300 



ATTENUATION OF RADIO WAVES 



Figure 7.23. Contours of constant path length for fixed outage time. 

 (After Hathaway and Evans, 1959). 



hops of a system are electrically long; they will have insufficient fading 

 margin (the number of dB the receiver input level can be reduced before 

 the noise exceeds the system objective) and hence contribute more than 

 their share of the outage time. So, this excess must be made up by 

 imposing tighter requirements on the remaining hops. To meet the 

 overall objective of the Bell System, it is necessary to know the contri- 

 butions of the long hops — those having a fading margin less than 40 dB. 

 Figure 7.24 shows the excess path loss due to rain, versus hours per year 

 for the Mobile area study. Since the shape of this curve is nearly 

 identical to Bussey's curve of cumulative distribution for point rates in 

 Washington, D.C. (if we assume the shape of this curve to be representa- 

 tive for other areas of the country), then the additional outage time for 

 path lengths given by figure 7.23 can be estimated for hops having a 

 fading margin less than 40 dB. The data shown on figure 7.24 have been 

 rationalized and are shown in figure 7.25 as an estimate of additional 

 outage time. 



Sometimes it is practical to shorten a proposed path to bring the fading 

 margin up to 40 dB. An approximation of the necessary reduction path 

 length can be made if uniform rainfall rate is assumed over the path. 



