RADAR ANTENNAS 



267 



from the lens surfaces and impose severe restrictions on the accuracy of plate 

 spacings. The compromises that have been used in practice are N = 0.5 

 for which a = 0.577X and N = 0.6 for which a — 0.625X. 



Even with N' = 0.5 or 0.6 lenses become thick unless inconveniently lon<7 

 focal distances are used. Thick lenses are undesirable not only because they 

 occupy more space and are heavier but also because the plate spacing must 

 be held to a higher degree of accuracy if the phase correction is to be as 



± 0.4 



0.2 



0.75X 

 PLATE SPACING 



l.OOx 



Fig. 29 — Variation of Effective Index of Refraction with Plate Spacing in a Metal 

 Plate Lens. 



required. To get around these difficulties the technique of zoning is used. 

 Zoning makes use of the fact that if the phase of an electromagnetic vector 

 is increased or decreased by any number of complete cycles the effect of the 

 vector is unchanged. When applied to a metal plate lens antenna this 

 means simply that wherever the phase correction due to a portion of the 

 lens is greater than a wavelength this correction can be reduced by some 

 integral number of wavelengths such that the residual phase correction is 

 under one wavelength. If this is done it is evident that no portion of the 



