Gregory antennas are parabolic anten nas with two reflectors: a main reflector and a subreflector. Both reflectors, the main reflector and the subreflector, have the shape of a paraboloid.
The subreflector is located in front of the focal point of the parabolic mirror and focuses the radio beams onto the focal point of the main reflector. The double reflector principle increases the sensitivity for received radio signals and the bundling for radiated radio signals.
In addition to the Gregory antenna, there is another antenna concept with a double reflector: the Cassegrain antenna. It differs from the Gregory antenna in that the subreflector has the shape of a hyperboloid with two focal points. One focal point of the subreflector is focused on the focal point of the main reflector, and the second focal point focuses the radio beams toward the exciter, which is often located in a hole in the main reflector. The focal points are determined by the respective focal length of the paraboloids.