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BELL SYSTEM TECHNICAL JOUMAL 



tilt in one plane and a quarter wave offset in the other plane. To utilize 

 most efficiently the space afforded the antenna on the top of the relay tower, 

 the lens aperture was made square. Since an unstepped lens has, by its 

 nature, a circular aperture, the four corners of the horn aperture must be 

 filled in with lens material as sketched in Fig. 18. The step height is 

 designed for midband wavelength and in the present case follows the equa- 

 tion of Fig. 17 with the focal length reduced from the value used for the 

 main lens section by K\/{n — 1) where K is an integer. That integer K is 



CURVES FOR 

 VARIOUS INTEGERS, K 



Fig. 18— Filling in the four comers of a square horn aperture with lens material. 



selected which brings the step profile nearest the horn corners as indicated 



in Fig. 18. 



(c) Horn Shield 



Lenses can be energized by means of a small feed horn placed at the focal 

 point, but to obtain the best directional properties a full metallic shield 

 extending from the wave guide feed up to the sides of the lens should be 

 employed. The use of foam slabs in the construction of the delay lens 

 prevents this shield from extending completely up to the lens as indicated 

 by the extension of A-Ay (shown dotted) in Fig. 16. Optical formulas 



