Introduction to HF Radio Propagation
1. The ionosphere
The regions of the ionosphere
In a region extending from a height of
about 50 km to over 500 km, some of the molecules of the atmosphere are
ionised by radiation from the Sun to produce an ionised gas. This region
is called the ionosphere, figure 1.1.
Ionisation is the process in which electrons,
which are negatively charged, are removed from (or attached to) neutral
atoms or molecules to form positively (or negatively) charged ions and
free electrons. It is the ions that give their name to the ionosphere,
but it is the much lighter and more freely moving electrons which are important
in terms of high frequency (HF: 3 to 30 MHz) radio propagation. Generally,
the greater the number of electrons, the higher the frequencies that can
During the day there may be four regions
present called the D, E, F1 and F2 regions. Their approximate height ranges
D region 50 to 90 km;
E region 90 to 140 km;
F1 region 140 to 210 km;
F2 region over 210 km.
During the daytime, sporadic E (section
1.6) is sometimes observed in the E region, and at certain times during
the solar cycle the F1 region may not be distinct from the F2 region but
merge to form an F region. At night the D, E and F1 regions become very
much depleted of free electrons, leaving only the F2 region available for
communications; however it is not uncommon for sporadic E to occur at night.
Only the E, F1, sporadic E when present,
and F2 regions refract HF waves. The D region is important though, because
while it does not refract HF radio waves, it does absorb or attenuate them
The F2 region is the most important region
for high frequency radio propagation as:
it is present 24 hours of the day;
its high altitude allows the longest communication paths;
it usually refracts the highest frequencies in the HF range.
The lifetime of electrons is greatest in
the F2 region which is one reason why it is present at night. Typical lifetimes
of electrons in the E, F1 and F2 regions are 20 seconds, 1 minute and 20
Figure 1.1 Day and night structure of the
1.2 Production and loss of electrons in the ionosphere
Radiation from the Sun causes ionisation
in the ionosphere. Electrons are produced when this radiation collides
with uncharged atoms and molecules, figure 1.2. Since this process requires
solar radiation, production of electrons only occurs in the daylight hemisphere
of the ionosphere.
Figure 1.2 Production (top) and loss (bottom).
When a free electron combines with a charged
ion a neutral particle is usually formed, figure 1.2. Essentially, loss
is the opposite process to production. Loss of electrons occurs continually,
both day and night.
1.3 Observing the ionosphere
The most important feature of the ionosphere
in terms of radio communications is its ability to refract radio waves.
However, only those waves within a certain frequency range will be refracted.
The range of frequencies refracted depends on a number of factors (section
1.4). Various methods have been used to investigate the ionosphere and
the most widely used instrument for this purpose is the ionosonde, figure
1.3. Note that many references to ionospheric communications speak of reflection
of the wave. It is, however, a refraction process.
An ionosonde is a high frequency radar which
sends very short pulses of radio energy vertically into the ionosphere.
If the radio frequency is not too high, the pulses are refracted back towards
the ground. The ionosonde records the time delay between transmission and
reception of the pulses. By varying the oscillation frequency of the pulses,
a record is obtained of the time delay at different frequencies.
Figure 1.3 Ionosonde operation.
Frequencies less than about 1.6 MHz are interfered
with by AM broadcast stations. As the frequency is increased, echoes appear
first from the lower E region and subsequently, with greater time delay,
from the F1 and F2 regions. Of course, at night echoes are returned only
from the F2 region and possibly sporadic E since the other regions have
lost most of their free electrons.
Today, the ionosphere is "sounded" not only
by signals sent up at vertical incidence. Oblique sounders send pulses
of radio energy obliquely into the ionosphere (the transmitter and receiver
are separated by some distance). This type of sounder can monitor propagation
on a particular circuit and observations of the various modes being supported
by the ionosphere can be made. Backscatter ionosondes rely on echoes reflected
from the ground and returned to the receiver, which may or may not be at
the same site as the transmitter. This type of sounder is used for over-the-horizon
1.4 Ionospheric variations
The ionosphere is not a stable medium which
allows the use of one frequency over the year, or even over 24 hours. The
ionosphere varies with the solar cycle, the seasons, the circuit and during
any given day. So, a frequency which may provide successful propagation
now, may not do so an hour later.
