The Cassegrain antenna is a double-reflecting surface antenna whose main reflecting surface is a rotating paraboloid and the second reflecting surface is a rotating hyperboloid. The difference between the structure of the Cassegrain antenna and the ordinary parabolic antenna is not only the addition of a second reflecting surface but also the feeding of the feed to the back of the main reflecting surface, as shown in Fig. 3. Therefore, the Cassegrain antenna is sometimes referred to as a feedforward antenna.
Cassegrain antenna One of the advantages of the double reflecting surface is that the forming technique can be employed. If the shape of the rotating hyperboloid is corrected so that the aperture field distribution meets the requirements, and the principal surface is appropriately modified to correct the phase difference of the aperture field due to the change of the minor plane, the Cassegrain antenna will have higher electrical performance. However, the diameter of the second surface of the Cassegrain antenna is generally larger, which causes a large occlusion in the small-diameter antenna, so the Cassegrain structure scheme is rarely used in the small antenna. Compared with ordinary parabolic antennas, the Cassegrain antenna has the following advantages:
1. Flexible design, the two reflective surfaces have four independent geometric parameters that can be adjusted;
2. The use of a paraboloid with a short focal length achieves the performance of a longer focal length paraboloid, thus reducing the longitudinal dimension of the antenna, which is significant for large aperture antennas;
3. Reduce the leakage of the feed and the radiation of the side lobes;
4. When receiving the antenna as a satellite ground, the noise temperature due to the leakage of the feed is relatively low because the feed is directed to the sky. The disadvantage is that the second reflection will have a certain occlusion against the main reflective surface, which reduces the aperture efficiency of the antenna. Since the caliber is above 4.5M, the manufacturing cost is high, and debugging is somewhat complicated when receiving satellite signals.