258 BELL S YSTEM TECH NIC A L JOURNA L 



the feed to space and accept the residual standing wave, or if this is 

 too great to use an offset section of the parabolic surface. 



8.4 Feeds for Paraboloids 



We have seen that an antenna with good wave front characteristics and 

 consequently with a good beam and pattern can be constructed by illu- 

 minating a reflecting paraboloid with a properly designed feed placed at its 

 focus. In this section we will examine the characteristics which the feed 

 should have and some of the ways in which feeds are made in practice. 



A feed for a paraboloid should 



a. be appropriate to the transmission line with which it is fed. This is 

 sometimes a coaxial line but more commonly a waveguide. 



b. Provide an impedance match to this feed line. This match should 

 usually be obtained in the absence of the reflector but sometimes, for 

 narrow band antennas, with the reflector present. 



c. have a satisfactory phase characteristic. For a paraboloid the feed 

 should be, as far as phase is concerned, a true point source radiating 

 spherical waves. As discussed at the end of 8.1, if the wave front is 

 not accurately spherical, a compensating correction in the reflector can 

 be made. 



d. have a satisfactory amplitude characteristic. According to 8.2 this 

 means that the feed should have a major radiation lobe with its maxi- 

 mum striking the center of the reflector, its intensity decreasing 

 smoothly to a value about 8 to 10 db below the maximum in the direc- 

 tion of the reflector boundaries and remaining small for all directions 

 which do not strike the reflector. 



e. have a polarization characteristic which is such that the electric vec- 

 tors in the reflected wave front will all be polarized in the same di- 

 rection. 



f. not disturb seriously the radiation characteristics of the antenna as a 

 whole. The shadow efl'ect of the feed, the feed line and the necessary 

 mechanical supports must be small or absent . Primary radiation from 

 the feed which does not strike the reflector or reflected energy which 

 strikes the feed or associated structure and is then reradiated must be 

 far enough down or so controlled that the antenna pattern is as 

 required. 



In addition to the electrical requirements for a paraboloid feed it must of 

 course be so designed that all other engineering requirements are met, it 

 must be firmly suj^ported in the required position, must be connected to the 

 antenna feed line in a satisfactory manner, must sometimes be furnished with 

 an air tight or water tight seal, and so forth. 