Variations due to the solar cycle
The Sun goes through a periodic rise and
fall in activity which affects HF communications; solar cycles vary in
length from 9 to 14 years. At solar minimum, only the lower frequencies
of the HF band will be supported by the ionosphere, while at solar maximum
the higher frequencies will successfully propagate, figure 1.4. This is
because there is more radiation being emitted from the Sun at solar maximum,
producing more electrons in the ionosphere which allows the use of higher
Figure 1.4 Solar cycle and seasonal dependence
of E and F region frequencies for a near vertical incidence sky wave (NVIS)
circuit in the southern hemisphere.
There are other consequences of the solar
cycle. Around solar maximum there is a greater likelihood of large solar
flares occurring. Flares are huge explosions on the Sun which emit radiation
that ionises the D region causing increased absorption of HF waves. Since
the D region is present only during the day, only those communication paths
which pass through daylight will be affected. The absorption of HF waves
travelling via the ionosphere after a flare has occurred is called a short
wave fade-out (section 3.1). Fade-outs occur instantaneously and affect
lower frequencies the most. Lower frequencies are also the last to recover.
If it is suspected or confirmed that a fade-out has occurred, it may help
to try using a higher frequency. However, if a flare is very large, the
whole of the HF spectrum may be rendered unusable. The duration of fade-outs
can vary between about 10 minutes to over an hour depending on the intensity
and duration of the flare.
1.4.2 Seasonal variations
E region frequencies are greater in summer
than winter, figure 1.4. However, the variation in F region frequencies
is more complicated. In both hemispheres, F region noon frequencies generally
peak around the equinoxes (March and September). Around solar minimum the
summer noon frequencies are, as expected, generally greater than those
in winter, but around solar maximum, winter frequencies at certain locations,
can be higher than those in summer. In addition, frequencies around the
equinoxes (March and September) are higher than those in summer or winter
for both solar maximum and minimum. The observation of noon, winter frequencies
often being greater than those in summer is called the seasonal anomaly
(this is not observed in figure 1.4).
1.4.3 Variations with latitude
Figure 1.5 indicates the variations in
E and F region frequencies at noon and midnight from the poles to the geomagnetic
equator. During the day and with increasing latitude, solar radiation strikes
the atmosphere more obliquely, so the intensity of radiation and the electron
density production decreases towards the poles.
Figure 1.5 Representation of latitudinal
Note in figure 1.5 how the daytime F region
frequencies peak not at the magnetic equator, but around 15 to 20 degrees
north and south of it. This is called the equatorial anomaly. At night,
frequencies reach a minimum around 60 degrees latitude north and south
of the geomagnetic equator. This is called the mid-latitude trough. Large
tilts can occur in the vicinity of these phenomena which may lead to variations
in the range of sky waves that have reflection points nearby.
1.4.4 Diurnal variations
Operating frequencies are normally higher
during the day and lower at night, figure 1.6. With dawn, solar radiation
causes electrons to be produced in the ionosphere and frequencies increase
reaching their maximum around noon. During the afternoon, frequencies begin
decreasing due to electron loss and with evening, the D, E and F1 regions
become insignificant. HF sky wave communication during the night is therefore
by the F2 region and absorption of radio waves is lower because of the
lack of the D region. Through the night, frequencies decrease reaching
their minimum just before dawn.
Figure 1.6 E and F layer frequencies for
a Singapore to Ho Chi Minh circuit some time in a solar cycle.
1.5 Variations in absorption
The D region, which becomes insignificant
at night, attenuates waves as they pass through it. Absorption was discussed
in section 1.4.1 when describing how solar flares can cause disruptions
or degradations to communication paths which pass through daylight. Absorption
in the D region also varies with the solar cycle, being greatest around
solar maximum. Signal absorption is greater in summer and during the middle
of the day, figure 1.7. There is a variation in absorption with latitude,
with more absorption occurring near the equator and decreasing towards
the poles, although certain solar events will significantly increase absorption
at the poles. Lower frequencies are absorbed to a greater extent, so it
is advisable to use as high a frequency as possible.
Figure 1.7 Example of diurnal and seasonal
variations in absorption at Sydney, 2.2 MHz.
