A Cassegrain antenna is a double reflector antenna. It has a concave parabolic reflector as the main reflector and a convex secondary reflector or subreflector. In the receiving antenna, the incoming waves are received by the main reflector and reflected onto the subreflector. The subreflector bundles the rays onto the LNB converter, which is located directly in front of the parabolic reflector.
The convex secondary mirror is located approximately at the focal point of the main mirror, and the LNB converter at the focal point of the subreflector. In the case of the Cassegrain transmitting antenna, the radio beams are radiated from the exciter, which can be a horn radiator, onto the convex subreflector. The horn radiator is located in a hole in the center of the large parabolic mirror, it radiates onto the subreflector, which reflects the rays onto the parabolic mirror. This reflects the beams and radiates them parallel to the receiving station. In contrast to the Cassegrain antenna, the subreflector of the Gregory antenna, also a double reflector antenna, is not hyperboloid but paraboloid.