BELOW THE INTERFERENCE REGION 



109 



Referring to equation (162) and Section 5.7.1, 



A = 2AoA,F,(HLgg'UHLgg'h, (184) 



■with ggf' = 1 for h < h^. 

 he is given by Figure 35, 

 20 log gg' is given by Figure 36, 

 20 log Ao is given by Figure 3 in Chapter 2, 

 F, is given by Figures 31, 32, and 33. 



2. Plane earth factor A i. This has been discussed in 

 Section 5.7.1. Ai is a function of p'd; p' is given in 

 Figure 44 and 20 log Ai in Figure 45. The curve 

 shift of Ai with X in the \THF band is less than 1 db. 

 When p'd > 50, Ai = 1/p'd, as in the dielectric 

 case. 



3. The low height-gain H^ is a function of Ih (see 

 Section 5.7.1). I is given by Figure 46, Hi^ or 20 log Hi 

 by Figure 47. H^ depends on the curve parameter 



Q = 6,/60o-X whicli for sea water is — . Since 



3X 



Hi,~^lh for Ih > 4, 20 log H^ can be found from 



Figure 34. 



4. hi, ho > 4/Z and d > 50/p'. As in Section 5.7.1 

 [noting especially equations (151) and (153)], 

 equation (184) reduces, when h > 4/Z and d > 50/p', 

 to 



A =Yp9g'hUgg'hh (185) 



Table 8. Values of 4/Z and 50/p' for various wavelengths. 



Equation (185) may be used generally, provided 

 (gg'h) is replaced by H^/l when h < 4/1, and the 

 right-hand member is multiplied by Aip'd or, in 

 decibels, 20 log ^i + 20 log p'd are added, when 

 d < 50/p'. In this formula d < 50/p' is given in 

 meters. 



5. Horizontal versus vertical polarization. It is of 

 interest to compare the gain of vertically and hori- 

 zontally polarized waves over sea water at VHF. 

 (It has been pointed out earlier that there is no 

 marked diiference in attenuation between horizon- 

 tally and vertically polarized waves for wavelengths 

 less than one meter.) Equation (184) is valid for 



horizontally polarized waves also by using the 

 appropriate F^ curve and putting g' = I and can be 

 made the basis of a comparison between vertical 

 and horizontal polarization. See, for comparison, 

 equation (IGO). 



.05 

 .04 



.01 



.005 

 .004 



.003 

 .002 



.0005 

 .0004 



.0003 

 .0002 



123456789 10 II 

 WAVELENGTH X METERS 



Figure 44. p' versus X for sea water, vertical polariza- 

 tion. 



'0 20 30 40 30 60 TO fiO 90 100 HO >20 |30 



p'd 



Figure 45. Plane earth factor Ai, for sea water, 

 vertical polarization. Note: All db values are negative. 



