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BELL SYSTEM TECHNICAL JOURNAL 



impressed E and H vectors. The supporting sheets are again polystyrene 

 foam. The foil disks have negligible thickness so that the magnetic lines 

 are unaffected as shown in Fig. 5. Equation (4) is therevore valid even at 

 radio frequencies and the index of refraction of this material is obtainable 



1 A 3 



from (4) and (2) with a equal to — ^ , as shown in the last section. 



o 



(c) Strip Array 



Both the sphere and disk type lenses have the advantage that they will 

 perform equally well on horizontally or vertically polarized waves. If the 

 lens is required to focus only one type of wave polarization the disks can be 

 replaced by thin, flat, conducting strips extending in length in the direction 



Fig. 5 — Disturbance of the magnetic field is avoided by the use of disks instead of 

 spheres. 



of the magnetic vector of the apphed field. A simple method of constructing 

 such a lens is shown in Fig. 6. Slabs of dielectric foam are slotted to a 

 depth equal to the strip width and each slab is marked with the profile 

 contour necessary to produce the final plano-convex lens. The metal 

 strips are then cut to the length indicated on the profile and inserted in the 

 slots. With the strips in place the unit slabs are stacked on top of one 

 another and held in a mounting frame to form the complete lens. Figure 7 

 shows one slab of a 10-foot strip type lens being assembled and Fig. 8 shows 

 a six-foot square lens half assembled. Figure 9 shows directional patterns 

 of a 3-foot diameter lens of this type made up of J inch x..005 inch strips 

 spaced \'^ in the slabs and the slabs \Y thick. The three patterns were 

 taken over a 12% band of frequencies with the lens purposely illuminated by 



