Design of a Multimode Feed Horn Applied in a Tracking Antenna

This paper proposes a multimode feed horn applied to a Cassegrain antenna operating at W-band. The horn is easily fabricated due to the simple structure. Comparisons are made between the performances of a four-mode feed horn and a seven-mode feed horn. The seven-mode feed horn is finally adopted for two reasons. First, the construction size is larger, and thus, it is easier to machine. Second, the active voltage standing-wave ratio is acceptable lower. A good compromise between the sum and difference beams is also achieved. The multimode feed horn is fabricated as part of the Cassegrain antenna. The simulation and measurement results of the Cassegrain antenna have shown good performances. The antenna efficiency reaches up to about 47%, and the 3-dB beamwidths are approximately 1.5° at operating frequencies. The sidelobe level is less than -15 dB in azimuth plane and less than -18 dB in elevation plane.