Gregorian Antenna

A Gregorian antenna is a dual-reflector antenna configuration in which a concave ellipsoidal sub-reflector is placed beyond the prime focus of a main parabolic reflector. One focal point of the ellipsoid coincides with the focus of the parabolic main reflector, while the other coincides with the phase center of the feed antenna. Unlike a Cassegrain antenna, which uses a convex hyperboloidal sub-reflector, the Gregorian employs a concave ellipsoidal sub-reflector. The Gregorian configuration can reduce spillover and sidelobe levels and allows convenient placement of absorbing baffles.