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Introduction to HF Radio Propagation


1. The ionosphere

1.1. The regions of the ionosphere

In a region extending from a height of about 50 km to over 500 km, some of the atoms and molecules of the atmosphere are ionised by radiation from the Sun. This region is called the ionosphere (Figure 1.1).

Ionisation is the process in which neutral atoms or molecules gain or lose electrons to become electrically charged. Of specific interest to High Frequency (HF: 3 to 30 MHz) radio communications via the ionosphere (sky wave), negatively charged electrons are removed from atoms or molecules to leave positively charged ions. It is the ions that give their name to the ionosphere, but it is the freely moving electrons which are important. The free electrons in the ionosphere refract (bend) HF radio waves, allowing the upper atmosphere to be used for communications between locations.

In the day hemisphere there may be four regions present, the D, E, F1 and F2 regions. Their approximate height ranges are:

  • D region - 50 to 90 km;
  • E region - 90 to 140 km;
  • F1 region - 140 to 210 km;
  • F2 region - over 210 km.

The E, F1 and F2 regions are capable of refracting waves in the HF range back towards Earth. The D region, through which HF waves must travel to reach these higher regions, absorbs or attenuates (reduces signal strength) HF waves (Section 1.5). Only the F2 region persists in the night hemisphere and is then called the F region.

In winter at solar maximum the F1 region may not be distinct from the F2 region with the two regions merging to form an F region (see F1 trace in Figure 1.5).

Sometimes a thin, dense layer of electrons, called sporadic E, can occur at E region altitudes. Sporadic E can occur day or night and may last minutes to hours, having a spatial extent of a few to hundreds of kilometres.

The F2 region is the most important region for HF radio propagation because:

  • it is present 24 hours of the day;
  • its high altitude allows the longest communication paths and least hops;
  • it usually refracts the highest frequencies in the HF range.
Day and night structure of the ionosphere.

Figure 1.1.  Day and night structure of the ionosphere.


1.2. Production and loss of electrons in the ionosphere

Extreme UltraViolet (EUV) and x-ray radiation from the Sun are the main sources of ionisation in the ionosphere. Electrons are produced when this radiation collides with the neutral atoms and molecules (Figure 1.2). Since this process requires solar radiation, production of electrons mostly occurs in the day hemisphere of the ionosphere.

When a free electron combines with a charged ion a neutral particle is usually formed, (Figure 1.3) and the electron is lost from the ionosphere. Loss of electrons occurs continually, both day and night.

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 minutes, respectively.

Electron production.

Figure 1.2.  Electron production.


Electron loss.

Figure 1.3.  Electron loss.


1.3. Observing the ionosphere

The most important feature of the ionosphere is its ability to refract radio waves. However, only waves within a certain frequency range are refracted.

Various instruments have been designed to observe the ionosphere, and the most widely used instrument for this purpose is the ionosonde (Figure 1.4). The ionosonde is an high frequency radar which "sounds" the ionosphere by transmitting short pulses of energy at different HF frequencies vertically. If the frequency of a pulse is not too great or highly attenuated by the D region, the returning pulse may be detected. The ionosonde records the time delay between transmission and reception of pulses over a range of different frequencies so that the approximate heights at which the pulses are refracted can be calculated. A picture (ionogram) of the heights at which frequencies are refracted can be created.

In general, lower frequencies are refracted from lower in the ionosphere (E region altitudes) and with increasing frequency, the pulses return from higher altitudes. At night only refraction from the F region occurs although sporadic E may refract quite high frequencies day or night if its electron density is high.

Ionosonde operation.

Figure 1.4.  Ionosonde operation.


The ionosphere can also be sounded by oblique ionosondes which send pulses of radio energy obliquely (at angles off vertical) into the ionosphere and which are received at some distant location. This type of sounder can monitor propagation on a particular sky wave path and observe the various propagation modes supported (Section 2.5). Backscatter ionosondes transmit energy obliquely which is refracted from the ionosphere and reflected off the ground before returning via the ionosphere to the receiver site which need not be co-located with the transmitter. This type of sounder is used for over-the-horizon radar.

