Use of imaging with spatial filtering in reflector antennas

The classical problem of illuminating efficiently the aperture of a reflector antenna is discussed. We combine a small feed with an ellipsoidal reflector producing, directly on the main reflector, a magnified imagef(x, y)of the feed aperture distributionf₀(x₀, Y₀). The transformation relating the feed distribution to its image is approximately frequency-independent, since it is given accurately by the imaging laws of geometric optics. Thus the antenna is efficient over a wide range of frequencies. Furthermore, since the entire image distributionf(x, y)is reflected by the main reflector with negligible spillover, the antenna far field is approximately a replica of the focal plane field and, therefore, its sidelobes can be reduced substantially by using spatial filtering in the focal plane.