BELOW THE INTERFERENCE REGION 



95 



If hi is small compared with h^ and hi', the result is 

 approximately 



\vh 



ere 



H-^) 



(168) 



h'>. 



KA- 



Recei\-ers situated between h^ and /)/,' were formerh' 

 below the line of sight but are now above it (assum- 

 ing k > 4/3), mth a consequent substantial gain in 

 signal strength at some points and loss at others. 

 The chances of detection in the region, howevei-, have 

 improved. 



// both antennas are low, a change of 4ff/3 to ka 

 gives a field strength change such that the field 



100 



50 



d' = d 



'&"' 



m" 



where k may now have any value, 

 change to A' where 



\2/3 



A'=A (ly 



(169) 



(170) 

 A itself will 



(171) 



Figure 30 illustrates the values of (3fc/4)" for various 

 values of k and n used in equations (169), (170), and 



10 



, 4 I 



0.5 



0.' 



0.1 



0.5 I 



50 100 



500 1000 



5000 10,000 



Figure 30. Values of (3 fc/-i)". Nota: Values shown between 0.1 and 0.5 of the ordinate scale are minus values. 



which was formerly at a point d well \vithin the 

 diffraction region will now be found at a distance of 

 approximately 



2/3 

 d\ " ' 



<!T 



except for the decrease in free-space gain (20/3) log 

 3fc/4, occasioned by the increase in distance. If, for 

 instance, fc = 12, the free-space gain is equal 

 approximately to —7 db, and the ratio of the new 

 distance to the old is equal to (3fc/4)-'^ ^ ^~''^ ^ 4. 

 More generally, if A/Ao is known at a point 

 (/i2,rf) for a transmitter height li\ and for fc = 4/3, 

 the same value of A/Ao will be found at h^ , hn', d 



(171). Figure 43 gives the same information in 

 nomographic form. 



Hence if a coverage diagram is known for k = 4/3, 

 then the same diagram can be used for k ^ 4/3 if the 

 diagram is interpreted in terms of /),', d', and A'. 



.s.7.3 Graphs for the Case 



of the Dielectric Earth (8>>1) 



1. Fundamental formula for gain factor. This formula 

 is (see Sections 5.1.7 and 5.7.1) 



2 d- 



(172) 



wliere g = 1 when h < 30X' 



2/3 