1.4. Ionospheric variations

The ionosphere is not a stable medium that allows the use of one frequency over the year, or even over 24 hours. The ionosphere varies with the solar cycle, the seasons, the sky wave path used, and diurnally (through the day).

1.4.1. Variations due to the solar cycle

The Sun's activity and radiation output changes over a period of about 9 to 14 years, the "solar cycle". The slow change in radiation output in turn affects the ionosphere and the range of frequencies available for HF sky wave communications. At the start of a solar cycle (solar minimum) radiation output is low causing less ionisation in the ionosphere. Only the lower frequencies of the HF band are supported (refracted). Radiation output at solar maximum is much greater leading to higher concentrations of electrons in the ionosphere, and therefore, higher frequencies being supported. In fact, at very high solar activity, frequencies in the VHF band which would normally penetrate the ionosphere may be refracted and propagate as a sky wave. At solar maximum the usable HF frequency range is greater giving a larger choice of frequencies which reduces interference between users. The solar cycle ends at the next solar minimum.

Figure 1.5 shows how the highest frequencies reflected from the three regions at noon above Canberra, Australia, vary with solar activity (the max and min arrows indicate solar maximums and minimums). Frequencies are generally higher at solar maximum, with the trend strongest in the F2 region.

The relationship between solar cycles and noon E and F region frequencies at Canberra (35.3º S, 149.0º E). Vertical lines indicate the start of each calendar year.

Figure 1.5.  The relationship between solar cycles and noon E and F region frequencies at Canberra (35.3º S, 149.0º E). Vertical lines indicate the start of each calendar year.


1.4.2. Seasonal variations

Frequencies refracted from the E and F1 regions are usually higher in summer than winter. Figure 1.5 shows how frequencies are greater around the start of a year (summer in the southern hemisphere). However, the variation in F2 region frequencies is more complicated as frequencies are greatest near the equinoxes (March and September). Around solar minimum the summer noon frequencies are, as expected, generally greater than those in winter, but at solar maximum, winter frequencies tend to be higher than summer frequencies; called the "seasonal anomaly".

1.4.3. Variations with latitude

Figure 1.6 indicates the variations in the highest frequencies refracted vertically from the E and F2 region at noon (Day hemisphere) and midnight (Night hemisphere) between the geomagnetic equator and poles. At noon, with increasing latitude the solar radiation strikes the atmosphere more obliquely, so the intensity of radiation and the free electron density production decreases with increasing latitude. However, the F2 region frequencies peak approximately 15º to 20º north and south of the geomagnetic equator. This "equatorial anomaly" is due to an increased electron concentration at these latitudes caused by the interaction of electric currents and the magnetic field at the equator.

Latitudinal variations.

Figure 1.6.  Latitudinal variations.


At night, frequencies are lowest around 60º north and south of the geomagnetic equator; this is called the "mid-latitude trough". Figure 1.6 indicates how rapidly the frequency can change with latitude near the mid-latitude trough and equatorial anomaly. A variation in the refraction point near these locations by a few degrees may lead to a substantial variation in the highest frequency supported.

1.4.4. Daily variations

At sunrise, EUV radiation produces electrons in the ionosphere; electron concentration increases sufficiently to allow the E and F1 regions to begin refracting HF sky waves, and higher frequencies to be supported by the F2 region (Figure 1.7). From sunrise to noon the EUV radiation increases in strength, increasing the production of electrons and allowing higher frequencies to be refracted by all regions. EUV radiation peaks around noon leading to the maximum electron concentration in the ionosphere occurring then or slightly later. As the afternoon progresses, the EUV radiation decreases, and the electron density and frequencies begin to slowly decrease. At sunset, with no EUV radiation to produce electrons, the E and F1 regions essentially disappear. The F2 region is sustained through the night, although with declining electron density, due to the long lifetimes of electrons and atmospheric winds. The electron density of the F2 region reaches a minimum just before dawn, as do frequencies.

Winter E and F region frequencies for Canberra at solar maximum.

