Special Modes of Propagation on Higher Amateur Radio Bands

Transequatorial Propagation

Transequatorial VHF propagation was first noted by radio amateurs in August 1947 when Mexican stations could work Argentine stations regularly across the equator in the late afternoon and early evening on 50 MHz. This occurred for few more years during the maximum of Solar Cycle 18. Transequatorial Propagation came down during the period of sunspot minimum and reappeared in 1955 and continued through the maximum years of Solar Cycle 19. We can expect similar conditions as we are heading for the maximum of Solar Cycle 25 in 2025.

Two types of transequatorial propagation has been described. One occurring during late afternoon and early evening with maximum distance of about 6000 km in low VHF band of 6 m. Second one was between 7 pm to 11 pm local time with contacts on 2 m and sometimes on 70 cm band. Mechanism by which transequatorial propagation occurs is the presence of equatorial anomaly, which is a high concentration of electrons on either side of the equator in the region of 10 to 20 degrees latitude.  The signals are reflected by one anomaly to the other and then back to the ground on the opposite side of the equator. So contacts are possible between stations equidistant from the equator.

Troposcatter Propagation

Tropospheric Scatter Propagation is known in short as Troposcatter Propagation. Troposphere is the lowest layer of the atmosphere. Though Troposcatter is typical at microwave or super high frequency or SHF band, it can also occur at VHF and UHF bands. The height of the troposphere is about 20 km near the equator, which comes to about 9 km in polar regions in summer. Height of the troposphere is more in summer than in winter. Troposphere can refract higher end of the radio spectrum. HF is reflected typically by the ionosphere, much above the troposphere. Tropospheric scatter was used mainly by the military prior to the era of satellites.

When the transmitting and receiving stations point their antennas to a common scatter volume in the troposphere as shown in the illustration, troposphere refracts the signal towards the receiving station. Troposcatter may work from frequencies as low as 144 MHz to 10 GHz and useful communication can occur from 100 km to 700 km. Typically the transmitting power has to be high and the receiver quite sensitive, as the signal loss is high in troposcatter. Yet it is supposed to be a reliable mode of communication at the higher end of the useful radio spectrum. The angle between the two radio horizon rays is known as the scatter angle. The volume of intersection of the two beams is known as the scatter volume. Scatter volume will be high for low gain antenna with a wider beam and lower for a high gain antenna with a narrower beam.

Auroral Propagation

Usually Aurora would mean a radio blackout on HF due to ionospheric activity. But Aurora can be used to our advantage for propagation on VHF, and rarely on UHF. Good directional antennas are required for auroral propagation. Beams are directed towards the auroral zone and not in the direction of the station being contacted. Distortion is quite likely in voice signals and CW communication may fare better.

Contacts with stations up to 1800 Km away may be possible with auroral propagation on VHF. Suffix A like 5,7A is given for auroral signal reports, though the readability may not be truly 5! Digital communications do not fare well on auroral propagation. Though being a natural phenomenon it can occur at any time, aurora is more likely in February, April, July and October. Of course, being in tropics, auroral propagation is out of question for me as aurora is a polar event!

Sporadic E propagation

Sporadic E, known in short as E_s, is an unusual form of radio propagation at VHF range using the E layer of the ionosphere. Normally only F layer of the ionosphere refracts radio waves, usually those in the HF range, allowing long distance radio communication. Sporadic E propagation occurs when small regions in E layer at 95 to 150 km reflects radio signals. This typically occurs from 3 weeks before to 3 weeks after the summer solstice, on VHF bands, which normally allows only line of sight propagation. This would be from early June to mid July in the Northern hemisphere. Sporadic E propagation is especially sought after by radio amateurs working on 6 m band for long distance communications.

Ionosphere containing ionized gas particles, is located mainly in the thermosphere layer of the atmosphere, though it can extend inwards to the mesosphere and outwards to the exosphere. The lower most layers of the atmosphere below the mesosphere are known as stratosphere and troposphere from above downwards. Ionosphere can extend from 60 to 300 km above the earth's surface. Three layers are F region, E layer and D layer. During daytime, F layer splits into F₁ and F₂ layers due to increased ionization. The two layers of F region merge together at night.

Existence of E layer was demonstrated by Marconi in 1901 when signals were shown to bounce off an electrically conducting layer at about 100 km altitude, during transmission between Europe and North America. It was Sir Edward Appleton in 1927 who named it the E layer, meaning electrical layer. Later additional conducting layers were discovered and named D layer inner to E layer and F layer outer to E layer. D layer disappears at night.

