Dual-mode Dual-aperture Deformative Cassegrain Antenna Feed In X-band

Monopulse radar has been increasingly widely used in recent decades because of its high accuracy, remote operating range, strong anti-interference capacity. The feed is the extremely important part of monopulse radar and determines the performance of the monopulse radar. Ways of designing and analyzing the feed mainly contain impedance matching method and the aperture field method. The main research objective of this article is broadband deformative Cassegrain antenna feed. Extend the bandwidth of the feed by impedance matching method and calculate the far-field radiation pattern of the feed by aperture field method.Firstly, the two kinds of folding magic T, which compose the sum-difference comparator are analyzed theoretically in this paper, and various impedance matching methods are used to realize impedance matching at each port of the two kinds of folding magic T. Good performance of the folding magic T are eventually obtained, and then the two kinds of the folding magic T are combined into a wideband sum-difference comparator. After accomplishing the design of model generator and transition section,sum-difference comparator, transition section and model generator were combined into a completed feed.Secondly, the aperture field method has been used to derive and calculate the far-field radiation pattern of the feed. In order to expand the bandwidth of the feed,some critical parameters of the feed are simulated and optimized repeatedly. The VSWRs of the feed’s sum-port, elevation difference-port, azimuth difference-port are eventually less than 2 within the operating band and the bandwidths are greater than20%. The simulated and calculated far-field radiation patterns results are compared and analyzed. Some discrepancies can be found between the calculated results and the simulated results due to the consideration of electromagnetic diffraction of the rectangular radiation aperture rim and the mutual coupling between the radiation ports in the simulation.Finally, the feed is fabricated and the scalar network analyzer is used to measure the VSWRs of the feed’s sum-port, elevation difference-port, azimuth difference-port and the isolation between the sum-port and the difference-port. The far-field radiation patterns were measured in an anechoic chamber. The measured VSWRs of the threeports are less than 2 in the operating frequency band, and at the same time, high isolation has been reached between the sum-port and the difference-port. The far-field radiation pattern of the simulated, the calculated and the measured were compared in this paper. The comparison shows that the measured azimuth difference pattern is poor and elevation difference pattern is better. Because of some errors in the interconnected processing and assembly process of the various components, and some inevitable measurement errors in the testing process, zero drift can be found in difference-pattern.However, the feed’s indicators generally meet the expected technical requirements, so the feed can be fed for the dual-band polarization-twisted Cassegrain antenna. Since the secondary antenna radiation pattern obtained by overall test are in line with technical requirements, which confirms the success of the design and processing of the feed, it indicates that the designed X-band dual-mode dual-aperture deformative Cassegrain antenna feed can be applied to the actual application.