Single offset reflector antennas with short focal lengths deployed conventionally in space missions are known to be of limited applicability to linearly-polarised shaped beam coverages because of their poor cross-polar properties. These are generally incompatible with the stringent specifications placed on such space missions, which typically dictate the use of polarisation-sensitive reflectors or dual offset reflector geometries (for example, Gregorian-type reflector geometries). Such geometries tend to suffer from significant disadvantage in terms of mass and the amount of accommodation space taken up for accommodating component parts.
It is to be understood that single offset reflectors generate high cross-polar levels because of their fundamental asymmetry. This can be reduced if the reflector focal length is increased relative to the aperture diameter. Recent studies of deployment of two reflector configurations have shown that long focal lengths are advantageous in improving overall scanning performance, so that boom-deployed long focal length systems can be suitably used in multi-beam applications. Such systems are being increasingly proposed for the emerging Ka band market. It has also been found that when the ratio focal length to diameter approaches 2, the cross-polar performance improves sufficiently that the antenna can be used for dual polarised shaped beams in Ku band, for example.