METALLIC DELAY LENSES 



65 



a low directivity teed at the focal point. This produced an almost uniform 

 illumination across the aperture at all three wavelengths so that the side 

 lobes were not too well suppressed. However, the deep minima in all three 

 directional patterns indicate the relative absence of curvature of the emerg- 

 ing phase front; this signifies that the strip dielectric has a negligible varia- 

 tion of refractive index over the indicated wavelength band. 

 (d) Sprayed Sheet Lenses 



The disk lens or strip lens can be constructed in the manner indicated in 

 Fig. 10, which shows two lenses made by spraying conducting paint on thin 

 dielectric sheets through masks. This results in round dot or square dot 



METAL STRIPS CUT 

 TO PROPER LENGTH 



SLOT 



FOAM 

 SLABS 



CIRC 

 OUT 



Fig. 6 — Construction of a delay lens employing metallic strips as the delay elements. 



patterns on the sheets and the size circle used on each sheet determines the 

 three-dimensional contour when the sheets are stacked up to form the lens. 

 Those in the photograph are spaced by wooden spacers as shown in the sketch 

 of Fig. 11; for large lenses it would probably be preferable to cement the 

 sheets to polystyrene foam spacers, thereby making a solid foam lens. 

 Because of the small size of the elements, the lens on the left in Fig. 10, was 

 effective at wavelengths as short as 1.25 cm, in addition to longer wave- 

 lengths (3, 7 and 10 cm). The lens of Fig. 12 was made by metal spraying 

 metallic tin directly on circular foam slabs through masks, and the foam 

 disks then cemented together. 



Strip lenses can also be constructed in this way. The lens in Fig. 13, 



