RA DA R A NTENNA S 259 



From the foregoing it is evident that a feed for a paraboloid is in itself a 

 small relatively non-directive antenna. Its directivity is somewhat less 

 than that obtained with an ordinary short wave array. It is therefore not 

 surprising that dipole arrays are sometimes used in practice to feed 

 paraboloids. 



A simple dipole or half wave doublet can in itself be used to feed a parabo- 

 loid, but it is inefficient because of its inadequate directivity. It is prefer- 

 able and more common to use an array in which only one doublet is excited 

 directly and which contains a reflector system consisting of another doublet 

 ov a reflecting surface which is excited parasitically. 



Dipole feeds although useful in practice have poor polarization charac- 

 teristics and although natural when a coaxial antenna feed line is used are 

 less convenient when the feed line is a waveguide. Since waveguides are 

 more common in the microwave radar bands it is to be expected that wave- 

 guide feeds would be preferred in the majority of paraboloidal antennas. 



The most easily constructed waveguide feed is simply an open ended 

 waveguide. It is easy to permit a standard round or rectangular waveguide 

 transmitting the dominant mode to radiate out into space toward the parabo- 

 loid. It will do this naturally with desirable phase, polarization and ampli- 

 tude characteristics. It is purely coincidental, however, when this results 

 in optimum amplitude characteristics. It is usually necessary to obtain 

 these by tapering the feed line to form a waveguide aperture of the required 

 size and shape. The aperture required may be smaller than a standard 

 waveguide cross section so that its directivity will be less. In this case it 

 may be necessary to 'load' it with dielectric material so that the power can 

 be transmitted. It may be greater, in which case it is sometimes called an 

 'electromagnetic horn'. It may be greater in one dimension and less in the 

 other, as when a paraboloidal section of large aspect ratio is to be illuminated. 



If a single open ended waveguide or electromagnetic horn is used to feed 

 a section of the paraboloid which includes the vertex, the waveguide feed 

 line must partially block the reflected wave in order to be connected to the 

 feed. To avoid this difficulty several rear waveguide feeds have been used. 

 In this type of feed the waveguide passes through the vertex of the parabo- 

 loid and serves to support the feed at the focus. The energy can be caused 

 to radiate back towards the reflector in any one of several ways, some of 

 which involve reflecting rings or plates or parasitically excited doublets. 

 The 'Cutler' feed is perhaps the most successful and common rear feed. It 

 operates by radiating the energy back towards the paraboloid through two 

 apertures located and excited as shown in Fig. 23. 



* C. C. Cutler, Loc. Cit. 



