270 ATTENUATION OF RADIO WAVES 



It is helpful to recall that when an incident electromagnetic wave passes 

 over an object whose dielectric properties differ from those of the sur- 

 rounding medium, some of the energy from the wave is (a) absorbed by 

 the object and heats the absorbing material (this is called true absorp- 

 tion), and (b) some of the energy is scattered, the scattering being gener- 

 ally smaller and more isotropic in direction the smaller the scatterer is 

 with respect to the wavelength of the incident energy. 



In the case of point-to-point radio communications we are interested 

 in the total attenuation of the scattering energy caused by losses resulting 

 from both the true absorption and the scattering. 



7.3. Attenuation by Atmospheric Gases 



The major atmospheric gases that need to be considered as absorbers 

 in the frequency range of 100 to 50,000 Mc/s are water vapor and oxygen. 

 For these frequencies the gaseous absorption arises principally in the 

 1.35 cm line (22,235 Mc/s) of water vapor and the series of lines centered 

 around 0.5 cm (60,000 Mc/s) of oxygen [4]. The variations of these 

 absorptions with pressure, frequency, temperature, and humidity are 

 described by the Van Vleck [4, 5] theory of absorption. The frequency 

 dependence of these absorptions is shown in figure 7.1 [4]. 



In connection with figure 7.1, the water vapor absorption values have 

 been adjusted to correspond to the mean absolute humidity, p, (grams of 

 water vapor per cubic meter) for Washington, D.C., 7.75 g/m^ The 

 reason for this adjustment is that water vapor absorption is directly pro- 

 portional to the absolute humidity [6] and thus variations in signal in- 

 tensity due to water vapor absorption may be specified directly in terms 

 of the variations in the absolute humidity of the atmosphere. 



It can be seen from figure 7.1 that the water vapor absorption exceeds 

 the oxygen absorption in the frequency range 13,000 to 32,000 Mc/s, 

 indicating that in this frequency range, the total absorption will be the 

 most sensitive to changes in the water vapor content of the air, while 

 outside this frequency range the absorption will be more sensitive to 

 changes in oxygen density. Only around the resonant frequency corres- 

 ponding to X = 1.35 cm is the water vapor absorption greater than the 

 oxygen absorption. The absorption equations and the conditions under 

 which they are applicable have been discussed by Van Vleck [4], and the 

 best values to use for this section of the report have been taken from 

 Bean and Abbott [3]. 



The Van Vleck theory describes these absorptions from 100 Mc/s to 

 50,000 Mc/s in the following manner. The oxygen absorption at 



