Rain Scatter Propagation

GHz bands rain scatter operation

Rain showers, referred to by meteorologists as rain “cells” can form very effective forward and back scatterers to signals on the GHz bands. Rainscatter affects all the GHz bands to a greater or lesser extent but it is generally accepted that the optimum bands are 5.7 and 10GHz. Why is that? Water forms in the atmosphere over a wide range of particle sizes; raindrops vary in size from about 3mm (a tropical downpour) to 0.5 mm (drizzle), cloud particles are between 100 and 1µm and fog or mist particles range from 10µm down to 1nm. A bit of basic physics tells you that scattering from particles (in this case water particles) increases with a fourth power law with frequency, up to particle sizes of around a tenth of a wavelength, above which the scattering remains constant with frequency. For the lowest microwave bands, a tenth of a wavelength is 23, 13, 9, 6, 3, and 0.12mm respectively, so you can now clearly see why 5.7GHz and 10GHz are the optimum bands. 10 GHz falls just below the one-tenth wavelength transition for typical rain and while 5.7GHz is a little long a wavelength even for a tropical downpour, RF power is more easily generated that at 24GHz and water absorption is much less. With the fourth power law, you can compare the expected signal at 10GHz, to that at 5.6 GHz (0.56 of the frequency). We would expect to receive (0.56)⁴ times the signal than at 10GHz, that is 9.83% of, or about 10dB less. If you calculate the dB loss for the lower bands and soon see why there are fewer RS QSOs as you go down in frequency.

Actually operating rain scatter is easy, both stations just need to be in range of a rain cell, (referred to as a scatterpoint or “scpt”) and point their antennas at it. Signals will be forward or backscattered between them. All modes work, with CW the easiest. SSB and CW have a characteristic “rasping” sound, not unlike Auroral signals, caused by Doppler spreading but if signals are strong enough NBFM gives almost perfect audio quality.

Rain scatter QSO range is determined by the height of the scatterpoint and your local takeoff angle to it and clearly some scatter points will be below your horizon. Typically rain cells are 2-4km above ground and QSOs can be from a few km to several hundred. To find where a scatter point is located there are a number of rain radar websites, or if you have a local beacon, tune to it, rotate your antenna and see what direction it peaks up on other than the direct path. With local heavy rain, often signals come in from all directions making multi way QSOs possible.

For a more detailed and very readable treatment refer to Mike G0MJW’s website (1) or Tom, WA1MBA’s excellent paper on the topic (2).