Chapter 7. Attenuation of Radio Waves 



7.1. Introduction 



The advent of tropospheric forward scatter techniques has made possi- 

 ble communications over longer distances with higher frequencies than 

 has been heretofore thought practicable. The limitations imposed by 

 gaseous absorption, and by scattering by raindrops, upon the power re- 

 quirements of a conmiunications system for this application become more 

 important with increasing distance and frequency. It has been common 

 in the past to evaluate propagation path attenuation due to absorption by 

 multiplying the ground separation of the terminals by the value of the 

 absorption calculated for surface meteorological conditions [1]^ or avoid 

 the problem by restricting the communications system to frequencies 

 that are essentially free of absorption [2]. This is in contrast to another 

 approach [3] which actually used the absorption along the ray path. 



The following sections of this chapter will be devoted to a descriptive 

 treatment of absorption of radio waves by raindrops and gaseous oxygen 

 and water vapor in the atmosphere. 



Unless otherwise specified, the following conditions will be assumed in 

 this chapter: (1) All attenuations will be expressed in terms of decibel 

 loss per unit length of the propagation path (dB/km). The attenuations 

 due to different causes are simply added to give the total attenuation in 

 decibels. (2) In this treatment average conditions of temperature, drop- 

 let size, and droplet distribution are assumed for the radio path in order 

 to approximate conditions met in practice. 



7.2. Background 



The attenuation experienced by radio waves is the result of two effects: 

 (1) absorption and (2) scattering. At wavelengths greater than a few 

 centimeters, absorption by atmospheric gases is generally thought to be 

 negligibly small except where very long distances are concerned. How- 

 ever, cloud and rain attenuation have to be considered at wavelengths 

 less than 10 cm, and are particularly pronounced in the vicinity of 1 and 

 3 cm. 



' Figures in brackets indicate the literature references on p. 308. 



2G9 



