Analysis of small parabolic reflector

Includes bibliographical references (pages 83-84)The purpose of this study is to investigate the radiation pattern\ud distribution produced by scattering from an electrically intermediate\ud sized parabolic reflector antenna. This investigation has been\ud prompted by the fact that very little has been written on pattern\ud prediction for this size class of antennas. Three methods are employed\ud for the actual numerical calculation of the patterns. These methods\ud are compared by considering the deviations from existing radiation\ud patterns obtained experimentally. Two methods are based on a prior\ud knowledge of the field distribution over the aperture; the scattered\ud field in the far zone being expressible in terms of the integrals of\ud the field vectors over the aperture surface. This is known in the\ud literature as the aperture-field method. The problem is first attacked\ud by assuming a uniformly illuminated circular aperture. Secondly, the \ud aperture fields are modified in accordance with the natural or "space"\ud tapering of these fields due to the parabolic reflector. The third\ud method, termed as the induced surface current method, computes\ud scattered fields by integration of surface currents induced on the\ud reflector by known incident fields from sources located at the focus\ud of the parabola. Due to the complexity of the integrands generated\ud by the latter two techniques, it was necessary to numerically integrate\ud the expressions to arrive at the far zone field values. Included in\ud all three models is the effects of aperture blockage due to the feed\ud assembly. All the techniques employ the assumption of a "point source"\ud feed; the simplest case being the superposition of electric and\ud magnetic dipoles forming a "Huygen's source" illumination. These\ud three methods and their implementation comprise Sections 2.0 and 3.0.\ud In Section 4.0 the accuracy achieved by these methods is compared\ud to measured data. Lastly, the techniques for achieving greater\ud accuracy in the results for the scattered field values is discussed\ud in Section 5.0