RADIO PROPAGATION 



47 



its image are substantially the same distance from the receiver so that 



6V5VP 



E 



d 



(5) 



The way in which the ground currents affect the antenna resistance 

 is given by the following equations which follow directly from more 

 general cases considered by Sterba.^ 



Rv 

 Ro 



Rh 

 Ro 



10 "^280 



V- 



280 



+ 



(6) 

 (7) 



where v is equal to 47r times the height of the antenna above the 

 ground in wave-lengths, and Rv and Rh are the radiation resistances of 



o 



1.2 



1.0 



< 



< 



a. 0.8 



^0.6 

 < 



^0.4 



0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 I.I 1.2 1.3 1.4 1.5 1.6 

 ANTENNA HEIGHT IN WAVELENGTHS (^/j^ 



Fig. 1 — Ratio of the radiation resistance of a short doublet antenna above perfectly 

 conducting ground to that of the same antenna in free space. 



short vertical and horizontal doublets above perfectly conducting 

 earth respectively, and Ra is the radiation resistance of the same 

 antenna in free space. For the same input power the received field is 

 inversely proportional to the square root of these ratios which are 

 plotted in Fig. 1. 



It is sometimes convenient to express the results in terms of the 

 ratio of transmitted power to useful received power. The useful re- 



