Horn-Reflector Antenna - Eliminating Weather-Cover Reflections

In the early 1940s, the pyramidal horn-reflector antenna was invented 1 at Bell Laboratories, Holmdel, New Jersey. It is now in extensive use in the Bell System 4-, 6-, and 11-GHz transcontinental microwave, common-carrier, radio-relay network. 2 This antenna is a combination of a square electromagnetic horn and a reflector that is a section of a paraboloid of revolution. The apex of the square horn coincides with the focus of the paraboloid. The antenna is essentially a shielded, offset, parabolic antenna, so that very little of the energy incident on the reflector is reflected back into the feed to produce an impedance mismatch. As used in system applications, the longitudinal axis of the antenna is normal to the earth's surface. This mounting position is shown in Fig. la. To prevent the entry of rain, snow, and other various foreign bodies into the antenna aperture and hence flowing down the waveguide transmission line, the aperture is covered by an essentially flat piece of glass fiber fabric (weather cover, Fig. la). The application of the weather cover was highly successful in excluding the elements. However, for microwave radio systems, the Bell System would like to have sidelobes down 40 to 50 dB or more. This has generally been obtained for far-out sidelobes except for those lobes associated with reflections from the weather cover. These weather-cover reflection 1333