72 BELL SYSTEM TECHNICAL JOURNAL 



mitter power. The second is good directional qualities so as to minimize 

 interference from outside sources and also interferences between adjacent 

 antennas. The third is a good impedance match so that reflections between 

 the antenna and the repeater equipment will not distort the transmitted 

 signal. These characteristics should preferably be attainable without the 

 imposition of severe mechanical or constructional requirements. 



The shielded lens type of antenna (lens in the aperture of a horn) lends 

 itself well to repeater work because of its moderate tolerance requirements, 

 its good directional properties associated with the shielding, and the desirable 

 impedance characteristic obtainable by tilting the lens. The delay lens 

 offers the additional advantage of broad band performance with the con- 

 sequent possibility of operating on several bands widely separated in wave- 

 length. In this section, construction details and performance of a 6-foot 

 square aperture strip type delay lens will be discussed. 



(a) Design of the Artificial Dielectric 



The operating frequency band envisioned for this antenna was 3700 to 

 4200 megacycles (X = 7.15 to 8.1 cm), and to keep the element length 

 sufficiently well removed from the half-wave resonant length a value of f " 

 for the strip width was chosen. The index of refraction of solid polystyrene 

 is approximately 1.61 and this introduces a reflection loss (mismatch loss) 

 at each surface of 0.225 decibels. To reduce this loss to 0.18 db. the arti- 

 ficial dielectric was designed to have an w of 1.5 as this still did not impart 

 too great a thickness to the lens. A combination of strip spacings which 

 yields an w of 1.5 was found to be f" in the horizontal direction and \^" 

 center to center spacing in the vertical direction as shown in Fig. 16. The 

 construction method of Fig. 6 was used which involved inserting .002'' 

 copper strip into slots cut in foam sheets. 



(b) Lens Design 



Several lens shapes were possible: (1) bi-convex, (2) plano-convex with 

 the flat side toward the feed, and (3) plano-convex with the curved side 

 toward the feed. For a given thickness and therefore weight of lens, the 

 third possibility produces the shortest over-all structure of lens plus horn 

 feed, and it was accordingly selected. The curved side is then a hyperboloid 

 of revolution as shown in Fig. 17 and for the chosen focal length of 60", the 

 profile, as calculated from the equation shown for n = 1.5, reaches a maxi- 

 mum thickness of 16". To eliminate the reflection from the lens into the 

 feed, a lens tilt could have been employed. It was found that a quarter 

 wave offset of one half of the lens relative to the other half could accomplish 

 this same purj)ose, because reflected rays from one half of the lens then 

 undergo a one half wavelength longer path in returning to the feed and the 

 reflections from the two halves cancel. As this process, however, is com- 

 pletely effective only at one frequency, the final lens design employed both a 



