128 



SITING AND COVERAGE OF GROUND RADARS 



Figure 20. Cornu spiral. 



power per unit area is proportional to i? 2 . Let W 

 denote peak power per unit area at the point P for 

 a certain arbitrary value of R. Then 



W = K ■ R 2 , (36) 



where K is a certain constant. When the whole wave 

 is acting, the integration limits extend from v = 

 - co to » = + co, that is, along the full length of 

 the Cornu spiral. The coordinate difference between 

 the foci of the spiral being (1,1) (see Figure 20) it 

 follows that their distance is R = y/2, so that the 

 corresponding peak power per unit area Wo is, by 

 equation (36), Wo = 2K which defines K as YiWo- 

 Hence it follows that equation (36) may also be 

 written as 



W 



= - 7? 2 

 2 K 



(37) 



shown a diffracting edge at M . At P the upper 

 half of the wave is effective, and on Figure 22 the 

 amplitude is AZ of length l/\/'2. The square of this 

 is one-half, which by equation (37) is multiplied by 

 Yi to get % for the power intensity at the edge of 

 the shadow. The electric field intensity is %. 



Consider next a point such as P' at a distance x 

 above P (see Figure 23). To be specific, the point 



SOURCE 



1548 Straight Edge Diffraction 



Using Cornu's spiral the diffraction pattern due to 

 a straight edge may be obtained. In Figure 23 is Figure 21. Division of wavefront into half-period zones. 



