Reconfigurable Antennas for Emerging Wireless Communication Systems

Abstract

This thesis explores a series of reconfigurable antennas with unique electrical characteristics for emerging wireless communication systems, including geostationary and navigation satellite communication systems, wireless sensor network systems and body-centric wireless communication systems. Reconfigurable antennas have become increasingly popular recently with the rapid development of modern wireless communication technologies. Antennas with polarization or pattern-reconfigurable characteristics have many advantages, including the enhancement of system capacity, multifunctional capabilities, avoidance of the multipath distortion in wireless channels, achievement of large radiation coverage and polarization coding. The significant challenges in reconfigurable antenna designs are narrow operating bandwidth, insufficient gain, complex structure, limited functionality, etc... The proposed reconfigurable antennas in this thesis have overcome the above constraints and are good candidates for the aforementioned wireless communication systems.
First, a wideband circularly-polarized reconfigurable antenna with the conical-beam radiation pattern is demonstrated for the geostationary satellite system. In this system, a conical-beam CP radiation pattern is preferred for receiving antennas mounted on moving objects on earth. The proposed antenna has overcome the narrowband challenge in such antenna designs. At the beginning, we designed a wideband and low-profile circularly polarized antenna with the conical-beam radiation pattern. The wideband characteristic is obtained by combining a wideband centre-fed monopolar patch and eight surrounding coupling loop stubs. The wide impedance bandwidth of 28% and AR bandwidth of 14.4% are realized by the simple structure. Later, we incorporated the original design with the polarization reconfigurability by introducing RF switches on the coupling loop stubs. It is the first antenna to realize wide bandwidth and switchable polarizations with the conical-beam radiation pattern.
Second, two reconfigurable antennas are developed for navigation satellite systems. This system requires antennas have the broadside CP radiation pattern with broad beamwidth. Conventional circular polarization-reconfigurable antennas with broadside patterns have very narrow bandwidths that are insufficient for modern wireless systems. To overcome this limitation, we designed a compact, wideband output phase reconfigurable feeding network to excite four sequentially placed monopoles. A wideband circular polarization-reconfigurable antenna is realized. The bandwidth of this antenna can reach 23.5% for both LHCP and RHCP modes, which is the widest among all reported designs. Unidirectional radiation patterns with stable gain and wide beamwidth are also observed. Furthermore, in order to achieve a larger CP radiation coverage for navigation satellite applications, another pattern-reconfigurable antenna with broadside and backfire unidirectional radiation is proposed. The antenna is realized by simply replacing the monopoles of the above design with a quadrifilar helical antenna (QHA). Switchable CP radiation patterns with larger beamwidth are achieved. This special pattern reconfigurability is very attractive for applications requiring large radiation coverage.
Third, the ±45°linear polarization-reconfigurable antenna and array are studied for indoor and outdoor wireless sensor network (WSN) systems. Initially, a wideband ±45°polarization-reconfigurable aperture-fed antenna element was designed for indoor applications of WSN. Wide bandwidth of 20% is realized by using the two stacked patches. The gain is very stable across the entire operating bandwidth. For outdoor applications as forest fire prevention, ±45°polarization-reconfigurable antenna array with high gain is highly desired. To realize this, we expanded the single element design into a 1 ×4 antenna array. An enhanced gain of 13.5 dBi is obtained. It is the first realized implementation of a polarization-reconfigurable antenna array owing to the simple biasing DC network and reconfigurable mechanism. It can be easily extended to larger array with more elements.
Fourth, multi-polarization reconfigurable antennas with 0°, +45°, 90°and -45° polarizations are investigated at the first time for body-centric wireless communication systems (BWCS). To mitigate the polarization mismatch in wireless systems for biomedical applications, external antennas with 0°, +45°, 90°and -45° polarizations are preferred. The reported multi-polarization reconfigurable antennas have no more than three switchable polarizations. To increase the functionalities, we developed a multi-dipole antenna system to realize four switchable polarizations. In addition, we designed a multi-slot antenna based on slot radiators that can realize the same reconfigurability but higher gain. Experimental results indicate that the polarization mismatching between the implantable and external antennas can be mitigated by adopting our techniques.

Date of Award	25 Aug 2016
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Hang WONG (Supervisor)