Figure 1.7.  Winter E and F region frequencies for Canberra at solar maximum.


1.5. Variation in absorption of HF sky waves

D region absorption on HF sky wave paths that pass through the day hemisphere is proportional to the solar x-ray flux. Attenuation is therefore greatest at solar maximum and when sky wave refraction points are near the sub-solar region (i.e., noon longitudes, summer hemisphere) (Figure 1.8). While absorption due to the D region is normally greatest in summer, absorption can be anomalously high at times during winter for periods of days.

Lower frequencies on sky wave paths passing through the day hemisphere are attenuated more than higher frequencies. Lower frequencies on such paths will generally have decreased signal strength, decreasing the chance of the signal being detected by the receiving system.

Sky wave paths that lie wholly within the night hemisphere do not experience D region attenuation, resulting in improved signal strength for the lower frequencies in the usable band.

Around solar maximum large solar flares are more likely to occur. Flares are huge explosions on the Sun which emit x-rays that strongly ionise the D region and further increase absorption of HF sky waves travelling through the day hemisphere.

The increase in absorption of HF sky waves after a flare has occurred is called a Short Wave Fade-out (SWF) or Sudden Ionospheric Disturbance (SID) (Section 3.1). Fade-outs occur instantaneously with lower frequencies more highly attenuated and taking longer to return to pre-flare signal strength. The duration of fade-outs can vary between about 10 minutes to several hours depending on the duration and intensity of the flare, with about 20 minutes being usual. Large flares are more likely to affect the whole HF band.

Some solar flares eject high energy protons that travel to the Earth's magnetosphere and are directed along the magnetic field lines into the polar D region. The protons increase ionisation in the D region which increases absorption of HF sky waves traversing the polar regions. This event is called a Polar Cap Absorption (PCA) event (Section 3.2) and may last as long as 10 days.

Example of daily and seasonal variations in absorption on 2.2 MHz at Sydney, Australia.

Figure 1.8.  Example of daily and seasonal variations in absorption on 2.2 MHz at Sydney, Australia.


1.6. Sporadic E

Sporadic E occurs at altitudes ranging from about 90 to 140 km, that is, at E region altitudes. While the normal E region is controlled by solar EUV radiation, sporadic E is the result of wind shears, meteors and other phenomena. The E region becomes significant for HF sky waves passing through the day hemisphere; sporadic E may form day or night and tends to appear for a few hours then disperse. The horizontal extent of sporadic E is probably of the order of tens to a few hundreds of kilometres with a vertical depth of a few kilometres.

The E region has an electron density that increases in the morning hours and decreases after noon. While sporadic E occurs at the same altitudes it can often have far greater electron density than the E region, and at times, the F region. Such high electron density allows sporadic E to refract quite high frequencies on occasions. On oblique paths sporadic E with high electron density may cause the radio wave to be refracted from it rather than the higher F region. This may alter the sky wave's footprint location and size.

Sometimes a sporadic E layer is partially transparent and allows transmission of the radio wave through to the F region or the ground (downwards wave); at other times sporadic E may refract (from either above or below) all the wave energy. A sporadic E layer that is partially transparent may lead to a weak or fading signal as the layer evolves (Figure 1.9). Transmission to the receiver site may be blocked completely on oblique paths when the sporadic E screens the F region (wave travelling up) or the ground (wave returning to Earth).

Sporadic E at low and middle 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.

Oblique sky wave path with no sporadic E present (red). Sporadic E forms along the path, its electron density is high enough to refract the wave and its structure is such that it refracts the entire signal (green). Sporadic E has electron density high enough to refract the wave and its structure is partially transparent (both red and green).

Figure 1.9.  Oblique sky wave path with no sporadic E present (red). Sporadic E forms along the path, its electron density is high enough to refract the wave and its structure is such that it refracts the entire signal (green). Sporadic E has electron density high enough to refract the wave and its structure is partially transparent (both red and green).


1.7. Spread F

Spread F is caused by electron density irregularities in the F2 region; the irregularities may last minutes to hours with a horizontal extent of a few to hundreds of kilometres. The irregularities distort signals passing through the affected region causing flutter fading on HF and scintillation (variations in signal strength and time of arrival) on higher frequencies used for ground-to-satellite communications.

