Receiving and transmitting properties of small grid paraboloid by moment method

The receiving and transmitting properties of a grid paraboloidal reflector of aperture diameter 2.73¿ and f/D ratio 0.395, operating at 410 MHz, are analysed using the moment method. For the receiving situation, with a normally-incident plane wave illuminating the reflector, the distributions of current, obtained in the inner grids only, show a close resemblance to those predicted by the physical optics approximation. It is shown that neither maximum scattered nor maximum total electric field on axis is received at the geometrical focus, but at a point between this focus and the vertex. This result is in agreement with that calculated by the physical optics method for small solid paraboloids of the same f/D ratio. Contours of scattered off-axis fields are also presented in the geometric and true focal planes of the paraboloid as well as in longitudinal planes of such a reflector. In the transmitting situation, the grid paraboloid is excited by a centrally driven couplet feed, positioned at different points along the axis. It is shown that the feed position affects considerably the distributions of current in the feed elements but not in the reflector grids, and confirms that defocusing the couplet feed results in an increased forward power gain.