Feed back antenna belongs to twice-reflected antenna, there're two reflectors, and the main reflector is paraboloid. On the focus point there is a sub-reflector, which is used for reflecting the Satellite signal once again and through the waveguide tube the signal is transmitted to the LNB at the back of the antenna. The main purpose for adopting feed back antenna is to enhance the antenna receiving efficiency and improve VSWR. The highest efficiency of feed back antenna can be getting to 75%, but the highest efficiency of common antenna just can be getting to 70%. Additionally, if the user adopts feed back antenna, LNB can avoid to be affected by high temperature in the hot area; however, the structure of feed back antenna is complex and also the production cost is much higher, so generally for bigger diameter antenna, it will adopt feed back structure so as to be convenient to do the feed assembly and adjustment; and for individual receiving, feed back structure will not be used.

According to the sub-reflector shape, feed back antenna can be classified into two kinds, one is Cassegrain antenna, and another one is Gregory antenna.

(1) Cassegrain antenna

The sub-reflector of Cassegrain antenna shows hyperboloid of revolution, and the Casegrain antenna is often used for commercial earth station. As shown in Fig.2-3