscatter propagation either
from the ionosphere or the auroral regions. Backscatter communication is unique in that
the stations in contact do not point their antennas at each other, but instead at the region
of high ionization in the ionosphere or towards the north (or south in the other
hemisphere) magnetic pole. The figure below shows the location of the north auroral
zone. During periods of high solar activity, this doughnut shaped region may expand to
the south, approaching the US-Canadian border in North America, and covering
Scandinavia in Europe.

During a sunspot minimum, the chromosphere is very quiet and its UV emissions are
very low. This leads to a major decrease in the ionization of the F2 layer and its MUF is
reduced accordingly. The MUF rarely rises to 20 MHz under these conditions and most
long distance communications must be carried out on the lower HF bands.

During periods of high sunspot activity, the best daytime bands are 12 and 10m. During
the evening hours, the best are 20, 17 and 15m. At the low end of the cycle, only 30 and
20m will provide long distance communications during daylight. After dark, 40m will
open for at least the early part of the evening. By the early morning hours, only 80m will
support worldwide communications.

IX.     Propagation Disturbances
Because HF propagation is so closely tied to solar activity, sudden changes in the sun can
have a profound effect how the ionosphere refracts radio waves. Besides sunspots, which
are to some degree predictable, there are other phenomena, such as solar flares or coronal
mass ejections (CME’s) that can happen without warning.

A solar flare is a plume of very hot gas ejected from the sun’s surface. It rises through the
chromosphere into the corona, disturbing both regions. X-ray emission from the corona
may increase and large numbers of charged particles are thrown out into space at high
velocity. The x-rays reach Earth in less than 9 minutes. If they are intense enough, the
ionosphere’s electron density will become so great that all HF signals are absorbed by it
and worldwide HF communications are blacked out.

The charged particles, which require 2 – 3 days to reach Earth, are trapped by the
terrestrial magnetic field. The particles spiral along the magnetic field lines toward the
magnetic poles, creating large auroral displays. Signals traveling through the auroral zone
are severely distorted, in some cases to the point of unintelligibility (auroral distortion is
what makes SM’s, LA’s, OH’s and UA9’s sound “watery”) . If the influx of charged
particles is great enough, the ionosphere itself is affected. The ionization density of the
layers increases dramatically, resulting in an HF communications blackout.

Generally speaking, ionospheric disturbances affect the lowest HF bands most severely.
Occasionally communications on 10m may be possible, but more often than not, one
must turn off the radio and wait for the disturbance to pass.

X.      Propagation Indices

Physicists use a variety of indices to quantify solar and geomagnetic. The most important
are:

K index – a local index of geomagnetic activity computed every three hours at a variety
of points on Earth. The K scale is shown below

K Index         Geomagnetic Condition
0                   Inactive
1                  Very Quiet
2                    Quiet
3                   Unsettled
4                    Active
5                 Minor Storm
6                 Major Storm
7                 Severe Storm
8              Very Severe Storm
9            Extremely Severe Storm

The best HF propagation occurs when K is less than 5. A K index less than 3 is usually a
good indicator of quiet conditions on 80 and 40m.
Ap index - a daily average planetary geomagnetic activity index based on local K indices.
The A scale is shown below:

Ap Index        Geomagnetic Condition
0-7                 Quiet
8-15                Unsettled
16-29                 Active
30-49              Minor Storm
50-99              Major Storm
100-400             Severe Storm

Good HF propagation is likely when A is less than 15, particularly on the lower HF
bands. When A exceeds 50 , ionospheric backscatter propagation is possible on 12 and
10m. When A exceeds 100, auroral backscatter may be possible on 10m.

These two geomagnetic indices are related to one another as follows:

K Index      Ap Index
0             0
1             3
2             7
3            15
4            27
5            48
6            80
7           140
8           240
9           400

Solar Flux – This index is a measure of 10.7 cm microwave energy emitted by the sun. A
flux of 63.75 corresponds to a spot free, quiet sun. As the flux number increases, the solar
activity increases. Single hop HF propagation is normally possible on bands below 20m
when the flux is greater than 70. Multi-hop propagation is possible on 80 – 20m when the
flux exceeds 120. Openings on 15 and 10 meters are common when the flux exceeds
180. Should the flux exceed 230, multi-hop propagation is possible up into the VHF
region.

Sunspot Number (Wolf Number)– This is the oldest measure of sunspot activity, with
continuous records stretching back into the 19th century. The sunspot number is computed
multiplying the number of sunspot groups observed by 10 and adding this to the number
of individual spots observed. Because the sun rotates and different areas of the sun are
visible each day, it is common to use 90 day or annual average sunspot numbers. The
lowest possible sunspot number is 0. The largest annual average value recorded to date
was 190.2 in 1957. As with solar flux, higher sunspot numbers equate to more solar
activity.

The chart below shows flux, sunspot numbers and A index for the past 80 days.
This chart shows the solar flux for the past 15 years (September 1986 – March 2002):

Actual monthly sunspot number
Smoothed Sunspot Number

XI.     Summary

This is meant to be a brief overview of HF propagation. There have been many books
written on this subject and a there are many computer resources available, particularly for
propagation forecasting. The Radio Society of Great Britain has an interesting website
devoted to propagation, www.keele.ac.uk/depts/por/psc.htm

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