302 



ATTENUATION OF RADIO WAVES 



Under this condition the attenuation due to rainfall should be directly 

 proportional to the path length. Thus the path length in figure 7.24 can 

 be shortened to correct for in.sufficient fading margin. 



Bell System results indicate that for their microwave relay links in the 

 extreme southeastern region of the United States rainfall will limit 11 

 Gc/s radio systems having a 40 dB fading margin to path lengths of 

 approximately 10 to 15 mi, depending on the number of hops, if normal 

 reliability objectives are to be met. Path lengths of 20 to 30 mi should 

 be acceptable in the central area and paths as long as 35 mi should be 

 acceptable in the northwestern part of the country. However, in their 

 existing point-to-point radio relay systems, the paths average about 23 

 mi due to other considerations than those of propagation. It appears 

 that the 11 Gc/s systems will not be penalized unduly except in the 

 southeastern part of the United States. 



An illustration of the correlation between the rainfall and path loss on 

 March 15, 1956, of the ]\Iobile study is presented in figure 7.26 in support 

 of the above conclusions. 



CALCULATED 

 (RAINFALL DATA) 

 MEASURED 



1200 

 TIME OF DAY 



Figure 7.26. Correlation between rainfall and path loss, March 15, 1956. 

 (After Hathaway and Evans, 1959). 



7.11. Attenuation by Hail 



Ryde concluded that the attenuation caused by hail is one-hundredth 

 that caused by rain, that ice crystal clouds cause no sensible attenuations, 

 and that snow produces very small attenuation even at the excessive rate 

 of fall of 5 in. an hour. However, the scattering by spheres surrounded 

 by a concentric film of different dielectric constant does not give the same 

 effect that Ryde's results for dry particles would indicate [24, 27]. For 

 example, when one-tenth of the radius of an ice sphere of radius 0.2 cm 

 melts, the scattering of 10 cm radiation is approximately 90 percent of 

 the value which would be scattered by an all-water drop. 