Around the polar regions absorption can affect
communications quite dramatically at times. Sometimes high energy protons
ejected from the Sun during large solar flares will move down the Earth's
magnetic field lines and into the polar regions. These protons can cause
increased absorption of HF radio waves as they pass through the D region.
This increased absorption may last for a number of days and is called a
Polar Cap Absorption event (PCA), section 3.2.
1.6 Sporadic E
Sporadic E may form at any time. It occurs
at altitudes between 90 to 140 km (in the E region), and may be spread
over a large area or be confined to a small region. It is difficult to
know where and when it will occur and how long it will persist. Sporadic
E can have a comparable electron density to the F region, implying that
it can refract comparable frequencies to the F region. Sporadic E can therefore
be used for HF communications on higher frequencies than would be used
for normal E layer communications at times. Sometimes a sporadic E layer
is transparent and allows most of the radio wave to pass through it to
the F region, however, at other times the sporadic E layer obscures the
F region totally and the signal does not reach the receiver (sporadic E
blanketing). If the sporadic E layer is partially transparent, the radio
wave is likely to be refracted at times from the F region and at other
times from the sporadic E. This may lead to partial transmission of the
signal or fading, figure 1.8.
Figure 1.8 Some possible paths when sporadic
E is present.
Sporadic E in the low and mid-latitudes occurs
mostly during the daytime and early evening, and is more prevalent during
the summer months. At high latitudes, sporadic E tends to form at night.
1.7 Spread F
Spread F occurs when the F region becomes
diffuse due to irregularities in that region which scatter the radio wave.
The received signal is the superposition of a number of waves refracted
from different heights and locations in the ionosphere at slightly different
times. At low latitudes, spread F occurs mostly during the night hours
and around the equinoxes. At mid-latitudes, spread F is less likely to
occur than at low and high latitudes. Here it is more likely to occur at
night and in winter. At latitudes greater than about 40 degrees, spread
F tends to be a night time phenomenon, appearing mostly around the equinoxes,
while around the magnetic poles, spread F is often observed both day and
night. At all latitudes there is a tendency for spread F to occur when
there is a decrease in F region frequencies. That is, spread F is often
associated with ionospheric storms (section 3.3).
2. HF communications
Types of HF propagation
High Frequency (3 to 30 MHz) radio signals
can propagate to a distant receiver, figure 2.1, via the:
ground wave: near the ground for short distances, about 100 km over land
and 300 km over sea. The range of the wave depends on antenna height, polarisation,
frequency, ground types, vegetation, terrain and/or sea state;
direct or line-of-sight wave: this wave may interact with the earth-reflected
wave depending on terminal separation, frequency and polarisation;
sky wave: refracted by the ionosphere, all distances.
Figure 2.1 Types of HF propagation.
2.2 Frequency limits of sky waves
Not all HF waves are refracted by the ionosphere,
there are upper and lower frequency bounds for communications between two
terminals. If the frequency is too high, the wave will penetrate the ionosphere,
if it is too low, the strength of the signal will be lowered due to absorption
in the D region. The range of usable frequencies will vary:
throughout the day;
with the seasons;
with the solar cycle;
from place to place;
depending on the ionospheric region used for communications.
While the upper limit of frequencies varies
mostly with these factors, the lower limit is also dependent on receiver
site noise, antenna efficiency, transmitter power, E layer screening (section
2.6) and absorption by the ionosphere.
2.3 The usable frequency range
For any circuit there is a Maximum Usable
Frequency (MUF) which is determined by the state of the ionosphere in the
vicinity of the refraction area(s) and the length of the circuit. The MUF
is refracted from the area of maximum electron density of a region. Therefore,
frequencies higher than the MUF for a particular region will penetrate
that region. During the day it is possible to communicate via both the
E and F layers using different frequencies. The highest frequency supported
by the E layer is the EMUF, while that supported by the F layer is the
The F region MUF in particular varies during
the day, seasonally and with the solar cycle. Long term data displays a
range of frequencies observed and some of the IPS predictions mirror this.
A range of F region MUFs is provided in the predictions and this range
extends from the lower decile MUF (called the Optimum Working Frequency,
OWF), through the median MUF to the upper decile MUF. These MUFs have a
90%, 50% and 10% chance of being supported by the ionosphere, respectively.