Spread F occurrence is greatest at night at all latitudes and at the equinoxes. At high latitudes, spread F can be prevalent during the day. Spread F occurs least at mid-latitudes. 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

2.1. Types of HF propagation

HF radio signals can propagate to a distant receiver, Figure 2.1, via the:

  • ground wave - near the ground for short distances, up to 100 km over land and 300 km over sea. Attenuation of the wave depends on antenna height, polarisation, frequency, ground types, 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.
Types of HF radio wave propagation.

Figure 2.1.  Types of HF radio wave propagation.


2.2. Frequency limits of sky waves

Not all HF sky waves are refracted by the ionosphere; there are upper and lower frequency limits for communications between two locations. If the frequency is too high, the wave will penetrate the ionosphere. If the frequency is too low and the communications path travels through daylight, the signal strength will be reduced due to D region absorption and may not be detected. The range of usable frequencies will vary:

  • diurnally;
  • with the seasons;
  • with the solar cycle;
  • with latitude.

2.3. The usable frequency range

For a particular sky wave path at a particular time, there is a Maximum Usable Frequency (MUF) that is refracted from the ionosphere back to Earth; this maximum frequency is usually determined by the state of the F region. Frequencies higher than the maximum frequency will penetrate the ionosphere. Over time, the maximum frequency varies because the electron concentration at the refraction location varies.

For the same sky wave path at a particular time, lower frequencies than the maximum frequency may propagate. These lower frequencies generally are refracted from lower altitudes in the ionosphere.

The lowest frequency that can propagate on a sky wave path at a particular time is dependent on the ionisation in the D region. Variations in D region ionisation cause this lowest frequency to change over time. Each time a sky wave traverses the D region the signal strength decreases; furthermore, the signal attenuation is greater at lower frequencies. Sky wave paths that lie completely within the night hemisphere may be able to use the lowest frequencies in the HF band since they are unaffected by the D region.

2.4. Hop lengths

The hop length is the distance across the ground from where a sky wave leaves the ground, is refracted once by the ionosphere and returns to Earth.

The upper limit of the hop length is set by the height of the ionosphere and the curvature of the Earth. At 0º elevation angle (horizontal), with E and F region heights of 100 and 300 km respectively, the maximum hop lengths are 2000 km (E region) and 4000 km (F region). For the same ionospheric heights, the maximum hop lengths decrease to 1800 km (E region) and 3200 km (F region) with an elevation angle of 4º (Figure 2.2). Distances between transmitting and receiving antennas greater than these maximum hop lengths require more than one hop. For example, a distance of 6100 km will require at least four hops via the E region and two hops via the F region. More hops will be required for larger antenna elevation angles.

In contrast, the path length is the distance across the ground between two tranceivers, e.g., the path length for the Sydney, Australia to Singapore sky wave path is 6295 km.

Hop lengths based upon an antenna elevation angle of 4º and heights for the E and F layers of 100 km and 300 km, respectively.

Figure 2.2.  Hop lengths based upon an antenna elevation angle of 4º and heights for the E and F layers of 100 km and 300 km, respectively.


2.5. Propagation modes or paths

A sky wave may travel different paths between two ground-based transceivers. These paths are dependent on the gain pattern of the transmitting antenna, the operating frequency and the state of the ionosphere.

The sky wave may propagate via just the E or F region (Figure 2.3).

Examples of propagation modes using one region only (one hop via the E region (pink) and F region (blue), two hops via the E region (green) and F region (red).

Figure 2.3.  Examples of propagation modes using one region only (one hop via the E region (pink) and F region (blue), two hops via the E region (green) and F region (red).


