RADAR AN TENNA S liT 



where Z is a loss factor (< 1). At a point Q on the Z axis the electric inten- 

 sity is obtained by adding the effects of all the Huygens sources in S. If 

 OQ is as great as in the above derivation for the gain of an ideal antenna then 

 we see from 17 that the electric intensity at Q is 



£x = i ^^^- £o I a{x, y)e"^^'-'US; Ey = 0; E, = 0. (26) 



Ad J 



The power flow per unit area at Q is given by 



^^T^rl^-I' (27) 



and Po the power flow per unit area at Q when Pt is radiated isotropically 

 is given by equation (3). 



The power gain of the antenna, by definition 1 is therefore 



Po 1207r / 47rrf2 x2 



f a{x, y)6'*^- 



dS 



/ a(x, y) 

 Js 



(28) 



dS 



The gain expressed in db is given by 



Gdb = 10 log.o G (29) 



We combine 12 and 28 to obtain 



A = L 



I a{x,y)e'*^'''''dS 



(30) 



/ a^{x, y) dS 



a formula for the effective area of the antenna. 



3.4 The Significance of the Pattern of a Radar A ntenna 



The accuracy with which a radar can determine the directions to a target 

 depends upon the beam widths of the radar antenna. The ability of the 

 radar to separate a target from its background or distinguish it from other 

 targets depends upon the beam widths and the minor lobes of the radar 

 antenna. The efficiency with which the radar uses the available power to 

 view a given region of space depends on the beam shape of the antenna. 

 These quantities characterize the antenna pattern. In the following sec- 

 tions means for the calculation of antenna patterns in terms of wave front 

 theory will be developed, and some illustrations will be given. 



