Dual offset shaped reflectors optimized for gain and XPD performance

Contoured beam antennas may conveniently be designed by shaping the reflector surface to produce a gain pattern which closely resembles the prescribed coverage area. The most appropriate technique is based on using physical optics (PO) to calculate the far-field pattern and optimize, using a minimax scheme, the surface shape until the desired performance is obtained. For most contoured beams the gain performance is of primary importance, and so it usually suffices to employ single-reflector systems which are fairly simple and thus attractive from a manufacturing and implementation point of view. However, if the antenna system is to operate in linear polarization with a stringent requirement on the polarization purity, it may be necessary to adopt polarization sensitive gridding of the reflector, or use a dual reflector configuration in which the cross polarization may be reduced by appropriately tilting the main- and sub reflector with respect to the feed. This paper presents the design of a dual offset reflector antenna for a typical European coverage divided into three regions with different gain requirements and a cross polar discrimination (XPD) constraint of 33 dB. The shaping is described in some detail and a comparison is made between designs optimized with and without XPD constraints.<>