Metallic Delay Lenses 



By WINSTON E. KOCK 



A metallic lens antenna is described in which the focussing action is obtained 

 by a reduction of the phase velocity of radio waves passing through the lens rather 

 than by increasing it as in the original metal plate lens. The lens shape accord- 

 ingly corresponds to that of a glass optical lens, being thick at the center and 

 thin at the edges. The reduced velocity or "delay" is caused by the presence of 

 conducting elements whose length in the direction of the electric vector of the 

 impressed field is small compared to the wavelength; these act as small dipoles 

 similar to the molecular dipoles set up in non-polar dielectrics by an impressed 

 field. The lens possesses the relatively broad band characteristics of a solid 

 dielectric lens, and since the conducting element can be made quite light, the 

 weight advantage of the metal lens is retained. Various types of lenses are 

 described and a theoretical discussion of the expected dielectric constants is given. 

 An antenna design which is especially suitable for microwave repeater application 

 is described in some detail. 



Introduction 



THE metal lens antennas described by the v^riter elsewhere^ comprised 

 rows of conducting plates which acted as wave guides ; a focussing effect 

 was achieved by virtue of the higher phase velocity of electromagnetic 

 waves passing between the plates. Higher phase velocity connotes an 

 effective index of refraction less than unity, and a converging lens therefore 

 assumes a concave shape. The relation between the index of refraction w, 

 the plate spacing, a, and the wavelength X 



n - Vl - (X/2a)2, (1) 



indicates that the refractive index varies with wave length. As a conse- 

 quence, such lenses exhibit ''chromatic abberation"; i.e. the band of fre- 

 quencies over which they will satisfactorily operate is limited. Although 

 some of these lenses may have ample bandwidth (15% to 20%) for most 

 microwave applications, others, having large apertures in wavelengths, may 

 have objectionable bandwidth limitations. For example, the lens of Fig. 1, 

 having an aperture diameter of 96 wavelengths, has a useful bandwidth of 

 only 5%. 



As a means for overcoming these band limitations, the metallic lenses of 

 this paper were developed. They are light in weight and possess an index of 

 refraction which can be made sensibly constant over any desired band of 

 microwave frequencies. They avoid the weight disadvantages of glass or 

 plastic lenses, and retain the tolerance and shielding advantages of the lens 



» W. E. Kock, Bell Laboratories Record, May 1946, p. 193; Proc. I. R. E., Nov. 1946, 

 p. 828. 



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