IPS predictions usually cover a period of one month, so the OWF should
provide successful propagation 90% of the time or 27 days of the month.
The median MUF should provide communications 50% or 15 days of the month
and the upper decile MUF 10% or 3 days of the month. The upper decile MUF
is the highest frequency of the range of MUFs and is most likely to penetrate
the ionosphere, figure 2.2.
Figure 2.2 Range of usable frequencies. If
the frequency, f, is close to the ALF then the wave may suffer absorption
in the D region. If the frequency is above the EMUF then propagation is
via the F region. Above the FMUF the wave is likely to penetrate the ionosphere.
The chances of successful propagation discussed
above rely on the monthly prediction of solar activity being correct. Sometimes
unforeseen events occur on the Sun resulting in the monthly predictions
being inaccurate. One of the roles of the Australian Space Forecast Centre
(ASFC) at IPS is to provide corrections to the monthly predictions, warning
customers of changes in communication conditions.
The D region does not allow all frequencies
to be used since the lower the frequency the more likely it is to be absorbed.
The Absorption Limiting Frequency (ALF) is provided as a guide to the lower
limit of the usable frequency band. The ALF is significant only for circuits
with refraction points in the sunlit hemisphere. At night, the ALF is zero,
allowing frequencies which are not usable during the day to successfully
2.4 Hop length
The hop length is the ground distance covered
by a radio signal after it has been refracted once from the ionosphere
and returned to Earth, figure 2.3. The upper limit of the hop length is
set by the height of the ionosphere and the curvature of the Earth. For
E and F region heights of 100 km and 300 km, the maximum hop lengths with
an elevation angle of 4 degrees, are 1800 km and 3200 km, respectively.
Distances greater than these will require more than one hop. For example,
a distance of 6100 km would require a minimum of 4 hops by the E region
and 2 hops via the F region with such an elevation angle. More hops may
be required with larger antenna elevation angles.
Figure 2.3 Hop lengths based upon an antenna
elevation angle of 4 degrees and heights for the E and F layers of 100
km and 300 km, respectively.
2.5 Propagation modes
There are many paths or modes by which
a sky wave may travel from a transmitter to a receiver. The mode by a particular
layer which requires the least number of hops between the transmitter and
receiver is called the first order mode. The mode that requires one extra
hop is called the second order mode. For a circuit with a path length of
5000 km, the first order F mode would require at least two hops (2F), while
the second order F mode would then require three hops (3F). The first order
E mode has the same number of hops as the first order F mode. If this results
in a hop length of greater than 2050 km, which corresponds to an elevation
angle of 0 degrees, the E mode is not possible. This also applies to the
second order E mode. Of course, the E region modes will only be available
on daylight circuits.
Simple modes are those propagated by one
region, say the F region, figure 2.4. More complicated modes consisting
of combinations of refractions from the E and F regions, ducting and chordal
modes are also possible, figure 2.5.
Figure 2.4 Examples of simple propagation
Chordal modes and ducting involve a number
of refractions from the ionosphere without intermediate reflections from
the ground. There is a tendency to think of the regions of the ionosphere
as being smooth, however, the ionosphere undulates and moves, with waves
passing through it which may affect the refraction of the signal. The ionospheric
regions may tilt and when this happens chordal and ducted modes may occur.
Ionospheric tilting is more likely near the equatorial anomaly, the mid-latitude
trough and in the sunrise and sunset sectors. When these types of modes
do occur, signals can be strong since the wave spends less time traversing
the D region and being attenuated during ground reflections.
Figure 2.5 Some other propagation modes.
Because of the high electron density of the
daytime ionosphere in the vicinity of 15 degrees of the magnetic equator
(near the equatorial anomaly), transequatorial paths can use these enhancements
to propagate on higher frequencies. Any tilting of the ionosphere may result
in chordal modes, producing good signal strength over long distances.
Ducting may result if tilting occurs and
the wave becomes trapped between refracting regions of the ionosphere.
This is most likely to occur in the equatorial ionosphere, near the auroral
zone and mid-latitude trough. Disturbances to the ionosphere, such as travelling
ionospheric disturbances (section 2.9), may also account for ducting and
chordal mode propagation.
