132 



SITING AND COVERAGE OF GROUND RADARS 



Figure 31. The Cornu spiral applied to obstacles and slits. 



and the relative power intensity is 0.092. This same 

 result may be computed from the table of Fresnel 

 integrals by obtaining the values of AC and AS for 

 v = 0.9 and v = 1.4. The sum of the squares of AC 

 and AS is R 2 . Typical patterns for slits of several 

 widths are shown in Figure 32. It will be noted that 

 there is little radiation outside the slit. 



i54.il Diff ract j on Dy a N arrow Obstacle 



The effect of a narrow object with parallel sides 

 may be determined with the Cornu spiral. In the 

 case of the slit only a fixed length slid along the 

 spiral is effective, the remainder being shielded by 

 the edges of the slit. With an obstacle, however, a 

 fixed length slid along the spiral represents the 

 ineffective portion. If the obstacle is of such size 

 that it covers an interval Av = 0.5 on the spiral, 

 Figure 31, the segment Av may be located as JK. 

 The radiation at the point considered will be due 



Av-1.5 



-5 +5 



Av=4.6 



Av: 2.5. 



•4 +4 



A v = 3.9 



-3 +3 



Av=l2 



-3 +3 



Av = 5.2 



2 yw\ 



-3 +3 



Av=6.2 



T 1 ^ 



-3 +3 



■ « 1 i 



-6 -3 +3 +6 



Figure 32. Diffraction patterns of slits. 



