Microwave DX Propagation

Some people believe that microwaves are only good for working local line of sight paths. This is not true. Contacts over many hundred km are routine and with enhanced conditions distances over a thousand km are possible.

There are four main propagation modes that allow extended paths to be worked:-

• Tropospheric Scattering. Most microwave station-to-station contacts beyond the line-of-sight take place using a very reliable propagation phenomenon known as ‘tropospheric scatter’, or more informally, ‘troposcatter’. This involves scattering of the microwave signal from natural discontinuities in the upper part of the lowest layer of the atmosphere, the troposphere. SSB/CW stations on sites without major local obstructions, running 1W on 10GHz to a 60cm diameter TV dish, can reliably communicate over about 250km. This performance is obtained because the dish concentrates the majority of the transmitted energy into a narrow cone, typically around 2° wide in both the vertical and horizontal directions for a small TV dish at 10GHz. The problem in finding contacts is that, unless they are very local, the station will only hear, and be heard by, other stations within that beam. Some simple maths shows that, even ignoring the potential for errors in vertical alignment, the chance of two stations directing their beams at each other is small: about 1 in 32400! So, the practice of ‘talkback’, (direct communication by another medium, being another Amateur band, phone or internet) has developed.
• Rainscatter: The scattering of RF energy from rainfall or other forms of precipitation is common and can result in contacts over several hundred km. More about this can be found at – www.mike-willis.com/Tutorial/rainscatter.htm
• Tropospheric Ducting:  In various forms this can vary from brief lifts in a specific direction to widespread openings covering much of Europe. See here for forecasts and maps – www.dxinfocentre.com/tropo_nwe.html
• Aircraft Scatter: Scattering from aircraft is a regular occurrence. With feedback from aviation websites, and the use of suitable software, this can be predicted surprisingly accurately. Contacts of up to about 800km can be made by this method. A useful resource is www.airscout.eu

Moonbounce (EME):  This provides the opportunity to make intercontinental contacts on the microwave bands. UK stations are active using moon reflection on all bands to 24GHz. Microwave ‘moonbounce’ does not require enormous antennas or transmitter power required on the lower frequencies – although both can help!. It is possible to make marginal EME QSOs with some of the larger stations using just a 1m TV dish and 10W on 10GHz, while an optimised 2.4m dish with 50W on the same band can provide SSB echoes. On the ‘low-bands’ at 1.3 and 2.3GHz, a 1.8m mesh dish with ~200W will provide many QSOs using CW or digital modulation schemes.

Satellites: There are numerous Amateur satellites. A number carry GHz band transponders and beacons. A notable system is QO-100 which is a geostationary transponder providing wide coverage over Region-1.