| Researches on antennas for special applications and a monopulse antenna and feed system are presented in this dissertation. The research contents are listed as follows:1. Antennas for the special applicationsThe designs of an on-board interference antenna, a vehicle-mounted communication antenna, and four multi-band multi-polarization antennas for WLAN/WiMAX communication systems are discussed in this dissertation.(1) An on-board miniaturized broadband dual-polarization multiband antenna is developed. According to the requirements of miniaturization, broadband, dual polarization, high gain, wide beam, and strong ability to resist vibration on an on-board antenna, two separate orthogonal placed feed networks are used to feed the aperture stacked patch antenna for obtaining dual polarization and wideband properties. Two thick substrates are proposed to reduce the sizes of the two radiated patches and to widen the beamwith. Finally, the antenna is embedded into a metal cavity to decrease the backward radiation and increase the gain of the main beam. The measured results of the experimental prototype can meet the design requirements.(2) A vehicle-mounted lightweight broadband low-profile and high radiation efficiency antenna is proposed. According to the requirements of wideband, low-profile, unidirectional radiation, high radiation efficiency and light-duty on a vehicle-mounted antenna, a planar elliptical printed dipole antenna element is presented to realize wideband and low-profile properties. A reflection floor and a circular patch director are loaded to obtain unidirectional radiation patterns and improve the impedance and radiation performances. The dual-rectangular shape supporting plates are utilized to reduce the overall weight of the array. A wideband and low-loss parallel double-transmission-line feed network is used to feed the array for improving the radiation efficiency. The measured results of the experimental prototype can satisfy the design requirements.(3) A study on multi-band multi-polarization antennas is presented. The multi-band multi-polarization antennas contain a dual-band dual-polarization antenna, a dual-band CP antenna, a dual-band dual CP antenna, and a compact tri-band antenna.Firstly, a dual-band dual-polarization antenna is presented. By loading a shorting metallic cylinder in the patch center, the monopolar patch mode and two near-degenerate orthogonal modes are excited. The antenna radiates an omnidirectional vertical polarization pattern and a unidirectional CP pattern at two resonant frequencies, respectively.Secondly, a dual-band dual-polarization antenna is presented. By loading a shorting metallic cylinder in the patch center and curved branches in the patch edge, two resonant modes are excited. The antenna radiates an omnidirectional RHCP pattern and a unidirectional PHCP pattern at two resonant frequencies, respectively.Thirdly, a dual-band dual circular polarization antenna is presented. By loading two kinds of curved slots symmetrically, the TM01 and TM02 modes of the circular patch are excited. The antenna radiates an omnidirectional LHCP and RHCP patterns for each band, respectively.Finally, a compact tri-band antenna with omnidirectional radiation patterns is presented. By loading two spiral ring strips on both sides of the substrate and connecting them through a via-hole, three resonant modes are excited. By introducing the spiral ring structure, the effect of the other surface currents on the radiation pattern is reduced and very good radiation pattern characteristics can be obtained.2. A monopulse antenna and feed systemThe monopulse antenna and feed system contains a monopulse sum-difference comparator, an eight-mode monopulse horn feed and a monopulse reflector antenna.(1) Research on a monopulse sum-difference comparator. A light and compact monopulse sum-difference comparator consisting of four modified waveguide magic T is designed. The four output ports of the comparator and four input ports of the monopulse horn feed can be connected directly to reduce the size and weight of the sum-difference feed network.(2) Research on a monopulse horn feed. A compact multi-mode monopulse horn feed with the required aperture field distributions is proposed. Compared with the multiple-horn feed, the proposed feed has a more compact structure, small volume, light weight and high efficiency.(3) Research on a monopulse reflector antenna. According to the project requirements, a modified Cassegrain antenna with the monopulse comparator and horn feed is designed and simulated by using Ansoft HFSS-IE. The simulated results can satisfy the design requirements, which lay a good foundation for the project.
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