■66 



CALCULATION OF RADIO GAIN 



4. Ultra-short leaves in the diffraction region. 

 Dielectric earth. For X < 10 meters (/ > 30 mc) and 

 for either polarization, land acts as a dielectric earth 

 or absorbing earth in contradistinction to a conducting 

 earth. Propagation over a dielectric earth is practi- 



10 20 30 ' 40 50 SO 70 



d IN KILOMETERS — 



SO 90 100 



Figure 5. Field .strength ratio versu.s distance for 

 vertical polarization over dry soil for h, = 100 meters 

 and /(•> = 0. 



2 



I 

 0.5 



OJ 

 OX) 5 



I 0.01 

 0.005 



0.001 



o.ooos 



30 40 50 60 70 



_^ d IN KILOMETERS 



60 90 100 



Figure 6. Field strength ratio versus distance for 

 vertical polarization and heights h\ = h^ = 100 meters. 



cally independent of earth constants. For a given 

 type of polarization, the chief variables affecting 

 gain are then the heights of the antennas, their 

 distance apart, and the wavelength. Within the 

 diffraction region, the effect of increasing wavelength 

 is to increase the field strength. This is illustrated 

 by the curves in Figures 5 and 6. The dielectric earth 

 is characterized by a value of 5 > > 1 . S is given 

 by equation (193). 



While .sea water has a relati\'el.y high conductivity, 

 radio wave propagation over it is the same as that 

 over a dielectric earth in the case of horizontal polar- 



ization for X < 10 meters, and in the case of vertical 

 polarization for X < 1 meter. Consequently, verti- 

 cally polarized radiation of wavelength range 1 to 10 

 meters over sea water is given special treatment in 

 Section 5.7.4. 



In the same range, 1 to 10 meters, for vertically 

 polarized radiation and for distances less than those 

 given in Table 3, propagation conditions over land 

 also deviate slightly from those corresponding to a 

 dielectric earth. 



5. Optical region. In the optical-interference 

 region, the lobes for the shorter waves are more 

 numerous, narrower, and lower, as can be seen from 

 the oscillatorj^ part of the field strength versus 

 distance curves of Figure 6. 



The dependence of reflection coefficients upon 

 polarization, wavelength, and ground constants is 

 discussed in Section 4.2. 



6. Horizontal versus verticcd polarizcdion. In the 

 optical region, for rays at small grazing angles, there 

 is not much difference between the two types of 

 polarization. For larger grazing angles, the diffei'- 

 ence is more marl^ed (see Section 4.2 on reflection 

 coefficients, and see Section 5.2.4). 



-SHADOW ZONE 



ELEVATED ANTENNA 

 20 L0G(g2A2)+B 



(APPLIES ONLY WELL 

 BELOW LINE OF SIGHT) 



OPTICAL REGION - 



\- iOX ' 

 B = RADIO GAIN AT h,^ 

 FOR /see TABLE 4 



v^ 



ho IN METERS 



Figure 7. Gain versus height at distances beyond the 

 radio horizon. 



It has been pointed out in the previous paragraph 

 that ■\\'ithin the diffraction region for X < 10 meters 



