Researches On Shaped Reflector Antennas And Feed Systems

In recent decades, the shaped reflector antennas and feed systems have been widely used in the satellite communication systems. With the rapid development of the satellite communication techniques, more demands are required on the performance of the shaped reflector antenna and its feed systems. The shaped reflector antennas can serve as satellite antennas to provide the radiation patterns to cover the required coverage region. Moreover, it can work as ground terminal antennas with high efficiency, low sidelobe and low cross-polarization. Based on recent progresses of both home and abroad and related to the research projects, this dissertation presents a systematic investigation on the analysis and design of the shaped reflector antenna, and the feed systems including feed horns and waveguide-fed devices are studied. The author’s major contributions are as follows:1. The feed horns are studied. Firstly, according to the requirements of reflector antenna systems, four axial corrugated horns with different beamwidth are designed with compact size and good radiation characteristics. Then the Ku-band corrugated horns composed of two kinds of corrugations are designed with compact size and low sidelobe. With nearly constant beamwidth over the operating band, the horns can be used as the feed of the dual-offset shaped reflector antennas. Finally, the coaxial horn loaded with a dielectric rod is proposed to cover the Ku receiving/transmitting band and Ka transmitting band simultaneously, which meets the multiband requirements. The loaded metal rings improve impedance matching in Ku-band, and the corrugations and the dielectric rod control the radiation characteristics in Ku-band and Ka-band respectively. Thus, the horn achieves good impedance matching and radiation characteristics over these bands. The measured and simulated results show that these horns have good performance.2. The design techniques of the waveguide-fed devices are studied. Firstly, the operating principle of the ortho-mode transducers (OMTs) is analyzed, and the Ku-band OMTs and Ka-band OMTs are designed. To meet the broadband requirement, a Ku broadband OMT based on a symmetric double ridged waveguide junction with two symmetric side branches is proposed and designed. With the symmetry of the structure, the OMT makes the higher order modes evanescent and achieves wide bandwidth. Moreover, the OMT is compact in size. Secondly,a Ka-band square waveguide iris-type polarizer is designed. To improve the isolation between the transmitting port and the receiving port and make the feed system compact, the Ku-band and Ka-band directly coupled non-centered resonant irises bandpass filters and the Ku broadband E-plane single-side irises lowpass filter are designed by combining DEGL algorithm with mode-matching method (MMM). These filters achieve good return losses in the passband and high attenuations in the stopband. The measured and simulated results show that the designed devices have good performance.3. The synthesis technique of single offset shaped reflector antenna with a single feed is studied. Firstly, an approach combining physical optics (PO) with physical theory of diffraction (PTD) is investigated for analyzing the radiation characteristics of the single offset reflector antenna. Secondly, the method of the shape of the reflector parameterized by Zernike polynomial expansion is studied. Then with these methods a single offset reflector antenna with a single feed is shaped to cover the square region. Finally, a contoured beam covering the China’s territory in Ku-band with the desired directivity is obtained by shaping the single offset reflector antenna. The calculated results are validated by commercial software GRASP9.3.01.4. The synthesis technique of dual offset shaped reflector antenna with a single feed is studied. Firstly, the structure of offset Gregorian antenna with zero cross-polarization radiation condition is introduced. Secondly, the analysis procedure of the radiation characteristics of the offset Gregorian antenna employing PO/PTD methods is studied. The geometry of the offset Gregorian antenna with zero cross-polarization radiation condition is parameterized by Zernike polynomial expansion. Then it is shaped employing DEGL algorithm and PO/PTD methods. After optimization a contoured beam is achieved to cover the China’s territory in Ku-band. Finally, with the Gaussian feed illuminating, a dual offset reflector antenna is shaped to have high efficiency, low sidelobe and low cross-polarization for VSAT satellite communication systems. Then the spherical-wave expansion coefficients are used to accurately describe the radiation characteristics of the feed, and the shape of the antenna is further optimized. The calculated results are validated by commercial software GRASP9.3.01.