Communications, Radar & Electronic Warfare (201.. - Index of

144__VHF to SHF Radio Prediction 173absorption per kilometre over the range 1-350 GHz, for sea level paths (see ITU-RP.676 for more accurate methods of determining the actual values). The thick lineshows total attenuation and the two dashed lines show the contribution due to dry airand water particles suspended within the air.For example, the attenuation at 20 GHz, the attenuation per km is approximately0.3 dB . Thus for a 10 km communications link, the loss along the link would be 3 dB,or for a radar system would be twice this to account for the send and receive paths,i.e. 6 dB .Atmospheric attenuation is always present, but other effects are intermittentlypresent such as rainfall, fog, ice and snow. Figure 10.14 shows some attenuation figuresfor different frequencies, polarisation and rainfall rate.Rain, particularly heavy rain, is most often not present along the entire length of alink. Heavy rain regions are referred to as 'cells'. If, for example, there is a 5-km longcell of rain falling at the rate of 45 mrnJh in the direct path of a link using 20 GHz withveliical polarisation then the attenuation per kilometre can be read from the graph to beapproximately 4 dB per km. Thus the total attenuation for the cell is 5 x 4 = 20 dB. Fora radar link in which the cell is in the path of both the transmit and receive paths, thetotal additional attenuation would be twice this; 40 dB . Similar calculations can becarried out for different types of precipitation. Rainfall and other precipitation mayalso affect the temperature and pressure at the altitudes between the clouds and theground. This can lead to localised variations in the vertical refractive index, which willalso influence propagation.Rain attenuation dB/km302S r-----------------------------------------26 "24 /" .r ,.' r----:--::-=:-:-:--:-...,-..,--,22 -·.10GHzVerllcalpol1 /"" ..,;;;"-,,::;r'- _ • . 10GHz HOrizontal polE ./ ,.. __ '"ai 2 1S O • • • . 20GHz Vertical pol/"".,-'-' ,. ;;;> -"- - 20GHz HOrizontal pol"0.§ 16./ 1"' . -" ... '- 30GHz Vertical pol/"" .. . .,.. ... ' ,.. ____- •• 30GHz HOrizontal pol§ 12- ;;;;..,::-' • :. .:--;...._. 40GHz Vertical pola5 10 • • -" ,.. ' .- - - • - _. 40GHz HOrlzonlal pol S.-:'- - :... . - -:. '• .. - - - - 50GHz Vertical pol,- #:: .. .. /::: r: - - -.. .. - - 50GHz HOrizontal pol6A.-:-;-" ___2 i _O .. -- ..o: -; - :,-== -::-,..- _ .. --'"-:-. _ ' - - '. ---"---=-..;------.--: )-:---.---:-=.:-:----- .. II-----"' .. :: .. ..o 10 20 30 40 50 60 70 so 90 100Rain rate mm/hFigure 10.14Graph of rain rate against attenuation for different frequencies and polarization.