BELOW THE INTERFERENCE REGION 



123 



If the antennas are elevated, F^ can still be used 

 for the first mode for great distances where the first 

 mode gives most of the field. 



5.7.7 



Sample Calculation 

 for Very Dry Soil 



The general solution given in Section 5.7.6 is here 

 illustrated for the case of doublet antennas, either 



function of distance (/ for doublets at zero height. 

 Figure 60 gives the first mode height-gain factors for 

 transmitter and receiver heights, h and Ih, respec- 

 tively. 



To obtam the radio gain under different conditions 

 it is merely necessary to add the decibel gains of the 

 transmitter and receiver antennas, the radio gain for 

 zero height (Figure 59), the height-gain factor 

 (Figure 60) for the transmitter, and a similar figure 



10,000 100,000 



DISTANCE d IN METERS 



10,000,000 



Figure 59. Free-space radio gain Aa ( ) and radio gain .1. in decibels, for propagation over very dry soil with 



doublet antennas on the ground ( for horizontal polarization and for vertical polarization). Numbers on the 



curves give the wavelength X. NMe: Radio gain is independent of the radiated power. 



hoiizontally or vertically polarized, placed at various 

 heights over an earth assumed to be very dry soil 

 for which the constants are e, = 4, and a = 0.001 

 mho/meter. The following graphs cover, in decimal 

 steps, the frequency range of 30,000 to 0.03 mc or 

 wavelengths X = 0.01 to 10,000 metei-s. 



Figure 59 gives the free space radio gain Ao and 

 the radio gain A decibels over very dry soil, as a 



for the receiver (Figure 60). This process, howe\'er, is 

 subject to the restriction mentioned in the next 

 paragraphs. 



The addition of the factors given in the preceding 

 paragraph is valid all the way up to the maximum of 

 the first lobe, where the field is given by the sum of the 

 direct and reflected ra3^s, provided that the antennas 

 have comparable heights. 



