pA = 
CALCULATION OF RADIO GAIN 
TRANSMITTER LOW 
381 
OPTICAL-INTERFERENCE REGION 
LINE OF SicHT 
TRANSITION 
Vn 
WNIN eo 
SN Veni wv 
TRANSITION LINE OF SIGHT 
Ficure 3. Field regions as related to transmitter heights for horizontal polarization or ultra-short waves. Low antenna 
tegion = Ad > 2hih2; hi, he < 30A2/3. 
150 
al 
EIN 08 ABOVE 14 vVAMETER 
POLARIZAT 10) 
ISOTROPIC RADIA 
d IN KILOMETERS 
® OPTICAL FORMULA 
IX DIFFRACTION FORMULA 
E IN DB ABOVE 4 KLV/METER 
Ficure 4, Variation of field strength with distance for propagation on vertical polarization with a wavelength of 70 cm 
over dry soil. The point ‘‘due to minimum” results from minimum in reflection coefficient at the pseudo-Brewster angle. 
At the ground, f(h) = 1, so that if both antennas 
are close to, the ground, the distance dependence is 
given by F'(d) only. 
3. One or both antennas elevated; h > h,= 30d". 
For elevated antennas, h > h, and the height-gain 
functions of f(h) vary with the modes. Conse- 
quently, it is not possible to separate the height and 
distance effects as in the previous paragraph. 
In the optical-interference region, it is more ad- 
vantageous to_use the method of combining the 
direct and reflected waves. This is equivalent to 
the rigorous solution which is illustrated by the dots 
in Figure 4. 
Simple graphical aids can be given for points well 
within the diffraction region where the first mode 
predominates. The ranye of usefulness of the first 
mode can be extended by plotting the field strength 
given by the first mode as a function of height (or 
distance) and plotting a similar curve by using the 
ray method as far as the lowest (or first) maximum 
(see Figure 7). Then by joining these partial curves 
into a smooth overall curve, a fairly good value of 
