RADAR ANTENNAS 



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parabolic cylinder and the point source S generates a line source at the focal 

 line of the parabolic cylinder (Fig. 19). The energy spreading cylindrically 

 from the line source is focussed by the parabolic cylinder in such a way that 

 a Uniphase wave front over a plane area is again produced. Parabolic 

 cylinders and paraboloids are both used commonly in radar antenna practice. 

 In the discussion so far it has been assumed that the primary source is 

 effectively a point source and that the reflector is exactly parabolic. If the 

 primary source is not effectively a point source, in other words if it produces 

 waves which are not purely spherical, then the reflector must be distorted 

 from the parabolic shape if it is to produce perfect phase correction. When 



Fig. 18 — -Parabola. 



this occurs the correct reflector shape is sometimes specified on the basis of 

 an experimental determination of phase. 



8.2 Control of Amplitude 



When a primary source is used to illuminate a parabolic reflector there 

 are two factors which affect the amplitude of the resulting wave front. One 

 of these is of course the amplitude pattern of the primary source. The other 

 is the geometrical or space attenuation factor which is different for different 

 parts of the wave front. In most practical antennas each of these factors 

 tends to taper the amplitude so that it is less at the edges of the antenna 

 than it is in the central region. The effective area of the antenna is reduced 

 by this taper. 



In any finite parabolic antenna some of the energy radiated by the primary 



