266 BELL S YSTEM TECH NIC A L JOVRNA L 



spacers, if small enough in cross section, do not disturb things unduly. 

 The velocity of the TEM mode is, on the other hand, almost independent of 

 the plate spacing. This mode is, however, more likely to cause trouble by 

 leaks through joints and cracks in the plates. 



9. Metal Plate Lenses 



At visible wavelengths lenses have, in the past, been far more common 

 than in the microwave region, due chiefly to the absence of satisfactory lens 

 materials. A solid lens of glass or plastic with a diameter of several feet is a 

 massive and unwieldy object. By zoning, which will be discussed below, 

 these difticulties can be reduced but they still remain. 



A new lens technique, particularly effective in the microwave region was 

 developed by the Bell Laboratories during the war.^ It is evident that any 

 material in which the phase velocity is different from that of free space can 

 be used to make a phase correcting lens. The material which is used in this 

 new technique is essentially a stack of equally spaced metal plates parallel 

 to the electric vector of the wave front and to the direction of propagation. 

 Lenses made from this material are called 'Metal Plate Lenses'. 



When the spacing between neighboring plates is between X/2 and X only 

 one mode with electric vector parallel to the plates can be transmitted. 

 This is the TEoi mode for which the phase velocity is given in Sec. 8.5. 

 When the medium between the plates is air this equation can be converted 

 into the expression 



N= i/l 



\2a[ 



for the effective index of refraction. Here X is the wavelength in air and a 

 is the plate spacing. 



As a varies between X/2 and X, A' varies as indicated in Fig. 29. In the 

 neighborhood of a = X, N is not far from 1 and as a approaches X/2, N ap- 

 proaches 0. Since A^ is always less than 1 we see that there is an essential 

 difference between metal plate lenses and glass or plastic lenses for which N 

 is always greater than 1. This difference is seen in the fact that a glass lens 

 corrects phases by slowing down a travelling wave front, while a metal lens 

 operates in the reverse direction by speeding it up. This means that a 

 convergent lens with a real focus must be thinner in the center than the 

 edge, the opposite of a convergent optical lens (Fig. 30). 



Unless the value of A^ is considerably different from 1 it is evident that 

 very thick lens sections must be used to produce useful phase corrections. 

 For this reason values of 'a' not far from X/2 should be chosen. On the other 

 hand values of *a' too close to X/2 would cause undesirably large reflections 



9 W. E. Kock, "Metal Lens Antennas", Proc. I. R. E., Nov., 1946. 



