596 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1957 



where external noise is generally controlling and where antenna dimen- 

 sions and heights are comparable to or less than a wavelength. The free 

 space field intensity is independent of frecjuency and its magnitude for 

 one kilowatt radiated from a half -wave dipole is shown on the left hand 

 scale on Fig. 1. 



The concept of free space transmission assumes that the atmosphere is 

 perfectly uniform and nonabsorbing and that the earth is either infinitely 

 far away or its reflection cbeflacient is negligible. In practice, the modify- 

 ing effects of the earth, the atmosphere and the ionosphere need to be 

 considered. Both theoretical and experimental values for these effects 

 are described in the following sections. 



II. TRANSMISSION WITHIN LINE OF SIGHT 



The presence of the ground modifies the generation and the propaga- 

 tion of radio waves so that the received power or field intensity is or- 

 dinarily less than would be expected in free space.^ The effect of plane 

 earth on the propagation of radio waves is given by 



Induction Field 

 and Secondary 

 Direct Reflected "Surface Effects of the 

 Wave Wave Wave" Ground 



E_ 



.^'^ \ /^ D^ ^-1 J^ 



1 + Re'^ + (1 - R)Ae'^ + • • • (3) 



where 



R = reflection coefficient of the ground 

 A = "surface wave" attenuation factor 

 4Thih-2 



A 



\d 



hi,2 = antenna heights measured in same units as the wavelength 

 and distance 



The parameters R and A vary with both polarization and the electrical 

 constants of the ground. In addition, the term "surface wave" has led 

 to considerable confusion since it has been used in the literature to 

 stand for entirely different concepts. These factors are discussed more 

 completely in Section IV. However, the important point to note in this 

 section is that considerable simplification is possible in most practical 

 cases, and that the variations with polarization and ground constants 