There is a tendency to consider the ionosphere as consisting of smooth, reflective layers however, the ionosphere undulates, affecting the height and angle of refraction of signals. Ionospheric tilting occurs near the equatorial anomalies, the mid-latitude trough, near the day-night terminators and when the ionosphere is disturbed due to sudden solar activity. Tilting can cause unusual propagation conditions such as ducting and chordal modes to occur (Figure 2.4). Chordal modes and ducting involve a number of refractions from the ionosphere without intermediate reflections from the ground. When chordal and ducting modes occur, signals can be strong since the wave spends less time traversing the absorbing daytime D region and being attenuated by ground reflections.

Some complex propagation modes, ducting (blue), chordal (red), F region propagation with an intermediate sporadic E refraction (green), propagation via the F then E region (pink).

Figure 2.4.  Some complex propagation modes, ducting (blue), chordal (red), F region propagation with an intermediate sporadic E refraction (green), propagation via the F then E region (pink).


2.6. E layer screening

HF sky waves that pass through the day hemisphere may be refracted by the E or F regions. Where the elevation angle of a propagation path via the F region is similar to that of a propagation path via the E region (e.g., two hops via the F region and three hops via the E region), E layer screening may occur. If the operating frequency is low enough, the signal will propagate via the E region and if high enough the signal will penetrate the E region and propagate via the F region. Propagation via the F region, particularly for longer paths, is usually preferable since signal strength will normally be greater.

Consider a particular propagation path where the elevation angle of one hop via the F region is similar to the elevation angle for two hops via the E region (Figure 2.5).

If the selected frequency is below the E region maximum usable frequency (EMUF) for the two hop E mode, the sky wave will travel via the E region (the wave is said to be screened from the F region). The wave will lose energy as it traverses the D region four times and reflects from the ground once. The wave will also be more highly absorbed in the D region due to the lower operating frequency.

If the selected frequency is above the EMUF for the two hop E mode but below the F region maximum usable frequency (FMUF) for the one hop F mode, the wave will penetrate the E region and propagate via the F region. The wave will lose energy as it traverses the D region twice (note, no intermediate ground reflection).

In the former case, the signal may successfully propagate via the E region. However, signal strength will probably be lower due to the higher absorption and reflection losses and, depending on the sensitivity of the receiving system, may not be detected.

Avoiding E layer screening is possible by selecting an appropriate frequency for F region communications. In cases where propagation is possible via both the E and F regions and the take-off angles of the two modes are similar, choosing a frequency well above the EMUF of the screening E mode should avoid E layer screening.

E layer screening occurs if communications are required by an F mode and the operating frequency is close to or below the maximum frequency that can propagate via the next higher E mode.

Figure 2.5.  E layer screening occurs if communications are required by an F mode and the operating frequency is close to or below the maximum frequency that can propagate via the next higher E mode.


Sporadic E may also screen a signal from the F region. At times it can 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 chance that the receiving antenna does not lie within the signal footprint (Figure 1.9 and Section 1.6). Screening due to sporadic E is possible on day and night sectors of sky wave paths.

2.7. Frequency, range and elevation angle

For oblique propagation (transceivers not co-located), there are three interdependent variables:

  • frequency;
  • range or path length;
  • antenna elevation angle.

Figure 2.6, Figure 2.7 and Figure 2.8 illustrate the changes to the propagation paths when each of these is fixed in turn.

Elevation angle fixed.

Figure 2.6. Elevation angle fixed.


Figure 2.6 (elevation angle fixed):

  • As the frequency is increased toward the MUF, the wave is refracted higher in the ionosphere and the ground range increases, paths 1 and 2;
  • At the MUF for that elevation angle, the maximum range is reached, path 3;
  • Above the MUF, the wave penetrates the ionosphere, path 4.
Path length fixed(base-to-base sky wave path).

Figure 2.7. Path length fixed(base-to-base sky wave path).


Figure 2.7 (path length fixed):

  • As the frequency is increased towards the MUF, the wave is refracted from higher in the ionosphere. To maintain a fixed length across the ground, the elevation angle must be increased as the frequency increases, paths 1 and 2;
  • At the MUF, the critical elevation angle is reached, path 3. The critical elevation angle for a particular frequency is the maximum elevation angle. If the elevation angle is increased on that frequency, the wave will penetrate the ionosphere;
  • At frequencies above the MUF, and elevation angles at or exceeding the critical elevation angle of path 3, the wave penetrates the ionosphere, path 4.
Frequency fixed.

