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BELL SYSTEM TECHNICAL JOURNAL 



source will fail to strike the reflector. The effective area must also be re- 

 duced by the loss of this 'spill-over' energy. 



The maximum effective area for a parabolic antenna is obtained by design- 

 ing the primary feed to obtain the best compromise between loss due to 

 taper and loss due to spill-over. It has been shown theoretically that this 

 best compromise generally occurs when the amplitude taper across the 

 aperture is about 10 or 12 db and that in the neighborhood of the optimum 

 the efficiency is not too critically dependent on the taper. 



This theoretical result is well justified by experience and has been applied 

 to the majority of practical parabolic antennas. It applies both when the 

 reflector is paraboloidal so that taper in both directions must be considered 



: — PARABOLIC 

 CYLINDER 



LINE SOURCE 

 ANTENNA 



Fig. 19 — A Parabolic Cylinder with Line Feed. 



and when the reflector is a parabolic cylinder with only a single direction 

 of taper. It is a fortunate by-product of a 10 or 12 db taper that it is gen- 

 erally sufficient to produce satisfactory minor lobe suppression. 



8.3 Choice of Configuration 



We have shown how a simple beam can be obtained through the use of a 

 paraboloidal reflector with a point source or alternatively through the use 

 of a reflecting parabolic cylinder and a line source. The line source itself 

 can be ])roduced with the help of a parabolic cylinder bounded by parallel 

 conducting plates. We will now outline certain practical considerations. 

 These considerations may determine which of the two reflector types will be 



' C. C. Cutler, Parabolic Antenna Design for Microwaves, paper to be [published in Proc. 

 of the I. R. E. 



