MICROWAVE REPEATER RESEARCH 207 



good impedance match (under 0.8 db standing wave ratio) over the desired 

 400-megacycle band of frequencies. The lens in the mouth of the horn also 

 provies a convenient support for a plastic impregnated Fiberglass sheet 

 which acts as a weatherproof cover and protects the lens against ice forming 

 between the plates. 



The lens itself, Fig. III-5, is based on waveguide principles and causes the 

 wave to be refracted by virtue of the fact that waves confined between plates 

 parallel to the electric vector acquire a phase velocity higher than their free 

 space velocity in accordance with the equation : 



t'lens = t'free space / A/ 1 " ( 2^) » (lH-l) 



where X is the wavelength and, a, the distance between the plates. The 

 index of refraction is thus less than one, and a converging lens must be made 

 concave. 



As seen in Fig. III-5, the lens is stepped to reduce its thickness. As a 

 consequence of this stepping, the efficiency at midband of the antenna 

 (50%), is a good deal less than the theoretical value of 81%. Furthermore, 

 the index of refraction varies with wavelength, as seen from equation III-l, 

 and this results in a defocussing of the lens, with a consequent drop in gain, 

 at wavelengths different from the design wavelength. This amounts to a 

 drop in gain of 1.5 db at the edges of a 400 megacycle band; however, its 

 other characteristics of impedance, side lobe suppression (70 db in the two 

 rear quadrants), crosstalk, and ease of construction, help to make up for 

 the gain deficiency. 



Measurements taken on the antenna when a thick coating of ice had 

 formed on the plastic cover indicated that the impedance match was im- 

 paired (the maximum standing wave ratio increased from .8 db to 1.6 db), 

 but that the gain was not appreciably affected (less than 1 db). Since 

 propagation experiments indicate that severe atmospheric fades are not 

 likely to occur during the winter months, some of the fading allowance can 

 be applied against ice loss. 



There was some doubt that the crosstalk figures quoted above could be 

 relied upon during heavy rainfalls, as there was indication that the signal 

 transmitted from one antenna might be reflected from the rain and thus 

 caused to enter an adjacent side-by-side antenna. Measurements during 

 a moderately heavy rain proved that this effect was small, but large enough 

 so that the 85 db side-to-side figure was approaching a limit for the 4000 

 megacycle band. 



The measurement of antenna characteristics involves microwave tech- 

 niques whose development is an important part of a research program. 



