272 



ATTENUATION OF RADIO WAVES 



T = 293 °K and standard atmospheric pressure in decibels per kilometer, 

 7i, is given by the expression: 



Ti 



(7.1) 



where X is the wavelength for which the absorption is to be determined 

 and where Ai'i and Ai'2 are line-width factors with dimensions of em~^ 

 This formula is based on the approximations of collision broadening theory. 

 This theory postulates that, although the electromagnetic energy is 

 freely exchanged between the incident field and the molecules, some of 

 the electromagnetic energy is converted into thermal energy during 

 molecular collisions and thus a part of the incident electromagnetic energy 

 is absorbed. The term in (7.1 )involving Aj'i gives the nonresonant 

 absorption arising from the zero freciuency line of oxygen molecules while 

 the terms involving Az^2 describe the effects of the several natural resonant 

 absorptions of the oxygen molecule which are in the vicinity of 0.5 cm 

 wavelength. The (2 ± 1/X) (cm~^) terms are the portion of the shape 

 factors that describe the decay of the absorption at frequencies away from 

 the resonant frequency (the number 2 is the reciprocal of the centroid 

 resonant wavelength 0.5 cm). 



The water vapor absorption at 293 °K arising from the 1.35 cm line, 

 72, is given by: 



72 



P 



3.5 X 10" 



X2 



AV3 



AX 1.35/ 



+ AV3 



+ 



Avs 



\X ^ 1.35/ 



+ AV3 



(7.2) 



where p is the absolute humidily and Afg is the line width factor of the 

 1.35 cm water vapor absorption line. The additional absorption arising 

 from absorption bands above the 1.35 cm line, 73, is described by: 



73 

 P 



0.05 Av4 



X2 



(7.3) 



