METALLIC DELAY LENSES 



75 



indicate, however, that for such large apertures, the amount of diffraction 

 (spreading of the waves outside of the pyramid formed by the dotted Hues) 

 will be small, and it is only necessary that the sides AAi, AiB and BC all be 

 conducting to insure good back lobe suppression and other desired properties 

 associated with a horn shield. • 

 (d) Performance 



The gain of this antenna over an isotropic radiator is plotted in Fig. 19. 

 The top curve is the theoretical gain of a uniform current sheet of the same 

 area (6' x 6'), the lower curve the gain of a 6' x 6' area having 60% effective 

 area. The points, which fall approximately on the lower curve, are the 

 experimentally measured gain values of this antenna at the frequencies 



39 



o 



IJ38 

 o 



34 



3920 



4020 4120 4220 



FREQUENCY IN MEGACYCLES PER SECOND 



4320 



Fig. 19 — Measured gain characteristics of the six-foot square shielded lens of Fig. 16. 

 The lower line indicates 60% effective area and the circles are experimental points. 



indicated. The constant percentage effective area indicates that the index 

 of refraction of the lens material remains quite constant over the indicated 

 frequency band. In contrast to this, the 10' x 10' metal plate lenses^ 

 {n < 1) exhibit, at the band edges, a falling off of IJ decibels from midband 

 gain for a 10% wavelength band. 



The magnetic plane pattern of the lens when fed by a 6-inch square feed 

 horn is shown in Fig. 20. The remarkable symmetry of the minor lobes in 

 Fig. 20 and also in Fig. 9 shows that the phase fronts of the waves radiated 

 by these antennas are very accurately flat. This result emphasizes the 

 tolerance advantages of the lens over the reflector. It is believed that the 



^ Loc. cit. 