Figure 2.8. Frequency fixed.


Figure 2.8 (frequency fixed):

  • At lower elevation angles the path length is greatest, path 1;
  • 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 wave returns when transmitted vertically into the ionosphere above the transmitting antenna, then there is no skip distance or skip zone around the transmitting antenna. However, if the wave penetrates the ionosphere above the transmitter, then as the elevation angle is increased from path 3, the range decreases. At some elevation angle between paths 3 and 4, the critical elevation angle for this frequency is reached. At take-off angles greater than the critical elevation angle, such as path 4, the wave penetrates the ionosphere resulting in an area into which neither the sky wave nor ground wave can propagate; this region is called the skip distance or skip zone. Communications via the sky wave into the skip zone may be possible with the use of a lower frequency.

2.8. Skip zones

The skip zone around a transmitter varies diurnally, with the seasons, and with solar activity. Skip zones are generally smaller in the day hemisphere, at solar maximum and around the equinoxes. The ionosphere during these times has increased electron density and so is able to support (refract) higher frequencies. At night and at solar minimum when the ionosphere is weaker, the same frequency may penetrate the ionosphere at the same antenna elevation angle, increasing the size of the skip zone (e.g., wave 3 in Figure 2.8 may penetrate the ionosphere at night).

At the outer edge of the skip zone there is usually a significant change in signal strength. Moving away from the transmitting antenna the signal strength will abruptly increase as a receiving antenna moves from within the skip zone into the sky wave's footprint.

Sidescatter may result in some of the sky wave propagating into the skip zone. Mountains or other terrain outside the skip zone may reflect the sky wave into the area.

2.9. Fading

Multipath fading results from spreading of the signal by the transmitting antenna causing the signal to propagate via a number of sky wave paths. If the signal propagates to the receiving antenna by two or more paths, and there are significant time and frequency shifts with similar signal strengths, fading is likely.

Travelling Ionospheric Disturbances (TIDs) are wave-like features passing through the ionosphere. As they travel through the refracting region of a sky wave path, the disturbance may change the altitude of refraction and cause the ionosphere to become tilted, focussing or spreading the signal and altering the wave's footprint (Figure 2.9). 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 events on the Sun and may travel large distances. Others are more localised, originating 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 its path through the ionosphere. Components of the signal orthogonally polarised to the receiving antenna are undetected. Polarisation fading can last for a fraction of a second to a few seconds and is likely to occur when linearly polarised receiving antennas are used.

Skip fading can occur when the ionosphere is changing rapidly (e.g., around sunrise and sunset, ionospheric storms) and the operating frequency is close to the MUF. Small changes in the electron density may cause the operating frequency to oscillate around the MUF, causing the signal to successively refract from and penetrate the ionosphere. A receiving antenna located near the outer edge of the skip zone will observe signal fade as the skip zone fluctuates in size.

Focussing and defocussing effects caused by tilting and travelling ionospheric disturbances (TIDs).

Figure 2.9.  Focussing and defocussing effects caused by tilting and travelling ionospheric disturbances (TIDs).


2.10. VHF and 27 MHz propagation

Very High Frequency (VHF: 30 to 300 MHz) and 27 MHz are used for line-of-sight or direct wave communications (e.g., FM radio, ship-to-ship, ship-to-shore and ground-to-air). In contrast to the medium frequency and HF bands where frequency choice can be crucial to success, frequencies in the VHF band usually perform similarly.

Since VHF and 27 MHz operate mainly by line-of-sight, mounting antennas as high as possible and free from obstructions such as hills and tall buildings is essential. Shore stations for maritime communications 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 27 MHz communications between two ground locations should concentrate radiation at low angles (towards the horizon).

VHF and 27 MHz do not usually suffer from atmospheric noise except during severe electrical storms. The Sun, environmental noise and galactic noise are the main sources of noise at VHF. Interference from other users can be a significant problem in densely populated areas.

