Polarization matching of wide angle conically scanned phased array beams

Multisatellite communication may require coverage of a segment of the geosynchronous arc as large as
60\deg
. A properly oriented ground station antenna can cover the above segment by means of a beam whose axis traces a conical surface (i.e., conically scanned beam). The antenna consists of two linear arrays feeding an imaging reflector arrangement through a polarization diplexer. This diplexer separates the incoming wave into two orthogonal polarizations without loss. In general, the polarization of the wave reflected by the diplexer does not remain the same as that corresponding to the feed array as the beam is scanned (polarization mismatch). This mismatch can cause an appreciable degradation of the antenna gain when the feed array is scanned in certain directions. For example, a ground station located at Seattle, WA could experience 3.6 dB reduction in gain due to polarization mismatch when the antenna beam is scanned along a
60\deg
segment of the geosynchronous satellite arc. The above polarization mismatch loss can be substantially reduced by the use of two properly inclined polarization rotators. These devices are sufficiently broadband to cover the entire 12/14 GHz satellite communication band. Design formulas for the inclination angle between the polarization rotators are presented. Applying these formulas we have reduced the above-mentioned polarization mismatch loss to less than 0.01 dB.