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Cassegrain antenna
In telecommunications and radar, a Cassegrain antenna is a parabolic antenna in which the feed antenna is mounted at or behind the surface of the concave main parabolic reflector dish and is aimed at a smaller convex secondary reflector suspended in front of the primary reflector. The beam of radio waves from the feed illuminates the secondary reflector, which reflects it back to the main reflector dish, which reflects it forward again to form the desired beam. The Cassegrain design is widely used in parabolic antennas, particularly in large antennas such as those in satellite ground stations, radio telescopes, and communication satellites.
==Geometry==
The primary reflector is a paraboloid, while the shape of the convex secondary reflector is a hyperboloid. The geometrical condition for radiating a collimated, plane wave beam is that the feed antenna is located at the far focus of the hyperboloid, while the focus of the primary reflector coincides with the near focus of the hyperboloid.〔 Usually the secondary reflector and the feed antenna are located on the central axis of the dish. However in ''offset Cassegrain'' configurations, the primary dish reflector is asymmetric, and its focus, and the secondary reflector, are located to one side of the dish, so that the secondary reflector does not partially obstruct the beam.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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