298 



ATTENUATION OF RADIO WAVES 



OjOI OjO! 01)3 01)5 007 0.1 02 0.3 05 0.7 I 2 3 5 7 10 20 30 50 70 100 200 300 500 TO IMC 



Total Path Attenuation in Decibels 



Figure 7.21. Combined rain and gaseous absorption to be exceeded 1 percent of the time. 



7.10. Rain Attenuation Effects on Radio 

 Systems Engineering 



Attenuation due to rainfall is obviously a dominating factor in deter- 

 mining the reliability of a communications system, especially at frequen- 

 cies in excess of 30 Gc/s. Rain varies greatly in frequency of occurrence 

 from one region to another, so it is important to have an effective method 

 of predicting the performance of a radio system in any region in order 

 that the communications engineer will be able to gain the widest possible 

 application and degree of reliability, consistent with cost, in any system 

 of his design. 



This section will be concerned, in the main, with the results of the 

 Bell System's [24] field experiment in the Mobile, Ala., area, which was 

 designed to establish a relationship between excess path attenuation and 

 instantaneous rate of rainfall and to seek out any relationship between 

 the profile of rate of rainfall along a radio path and rainfall measured 

 at a point. 



The main problem concerns the ability to predict outage time due to 

 rainfall (time the system noise exceeds the system objective) at 11 Gc/s 

 in all areas of the country. This is obviously a difficult problem since, 

 due to reasons of cost, it is not feasible to measure rainfall attenuation 

 in all parts of the country. Therefore, it is desirable to be able to use 

 what rainfall data are available and to couple the data, through what are 

 thought to be reasonable assumptions, to the relationships between rain- 

 fall and attenuations. 