2.6 E layer screening
For daytime communications via the F region,
the lowest usable frequency via the one hop F mode (1F) is dependent upon
the presence of the E region. If the operating frequency for the 1F mode
is below the two hop EMUF, then the signal is unlikely to propagate via
the F region due to screening by the E region, figure 2.6. This is because
the antenna elevation angles of the 1F and 2E modes are similar.
Figure 2.6 E layer screening occurs if communications
are required by the 1F mode and the operating frequency is close to or
below the EMUF for the 2E mode. Note the paths through the absorbing D
A sporadic E layer may also screen a wave
from the F region. Sometimes sporadic E can be quite transparent, allowing
most of the wave to pass through it. At other times it will partially screen
the F region leading to a weak or fading signal, while at other times sporadic
E can totally obscure the F region with the possible result that the signal
does not arrive at the receiver, figure 1.9 (section 1.6).
2.7 Frequency, range and elevation angle
For oblique propagation, there are three
range or path length;
The diagrams below illustrate the changes
to the ray paths when each of these is fixed in turn.
Figure 2.7: Elevation angle fixed
as the frequency is increased toward the MUF, the wave is refracted higher
in the ionosphere and the range increases, paths 1 and 2;
at the MUF for that elevation angle, the maximum range is reached, path
above the MUF, the wave penetrates the ionosphere, path 4.
Figure 2.7 Elevation angle fixed.
Figure 2.8: Path length fixed (point-to-point
as the frequency is increased towards the MUF, the wave is refracted from
higher in the ionosphere. To maintain a circuit of fixed length, the elevation
angle must therefore be increased, paths 1 and 2;
at the MUF, the critical elevation angle is reached, path 3. The critical
elevation angle is the elevation angle for a particular frequency, which
if increased, would cause penetration of the ionosphere;
above the MUF, the ray penetrates the ionosphere, path 4.
Figure 2.8 Path length fixed.
Figure 2.9: Frequency fixed
at low elevation angles the path length (ground range) is greatest, path
as the elevation angle is increased, the path length decreases and the
ray is refracted from higher in the ionosphere, paths 2 and 3;
if the frequency will return when sent vertically up into the ionosphere,
then there is no skip distance. However, if this is not the case, then
as the elevation angle is increased beyond the critical elevation angle
for that frequency then the wave penetrates the ionosphere and there is
an area around the transmitter within which no sky wave communications
can be received, path 4. To communicate via the sky wave within the skip
zone, the frequency must be lowered.
Figure 2.9 Frequency fixed.
2.8 Skip zones
The skip zone is an area around a transmitter
in which neither the ground wave nor the sky wave propagate. Skip zones
can often be used to advantage if it is desired that communications are
not heard by a particular receiver. Selecting a different frequency will
alter the size of the skip zone and if the receiver is within the skip
zone and out of reach of the ground wave, then it is unlikely that it will
receive the communications. However, factors such as sidescatter, where
reflection from terrain outside the skip zone results in the wave transmitting
into the zone, may affect the reliability of this technique.
Skip zones vary in size during the day, with
the seasons, and with solar activity. During the day, solar maximum and
around the equinoxes, skip zones generally are smaller in area. The ionosphere
during these times has increased electron density and so is able to support
Multipath fading results from dispersion
of the signal by the transmitting antenna. A number of modes propagate
which have variations in phase and amplitude. These waves may interfere
with each other if they reach the receiver, figure 2.10.
Figure 2.10 Multipath fading. The signal
may travel by a number of paths which, if they arrive at the receiver and
are of similar amplitude with time delay, may interfere and cause fading.
Disturbances known as Travelling Ionospheric
Disturbances, TIDs, may cause a region to be tilted, resulting in the signal
being focused or defocused, figure 2.11. Fading periods of the order of
10 minutes or more can be associated with these structures. TIDs travel
horizontally at 5 to 10 km/minute with a well defined direction of travel.
Some originate in auroral zones following an event on the Sun and these
may travel large distances. Others originate in weather disturbances. TIDs
may cause variations in phase, amplitude, polarisation and angle of arrival
of a wave.
Polarisation fading results from changes
to the polarisation of the wave along the propagation path. The receiving
antenna is unable to receive components of the signal; this type of fading
can last for a fraction of a second to a few seconds.