27 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 this may occur:

  • around solar maximum and during the day, the ionospheric F region will often support long range sky wave communications on 27 MHz and above;
  • sporadic E layers can often refract 27 MHz and lower frequency VHF propagation over circuits of about 500 to 1000 nautical miles (900 to 1800 km) in length. This is most likely to occur at mid-latitudes, during the daytime in summer;
  • 27 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.

2.11. Medium frequency (MF) sky wave propagation

The Medium Frequency (MF: 300 kHz to 3 MHz) band is able to propagate via ionospheric refraction at night. D region absorption of sky waves decreases markedly between sunset and sunrise since the D region essentially disappears in the night hemisphere. MF frequencies that are normally highly attenuated by the D region during the day, such as frequencies in the AM radio band, are thus able to propagate much further.

2.12. Ground wave MF and HF propagation

It is possible to communicate up to distances of several hundred nautical miles on MF and low HF frequencies by transmitting the signal across the surface of the Earth.

The range of the ground wave is dependent on the frequency, terrain, ground type, transmitter power and antenna type.

Lower frequencies travel further across the ground, with mountains and valleys hindering this form of propagation. Sea water is the ideal ground type with attenuation increasing as the ground becomes drier and/or less salty; ice being least conductive. A greater transmitter power provides a stronger signal which will propagate further and normally polarised antennas directing and receiving energy at low take-off angles are ideal (e.g., vertical whips appropriate for the operating frequency).

2.13. Noise

Radio noise in a receiving system has internal and external origins. Internal or thermal noise is generated within the receiving system and is usually negligible when compared to external noise sources in the HF and lower frequency bands. External radio noise originates from natural (atmospheric and galactic) and environmental (man-made) sources.

Atmospheric noise, caused by lightning discharges in thunderstorms, is normally the major contributor to radio noise. Thunderstorm occurrence is greatest at low latitudes with a seasonal variation. Atmospheric noise is normally directional at a receiving antenna, and varies in strength from being impulsive when storms are proximate to the receiving antenna, to a broad continuum from distant storms. Atmospheric noise levels are generally greater at lower frequencies.

Galactic noise in the HF band arises from celestial sources in the Milky Way galaxy and the Sun when active. Galactic noise levels decrease with increasing frequency. Galactic noise affects only those frequencies that are able to penetrate the ionosphere from the top side. Therefore, longer paths which use higher frequencies are most susceptible to galactic noise.

Environmental noise sources include ignition systems, industrial and domestic machinery (microwave ovens and computers), electrical cables and transmission lines. Interference can be included in this classification and may be intentional, due to unusual propagation conditions or the result of operators ignoring regulations. Environmental noise levels decrease with increasing frequency. Environmental noise tends to be normally polarised (horizontally polarised waves in the HF band do not travel great distances across the ground), so selecting a horizontally polarised receiving antenna may be helpful.

All noise can be reduced by use of a narrower bandwidth and a directional receiving antenna (with good gain in the direction of the transmitting source and preferably low gain in the direction of any unwanted noise source). Initial receiver site investigation and identifying potential noise sources are important factors in establishing a successful communications system.

2.14. Universal time

Unless transceivers are always to operate in the same local time zone, it is considered more appropriate to use Universal Time (UT) for communication schedules. Universal Time equates to Greenwich Mean Time (GMT) and Zulu (Z) time. The Universal Time, 0000 UT, occurs at midnight at Greenwich, England.

3. Effects of solar events on HF communications

3.1. Short wave fade-outs (SWFs)

Also called daylight fade-outs or sudden ionospheric disturbances (SIDs), large solar flares emit x-rays which increase the ionisation in the D region and attenuate HF sky waves passing through the layer (Figure 3.1). If the flare is large enough, the whole HF spectrum may be compromised for a period of time. Flares, and so fade-outs, are more likely to occur around solar maximum. The main features of SWFs are:

  • Only HF paths that pass through the day hemisphere are affected;
  • HF paths with refraction points near the sub-solar region are most affected;
  • Attenuation 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 remain usable, depending on the strength of the flare (Figure 3.2);
  • Fade-outs have a fast onset. The signal strength decreases very quickly over seconds to minutes and slowly recovers;
  • Fade-outs usually last from a few minutes to, on rare occasions, hours. The duration of the fade-out will depend on the duration and x-ray output of the flare, the sky wave path through the day hemisphere, and the operating frequency;
Fade-outs affect only those paths where the HF sky wave passes through the D region. That is, paths with daytime sectors. HF sky wave communications through the night hemisphere are unaffected.

