In telecommunication, a Cassegrain antenna is an antenna in which the feed radiator is mounted at or near the surface of a concave main reflector and is aimed at a convex secondary reflector slightly inside the focus of the main reflector.
Note 1: Energy from the feed unit illuminates the secondary reflector, which reflects it back to the main reflector, which then forms the desired forward beam.
Note 2: The Cassegrain antenna design is adapted from optical telescope technology and allows the feed radiator to be more easily supported.
Source: from Federal Standard 1037C
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