294 



ATTENUATION OF RADIO WAVES 



40 



20 



1 

 0.8 

 0.6 



0.4 



0.2 



O.t 

 0.08 



0.06 

 0.04 



0.02 



O.Ot 



0.01 0.02 0.04 0.1 0.2 0.4 0.6 1 2 



RAINFALL RATE IN INCHES PER HOUR 



Figure 7.19. Rain attenuation versus rainfall rate. 

 (Theoretical, after Ryde and Ryde). 



6 8 10 



There is little evidence that a rain with a known rate of fall has a unique 

 drop-size distribution though studies on this problem seem to indicate 

 that a most probable drop size distribution can be attached to a rain of 

 given rate of fall [22]. Table 7.8 shows the percentage of total volume 

 of rainfall occupied by raindrops of different diameters (cm) and varying 

 rainfall rates (mm/hr). Tables 7.8 and 7.9 are offered as an aid to 

 estimating, through a qualitative approach, the attenuation of radio 

 waves by raindrops [25]. 



Table 7.8 gives the decibel attenuation per kilometer in rains of different 

 rates of fall and radio wavelengths between 0.3 and 10 cm. In table 7.9, 