Figure 3.1.  Fade-outs affect only those paths where the HF sky wave passes through the D region. That is, paths with daytime sectors. HF sky wave communications through the night hemisphere are unaffected.


Higher frequencies are least affected by fade-outs and take less time to recover to pre-flare signal strength.

Figure 3.2.  Higher frequencies are least affected by fade-outs and take less time to recover to pre-flare signal strength.


3.2. Polar cap absorption events (PCAs)

Some solar flares emit high energy protons as well as x-rays. These fast protons have variable speeds, some relativistic, others taking up to ten days to reach Earth. When the protons enter the Earth's magnetosphere, they are directed along the magnetic field lines towards the poles. The protons ionise the polar D region. An increase in polar D region ionisation attenuates HF sky waves that traverse these areas and is called a PCA. PCAs occur most often with larger solar flares which have a greater likelihood around solar maximum.

The increased attenuation of HF sky waves in the polar regions may begin as soon as a few minutes after the flare onset and last for as long as ten days. PCAs vary in strength over their course due to the temporal variability in protons arriving from the Sun and entering the D region.

Even the winter polar zone (a region of darkness) can suffer the effects of PCAs as the protons may actually produce a night D region.

The effects of PCAs on polar sky wave paths can sometimes be overcome by relaying messages on paths which avoid the polar regions.

3.3. Ionospheric storms

Solar events such as coronal holes and coronal mass ejections change the character of the solar wind, increasing its speed and altering its density and magnetic structure. When the disturbed solar wind impacts the Earth's magnetosphere, the geomagnetic field and electric fields and currents at ionospheric altitudes change. The response by the ionosphere, a change in electron density and height in the F2 region, is called an "ionospheric storm".

Ionospheric storms are characterised by a decrease in the electron density of the F2 region, although sometimes there is an increase in electron density for a few hours before the decrease.

The decrease in the F2 region electron density decreases the highest frequencies that can be refracted, the MUFs. Frequencies that are greater than the storm MUFs will penetrate the ionosphere.

Communications are likely to be further complicated because the F2 region increases in height during ionospheric storms, altering the location of the footprint of the sky wave.

Ionospheric storms vary in their behaviour at different latitudes and locations. They may last a number of days with high latitudes generally having larger decreases in F2 region electron density (and MUFs) than other latitudes. Unlike fade-outs, the effects of ionospheric storms begin more slowly, over hours, have a longer duration, and may require the use of lower operating frequencies.

Figure 3.3 shows the effects of sudden solar events on the MUFs of the different ionospheric regions above Norfolk Island in February 2012. Sudden solar events caused a strong geomagnetic storm on 15 February and moderate geomagnetic disturbances on 19, 20 and 27 February. The geomagnetic disturbance on 15 February was followed by a marked decrease in the F2 MUFs on 16 February with the F2 region recovering over the 17 and 18 February. The F2 MUFs also decreased on 21 February and 28 and 29 February in response to the more moderate geomagnetic effects. It is worth noting there was little or no change in the MUFs for the E and F1 regions due to the disturbances.

Norfolk Island (29.03º S, 167.97º E) MUFs for F2 region (blue), F1 region (red) and E region (green) in February 2012. Sudden solar events caused the geomagnetic field to become strongly disturbed on 15 February, and moderately disturbed on 19, 20 and 27 February.

Figure 3.3.  Norfolk Island (29.03º S, 167.97º E) MUFs for F2 region (blue), F1 region (red) and E region (green) in February 2012. Sudden solar events caused the geomagnetic field to become strongly disturbed on 15 February, and moderately disturbed on 19, 20 and 27 February.


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