Design aspects of prime focus parabolic reflector antenna

Long range applications require antennas that can radiate a pencil beam pattern. Parabolic reflector antennas are widely used for such applications due to their high gain and simplicity. A parabolic reflector has different feed configurations mainly Prime Focus, Cassegrain, and Gregorian configurations. Among other configurations, prime focus configuration has highest main lobe level and the lowest Side Lobe Level (SLL). The horn is widely used as a feed element. The effective utilization of reflector antenna can be accomplished through design of an optimum feed horn through trade-off between taper and spillover efficiencies. There are three basic types of rectangular horn antennas namely E-plane sectoral horn, H-plane sectoral horn, and Pyramidal horn depending on the plane in which the dimensions are flared. Since the reflector of a parabolic reflector antenna is rotationally symmetric, the feed horn is desired to have a symmetric pattern. A pyramidal horn gives the flexibility of designing an optimum horn that can symmetrically and efficiently illuminate the reflector. The design procedure of the prime focus parabolic reflector antenna and the feed antenna which can efficiently illuminate the aperture of the reflector is presented in this work. Practically, the field strength of the feed horn illuminating the reflector should be (9-10) dB less at the edges than that at the vertex of the reflector for maximum efficiency. For reflectors of less curvature (higher focal length to diameter ratio, F/D), the Space Attenuation Factor (variation in field strength on the surface of the reflector due to nonuniform distance from focus to the surface) can be neglected while designing the feed horn antenna. A feed horn antenna whose 10 dB beamwidth coincides with the half subtended angle (angle subtended by the reflector edge at the focus) of the reflector will be a suitable feed antenna for lower curvature reflectors. The axial lengths of the feed horn in E-plane and H-plane are calculated using the slant ratio required for the 10 dB beamwidth. The dimensions of the pyramidal feed horn are computed iteratively by equating the combination with almost identical axial lengths in both planes. The design procedure is explained in detail in this work. (author)