RADAR ANTENNAS 269 



Zoning is not without disadvantages. One disadvantage is obviously 

 that a zoned lens which is designed for one frequency will not necessarily 

 work well at other frequencies. It is in principle possible to design a broad 

 band zoned metal plate lens corresponding to the color compensated lenses 

 used in good cameras. So far, however, this has not been necessary since 

 band characteristics of simple lenses have been adequate. 



Another difficulty that zoning introduces is due to the boundary regions 

 between the zones. The wave front in this region is influenced partly by 

 one zone and partly by the other and may as a result have undesirable phase 

 and amplitude characteristics. This becomes serious only if especially short 

 focal distances are used. 



9.1 Lens Antenna Configuratio7is 



Any of the configurations which are possible with parabolic reflectors have 

 their analogues when metal plate lenses are used. Circular lenses illumi- 

 nated by point sources and cylindrical lenses illuminated by line sources are 

 not only theoretically possible but have been built and used. Since a lens 

 has two surfaces there is actually somewhat more freedom in lens design 

 than in reflector design. Metal Plate Lenses have usually been designed 

 with one surface flat, but the possibility of controlling both surfaces is 

 emerging as a useful design factor where special requirements must be met. 



Feeds for lenses should fulfill most of the same requirements as feeds for 

 reflectors. We find a difference in size in lens feeds in that they must gen- 

 erally be more directive because of greater ratios of focal length to aperture. 

 A difference in kind occurs because the feed is located behind the lens where 

 none of the focussed energy can enter the feed or be disturbed by it. As a 

 result some matching and pattern problems which arise in parabolic antennas 

 are automatically absent when lenses are used. 



In choosing a design for a lens antenna system with a given aperture one 

 must compromise between the large size which is necessary when a long focal 

 length is used and the more zones which result if the focal length is made 

 short. Most metal plate lenses so far constructed have had focal lengths 

 somewhere between 0.5 and 1.0 times the greatest aperture dimension. 



9.2 Tolerances in Metal Plate Lenses 



It is not difficult to see that phase errors resulting from small displace- 

 ments or distortions of a metal plate lens are much less serious than those 

 due to comparable distortions of a reflector surface. This follows from the 

 fact that the lens operates on a wave which passes through it. If a portion 

 of the lens is displaced slightly in the direction of propagation it is still 

 operating on roughly the same portion of the wave front and gives it the 

 same phase correction. If a portion of a reflector were displaced in the 

 same way the error in the wave front would be of the order of twice the 



