256 BELL SYSTEM TECHNICA L JOURNAL 



ofifset section as shown in Fig. 20. We can comment on these difficulties as 



follows : 



1. The presence of the feed in the {)ath of the reflected energy causes a 

 region of low intensity or 'shadow' in the wave front. The effect of 

 this shadow on the antenna pattern depends on the size and shape of 

 the feed and on the characteristics of the portion of the wave front 

 where it is located. Its effect is to subtract from the undisturbed 

 pattern a 'shadow pattern' component which is broad in angle. This 

 decreases the gain and increases the minor lobes as indicated in Fig. 21.^ 



\ V-FEED 



Fig. 20 — Offset Parabolic Section. 



2. Return of reflected energy into the feed introduces a standing wave 

 of impedance mismatch in the feed line which is constant in amplitude 

 but varies rapidly in phase as the frequency is varied. A mismatch at 

 the feed which cancels the standing wave at one frequency will add to 

 it at another frequency. A mismatch which will compensate over a 

 band can be introduced by placing a raised plate of proper dimensions 

 at the vertex of the reflector as indicated in Fig. 22, but such a jilate 

 produces a harmful effect on the pattern. In an antenna which must 

 operate over a broad band it is consequently usually better to match 



' Figures 21, 22, and 23 arc taken from V. C. Cutler, loc. cit. 



J 



