DIFFRACTION BY COASTS 



181 



lower angles, the di\'ergence factor D must be taken 

 into account (see Chapter 5). This is done by com- 

 puting D for the angles desired and replacing z by zD 

 in etiuation (21). 



8.4.5 



Cliff Site 



If the radar is sited on a cliff and if the land inter- 

 vening between the radar and the reflecting plane 

 (ocean) is rough, the equations of Section 8.4.3 

 apply. We shall now consider the case where the 

 radar is sited at some distance from the cliff edge 

 and where the ground between the radar site and the 

 cliff edge is reflecting. There are then two reflecting 

 planes, the lower of which might be the ocean, or 



s' s" 



OPAQUE SCREENS 



Figure 15. Diffraction from a cliff site. 



might be a reflecting land surface. In Figure 15, 

 this surface has been designated as ocean. The upper 

 plane is at a height H above the lower plane and the 

 transmitter at a height hi above the upper plane. 

 Assume that the azimuth chosen is perpendicular 

 to the direction of the cliff edge; the distance of the 



radar to the cliff edge is do. For any other azimuth 

 (angle y in Figure 13), replace do by do/cos y in the 

 following equations. 



Two images are shown in Figure 15 and two 

 fictitious opaque screens, one corresponding to each 

 image. The corresponding variables are distinguished 

 by single and double primes. The lobe numbers are 

 given by 



n = 



X 



^^, ^ 4(j? + h)^l^ ^ n"{H + fei) 

 \ hi 



The critical angle, i/'o = hi/do, is the same for both 

 image transmitters. Thus 



no" =-— 



ih^ 

 Xdo 



no' = 



4iH + hi)hi no"{H + hi) 



\do 



hi 



Further 



V2 



— V 



1 



«0 



1 + H/hi 



Again, the field is given by 



E = Foil - z'e- 



— z e 



II -j-nn '-Jt,' 



(23) 



The expression for the field strength [equation (23)] 

 is in a form where all the quantities involved may be 

 evaluated for any given height of transmitter, height 

 of the cliff, and any wavelength by using graphs and 

 tables given in earlier paragraphs. 



