292 ATTENUATION OF RADIO WAVES 



7.8. Attenuation by Rain 



Ryde and Ryde [21] calculated the effects of rain on microwave propa- 

 gation and showed that absorption and scattering effects of raindrops 

 become more pronounced at the higher microwave frequencies where the 

 wavelength and the raindrop diameters are more nearly comparable. In 

 the 10 cm band and at lower wavelengths the effects are appreciable, but 

 at wavelengths in excess of 10 cm the effects are greatly decreased. It is 

 also known that suspended water droplets and rain have an absorption 

 rate in excess of that of the combined oxygen and water vapor absorp- 

 tion [3]. 



In practice it has been convenient to express rain attenuation as a 

 function of the precipitation rate, R, which depends on both the liquid 

 water content and the fall velocity of the drops, the latter in turn depend- 

 ing on the size of the drops. 



Ryde studied the attenuation of microwaves by rain and .showed that 

 this attenuation in dB/km can be approximated by: 



Ku=K RirYdT {1.1) 



where Kr = total attenuation in dB 



K = con.stant 

 R{r) = rainfall rate 



r = length of propagation path in km, and 

 a = constant. 



Laws and Parsons [22] observed the distribution of drop sizes for 

 various rates of fall on a horizontal surface. The higher the rainfall rate, 

 the larger the droits, and also the greater the range in size of the drops. 

 However, in order to derive the size distributions occurring while the 

 drops are falling in the air, each rainfall rate must be divided by the 

 particular velocity of fall appropriate to the corresponding drop diameter. 

 Figure 7.18 shows the resulting distribution in air, expressed as the rela- 

 tive mass of. drops of given diameter, for three chosen representative 

 values of precipitation rate, p, namely, 2.5 mm/hr, 25 mm/hr, and 100 

 mm/hr [23]. The excess path loss per mile, according to Ryde, for the 

 three carrier frequency bands of 4, 6, and 11 Gc/s is shown on figure 7.19. 

 Figure 7.20 [24] is a scatter diagram showing transmission loss versus rain- 

 fall rate. For comparison, Ryde's equation is plotted on figure 7.20. 



The greatest uncertainty in predictions of attenuation due to rainfall, 

 using theoretical formulae as a ba.sis for calculation, is the extremely 

 limited knowledge of drop size distribution in rains of varying rates of 

 fall observed under differing climatic and current weather conditions. 



