270 



BELL SYSTEM TECHNICAL JOURNAL 



displacement. Quantitative arguments show that less severe tolerances 

 apply to all major structural dimensions of a metal lens antenna. 



It is true of course that the dimensions of individual portions of the metal 

 lens must be held with some accuracy. The metal plate spacing determines 

 the eflfective index of refraction of the lens material. Where A^ = 0.5 it is 

 customary to require that this be held to ±X/75, and where A' = 0.6 to 

 ±X/50. The thickness of the lens in a given region is less critical, and must 



be held to ± ttt., T7\ where it is desired to hold the phase front to ±X/16. 



10 (1 — A') 



Fig. 32 illustrates clearly the drastic way in which the location of a lens 

 can be altered without seriously afifecting the pattern. It shows, inci- 

 dentally, how a lens may behave well when used as the focussing element 

 in a moving feed scanning antenna. 



Fig. 32 — Effect on Pattern of Lens Tilting. 



9.3 Advantages of Metal Plate Lenses 



On the basis of the above discussion we can see that metal plate lenses 

 have certain considerable advantages. The most important of these is 

 perhaps found in the practical matter of tolerances. It is a comparatively 

 simple matter to hold dimensions of small objects to close tolerances but 

 quite another thing to hold dimensions of large objects closely under the 

 conditions of modern warfare. This advantage emerges with increasing 

 importance as the wavelength is reduced. 



Metal plate lenses have contributed a great degree of flexibility to radar 

 antenna art. When they are used two surfaces rather than one may be 

 controlled, and the dielectric constant can be varied within wide limits. 

 Independent control in the two polarizations may be applied. We can con- 

 fidently expect that they will become increasingly popular in the radar field. 



10. Cosecant Antennas 

 One of the earliest uses of radar was for early warning against aircraft. 



i 