Skip fading can be observed around sunrise
and sunset particularly, when the operating frequency is close to the MUF,
or when the receive antenna is positioned close to the boundary of the
skip zone. At these times of the day, the ionosphere is unstable and the
frequency may oscillate above and below the MUF causing the signal to fade
in and out. If the receiver site is close to the skip zone boundary, as
the ionosphere fluctuates, the skip zone boundary also fluctuates.
Figure 2.11 Focusing and defocusing effects
caused by tilting and travelling ionospheric disturbances (TIDs).
Radio noise arises from internal and external
origins. Internal or thermal noise is generated in the receiving system
and is usually negligible when compared to external sources. External radio
noise originates from natural (atmospheric and galactic) and man-made (environmental)
Atmospheric noise, caused by thunderstorms,
is normally the major contributor to radio noise in the HF band and will
especially degrade circuits passing through the day-night terminator. Atmospheric
noise is greatest in the equatorial regions of the world and decreases
with increasing latitude. Its effect is also greater on lower frequencies,
hence it is usually more of a problem around solar minimum and at night
when lower frequencies are used.
Galactic noise arises from within our galaxy.
Receive antennas with high angle lobes are more likely to be affected by
this type of noise.
Man-made noise emanates from ignition systems,
neon signs, electrical cables, power transmission lines and welding machines.
This type of noise depends on the technological advancement of the society
and the size of the population.
Interference from other users on the same
frequency may be intentional, such as jamming or due to propagation conditions.
Man-made noise tends to be vertically polarised,
so selecting a horizontally polarised antenna may help in reducing noise.
Using a narrower bandwidth, or a directional receiving antenna (with a
lobe in the direction of the transmitting source and a null in the direction
of the unwanted noise source), will also aid in reducing the effects of
noise. Selecting a site with a low noise level and determining the major
noise sources are important factors in establishing a successful communications
2.11 VHF and 28 MHz propagation
VHF and 28 MHz are used for line-of-sight
or direct wave communication, for example, ship-to-ship or ship-to-shore.
The frequency bands are divided into channels and one channel is usually
as good as the next. This is in contrast to medium frequency (MF: 300 kHz
to 3 MHz) and HF where the choice of a frequency channel may be crucial
for good communications.
Because VHF and 28 MHz operate mainly by
line-of-sight, it is important to mount the antenna as high as possible
and free from obstructions. Shore stations are usually on the tops of hills
to provide maximum range, but even the highest hills do not provide coverage
much beyond about 45 nautical miles (80 km) because of the Earth's curvature.
Antennas for VHF and 28 MHz should concentrate
radiation at low angles (towards the horizon) as radiation directed at
high angles will usually pass over the receiving antenna, except when communicating
with aircraft. VHF and 28 MHz do not usually suffer from noise except during
severe electrical storms. Interference results from many users wishing
to use the limited number of channels, and this can be a significant problem
in densely populated areas.
28 MHz and the lower frequencies in the VHF
band can, at times, propagate over large distances, well beyond the normal
line-of-sight limitations. There are three ways that this can take place:
around solar maximum and during the day, the ionospheric F region will
often support long range sky wave communications on 28 MHz and above;
sporadic E layers can sometimes support 28 MHz and lower frequency VHF
propagation over circuits of about 500 to 1000 nautical miles (1000 to
2000 km) in length. This kind of propagation is most likely to occur at
mid-latitudes, during the daytime in summer;
28 MHz and VHF can also propagate by means of temperature inversions (ducting)
at altitudes of a few kilometres. Under these conditions, the waves are
gradually bent by the temperature inversion to follow the curvature of
the Earth. Distances of several hundred nautical miles can be covered in
2.12 Medium frequency (MF) sky wave propagation
Both the MF (300 kHz to 3 MHz) and HF bands
can be used for long distance sky wave communications at night. During
the night the D region disappears, so absorption falls to very low levels.
This is why radio broadcast stations operating in the MF and 4 MHz bands
can be heard over long distances at night.
2.13 Ground wave MF and HF propagation
It is possible to communicate up to distances
of several hundred nautical miles on MF/HF bands at sea by using ground
The ground wave follows the curvature of
the Earth and its range does not depend upon the height of the antenna.
