SITING AND COVERAGE OF GROUND RADARS 85 
Figure 22. Vector sum of subzone contributions. 
P’ is chosen in the direction of SM, of Figure 23, 
where M, is the upper edge of the first half-wave- 
length zone. The illumination at the point P’ is, 
firstly, due to all wavelets emanating from the half 
wavefront above P’S. In addition, there is the 
contribution from the lower half of the wavefront 
extending from M, to Mo. The situation is, in fact, 
the same as if P’ were brought down to P and the 
diffracting edge were lowered from My to M’ (see 
Figure 24). The resultant amplitude F is represented 
on Figure 25 by ZB. Starting at the point P at the 
edge of the shadow (Figure 23) where the amplitude 
is AZ, if the point is moved upward, the tail of the 
Ss 
SOURCE 
Ficure 23. Diffraction at shadow line. 
amplitude vector moves to the left along the spiral 
while its head is fixed at Z. 
The amplitude goes through a maximum at b’, a 
Ficure 24. Diffraction in illuminated region. 
Figure 25. Method of using the Cornu spiral. 
