On the peak-to-edge directivity performance of a shaped reflector generating a circular spot beam

The edge-of-coverage directivity is an important figure of merit for a shaped beam antenna. In direct broadcast satellite applications the peak directivity is also an important parameter because of regulatory requirements. In antenna designs, the peak-to-edge ratio of the directivity performance is an indication of the quality of the antenna design. This is especially true in applications where the antenna is optimized for a uniform gain requirement over the coverage area. For simple pencil beams generated by unshaped parabolic reflectors, the peak-to-edge performance for optimum gain performance is about 4.5 dB, depending on the edge taper and the F/D ratio of the reflector. The edge of coverage performance can be improved by using a larger reflector and shaping the reflector to improve the ´gain-area´ product. For very large reflector apertures the improvement in the edge-of-coverage directivity is associated with a reduction in the peak directivity and the peak-to-edge ratio. This paper summarizes the results on a set of numerical experiments on the peak-to-edge performance of a shaped reflector antenna for a circular spot beam, as a function of the aperture size. Some of the results are interesting and are contrary to the original notion the authors had that any improvement in the edge-of-coverage directivity is always associated with a reduction in the peak directivity. An attempt has been made to generalize the results so that they can be applied to circular beams of arbitrary size generated from a reflector of arbitrary size.