Sporadic E propagation occurs in the frequency range of 25 to 150 MHz which would include amateur radio bands of 2 m, 6 m and 10 m. Communication may occur by a single hop along an E_s patch or by multiple hops. Hops can occur from the ground back to the next E_s patch in which case the signal loss is more as the second hop is from the earth which is lossy. One or more chordal hops can occur between the patches before the signal reaches the ground receiver. In that case, signal loss is lesser because lossy reflection from earth is not involved. Distance covered by hop from a single E_s patch can range from 800  to 2,200 km. Multiple chordal hops can take the signal from 2,200 to 15,000 km.

Rain Scatter Propagation

Radio waves can be scattered by irregularities within the troposphere. An effect which is important above 3 GHz, that is in the microwave range, is Rain Scatter. A generic name would be Hydrometeor scatter which will include scatter by different types of water particles within the atmosphere. Water in liquid form can be there as rain, mist and fog. Water in the form of ice can be there in clouds, hail and snow.

We know that frequencies in the microwave range typically have line of sight propagation. Rain Scatter can extend the propagation beyond line of sight. If the rain is 2-4 km above ground level, Rain Scatter propagation can be several hundred kilometers, as described by F5LEN in his post on X. When the particle size is much smaller than the wavelength, the type of scattering is known as Rayleigh scattering, with radiation pattern like that of a dipole. That will be more to the front and back rather than to the sides. Particles of size of the order of the wavelength produce Mie scattering, in which there is more of forward scattering with a forward radiation lobe. Dependence on the particle size is the reason why Rain Scatter does not occur at VHF and UHF range, only above that, in the SHF or Super High Frequency range which has been defined as 3 to 30 GHz by the International Telecommunication Union. Most of the scattering of microwaves is Rayleigh scattering.

There is a web application which helps radio amateurs in making Rain Scatter contacts at rainscatter.com. The application automatically downloads storm data from the National Weather Service in the United States and identifies possible propagation paths between stations. The map shown there is that of the United States of America, Mexico and up to part of South America. Though I tried reducing the size of the map to see if it covers rest of the world, I could not see. There is also a clickable icon labelled 'DX' which asks you to input grid. I tried entering my grid, but nothing happened, may be because my region is far away from the range of data which they have in the database! Of course, I have not heard of any radio amateur communicating in the microwave range in this region except on QO 100 Geostationary Amateur Radio Satellite. If you sign up on rainscatter.com and post your location, others who wish to have contacts will be able to see it.

It has been mentioned that 5.7 GHz and 10 GHz are the optimum bands for Rain Scatter propagation. In fact, 10 GHz has been called as the "Rainy Day Band". That is because of expected rain particle sizes favour these frequencies. While a tropical downpour which we experience in this region can have a rain particle size of 3mm, drizzle has a particle size of around 0.5mm. Fog or mist particles can be from 10 microns to 1 nanometer. From this description, I would expect lower frequency to be useful in my region, though I am yet to see information on that, as mentioned earlier. For actual operations, both stations should be in range for the rain cell which is the scatter point and antennas should be pointed at it. CW works better, while SSB can have distortion of audio as in Auroral Propagation due to Doppler spreading of signals. May be some of our friends in this region could turn their 10 GHz QO 100 antennas towards rain cells and try Rain Scatter QSOs! Wishing them all success as I hear heavy rain outside now.

Meteor Scatter Propagation 

Meteor Scatter is a special mode of propagation in which the ionization of atmosphere produced by the meteor trail refracts VHF and sometimes UHF signals so that long distance communication is possible without repeaters. MSK144 Protocol is a special mode available in WSJT-X for meteor scatter communication. MSK144 is a Minimum Shift Keying Frequency-Shift Keying which transmits 144 bit long packets at a baud rate of 2000 bps using frequencies 1000 and 2000 Hz. A sample audio and spectral display of MSK144 is available at the sigidwiki.com webapge, which is meant as a signal identification guide.

K9AN and K1JT has mentioned that meteor scatter communication was first described in 1953 as a means of communicating in the then dead 15 and 20 m bands. Later it was found that 6 m and 2 m bands are better with much lower background noise levels. Useful low elevation gain was also achievable with relatively modest antennas. Initial contacts were on CW and utilized relatively rarer blue whizzer meteor trails which lasted several seconds or longer. But current day contacts using digital technology with error correction can utilize those lasting less than 0.1 second and thus enables communication on VHF bands any day of the year to around 1300 miles, making these contacts independent of weather, solar activity, position of the moon etc.