614 THE bp:ll systkm technical journal, may 1957 



enough to block vision are roughly equivalent to a solid obstruction of 

 the same over-all size. 



IV. MEDIUM and LOAV FREQUENCY GROUND WAVE TRANSMISSION 



Wherever the antenna heights are small compared with the wave- 

 length, the received field intensity is ordinarily stronger with vertical 

 polarization than with horizontal and is stronger over sea water than 

 over poor soil. In these cases the "surface wave" term in (3) cannot be 

 neglected. This use of the term "surface wave" follows Norton's usage 

 and is not equivalent to the Sommerfeld or Zenneck "surface waves." 



The parameter A is the plane earth attenuation factor for antennas 

 at ground level. It depends upon the frequency, ground constants, and 

 type of polarization. It is never greater than unity and decreases with 

 increasing distance and frec^uency, as indicated by the following approxi- 

 mate equation : 24 , 25 



-1 



A ^^ — 



1 + .7 (sm d -\- z) 



where 



z = " '■ for vertical polarization 



Co 



z = \/eo — cos^ Q for horizontal polarization 

 f = e — J6O0-X 

 6 = angle between reflected ray and the ground 



= for antennas at ground level 

 e = dielectric constant of the ground relative to unity in free space 

 (X = conductivity of the ground in mhos per meter 

 X = wavelength in meters 



In terms of these same parameters the reflection coefficient of the 

 ground is given by^^ 



R = ^j^l^Ll (10) 



sin 6 -\- z 



When 6 <K \ z \ the reflection coefficient approaches — 1 ; when » | 2: | 