However, the range does depend upon the transmitter power and also upon
the operating frequency. Low frequencies travel further than high frequencies.
Thus under ideal low noise conditions (noon, during winter), it is possible
to communicate over distances of about 500 nautical miles at 2 MHz by using
a 100 W transmitter. At 8 MHz, under the same conditions and using the
same transmitter power, the maximum range is reduced to about 150 nautical
Note that ground wave propagation is much
less efficient over land than it is over sea because of the much lower
conductivity of the ground and other factors. Consequently, ranges over
land are greatly reduced.
Ground wave communications vary daily and
with the seasons. Greatest communication ranges are achieved during the
daytime in winter because background noise levels are lowest during these
Successful ground wave communications over
hundreds of nautical miles can only be achieved if the transmitting and
receiving antennas are chosen to direct and receive radiation at low angles.
Tall whips are ideal for this purpose.
3. The effects of solar disturbances
Short wave fade-outs (SWFs)
Also called daylight fade-outs or sudden
ionospheric disturbances (SIDs). Radiation from the Sun during large solar
causes increased ionisation in the D region which results in greater absorption
of HF radio waves, figure 3.1. If the flare is large enough, the whole
of the HF spectrum can be rendered unusable for a period of time. Fade-outs
are more likely to occur around solar maximum and in the first part of
the decline to solar minimum.
Figure 3.1 Fade-outs affect only those circuits
where the wave passes through the D region. That is, circuits with daytime
sectors. Night circuits are unaffected by fade-outs.
The main features of SWFs are:
only circuits with daylight sectors will be affected;
fade-outs usually last from a few minutes to sometimes two hours, with
a fast onset and a slower recovery. The duration of the fade-out will depend
on the intensity and duration of the flare;
the magnitude of the fade-out will depend on the size of the flare and
the position of the Sun relative to the point where the radio wave passes
through the D region. The higher the Sun with respect to that point, the
greater the amount of absorption;
absorption is greatest at lower frequencies, which are the first to be
affected and the last to recover. Higher frequencies are normally less
affected and may still be usable, figure 3.2.
Figure 3.2 Fade-outs affect lower frequencies
first and these are the last to recover. Higher frequencies are least affected
and with many fade-outs will be unaffected.
3.2 Polar cap absorption events (PCAs)
PCAs are attributed to high energy protons
which escape from the Sun when large flares occur and move along the Earth's
magnetic field lines to the polar regions. There they ionise the D region,
causing attenuation of HF waves passing through the polar D region. PCAs
are most likely to occur around solar maximum, however, they are not as
frequent as fade-outs.
PCAs may commence as soon as 10 minutes after the flare and last for up
to 10 days;
the effects of PCAs can sometimes be overcome by relaying messages on circuits
which do not require polar refraction points;
even the winter polar zone (a region of darkness) can suffer the effects
of PCAs. The particles from the Sun may actually produce a night D region.
3.3 Ionospheric storms
Due to events on the Sun, sometimes the
Earth's magnetic field becomes disturbed. The geomagnetic field and the
ionosphere are linked in complex ways and a disturbance in the geomagnetic
field can often cause a disturbance in the F region of the ionosphere.
These ionospheric storms sometimes begin
with increased electron density allowing higher frequencies to be supported,
followed by a decrease in the electron density leading to the successful
use of only lower than normal frequencies of the F region. An enhancement
will not usually concern the HF communicator, but the depression may cause
frequencies normally used for communication to be too high with the result
that the wave penetrates the ionosphere.
Ionospheric storms may last for a number
of days and mid and high latitudes are affected moreso than the lower latitudes,
generally. Unlike fade-outs, higher frequencies are most affected by ionospheric
storms. To reduce storm effects, a lower frequency should be used where
Ionospheric storms can occur throughout the
solar cycle and are related to coronal mass ejections (CMEs) and coronal
holes on the Sun. Figure 3.3 shows how an ionospheric storm has caused
frequencies in the main to be depressed at Canberra, Australia (a mid-latitude
station) from 24 to 28th. Higher frequencies would probably have been unsuccessful
over this time.
Figure 3.3 Canberra, Australia observed and
median vertical MUFs for latter part September 1998. Significant depressions
in F region frequencies occurred between 24 to 28 September due solar activity.