| In recent years, reconfigurable antennas have received much attention, which are the combination of antenna techniques, RF technologies, microelectronics and electromachines. Reconfigurable antennas can dynamically adjust their characteristics of frequency, polarization and pattern according to the operation environment, by changing the structure and aperture of the antennas. Compared with the conventional antennas, reconfigurable antennas have predominant advantages and are attractive candidates for future wireless communication systems, satellite communication systems and radar systems. Circularly polarized antennas are widely used in many sytems, such as satellite communication systems, GPS, remote sensing systems and electronic warfare systems. In circularly polarized communication system applications, microstrip antennas are particularly attractive due to their low profile, light weight, easy integration with RF circuits and easy to achieving circularly polarized radiation. However, microstrip antennas have main drawback of narrow bandwidth that restricts their wide usage. Therefore, the design of broadband circularly polarized microstrip antennas have been receiving more and more attention. The dissertation is mainly concerned with the design of reconfigurable microstrip antennas and broadband circularly polarized microstrip antennas. The author's major contribution are outlined as follows:1. The basic principle for the frequency reconfigurable antennas is studied. A novel frequency reconfigurable CPW-fed microstrip slot antenna is designed based on PSO algorithm combined with FDTD method. By controlling the PIN diodes on or off, the resonant length of the slot is changed and the frequency reconfiguration is achieved. A prototype antenna using PIN diodes is fabricated and measured. The central frequency of the proposed antenna can shifted between 1.5GHz, 3.22GHz and 3.55GHz.2. The basic principle for the pattern reconfigurable antennas is studied. A novel pattern reconfigurable microstrip slot antenna is presented. The proposed antenna includes four slot dipoles uniformly arranged along circumference. The slot antenna is fed through a circular slot located at the center of the dipoles and an open-end circular microstrip stub, which can achieve good impedance match in four operation modes. By controlling the switches mounted on the four slots on or off, the direction of null in the x-y plane can be changed in four pairs of directions of ? =22.5 and ? =202.5, ? =67.5 and ? =247.5, ? =112.5 and ? =292.5, ? =147.5 and ? =337.5, with the shape of the radiation pattern and boresight direction maintaining unchanged. 3. The basic principle for the pattern reconfigurable antenna array is studied. A novel two-path microstrip antenna array is proposed. The defected ground structure periodic CPW transmission line is used to replace the conventional phase shifter. By controlling the switches on the slots of the DGS structures, the excition phases of two elements are changed, so the beam steering of the antenna array is achieved. A prototype antenna is constructed and measured. In the experiment, metal tapes are used to replace the on and off states of the MEMS switch. The measurement results show that in five modes the boresight directions of the proposed antenna in H plane are inθ=±21,θ=±9 andθ=0directions.4. The wideband circular polarization techniques for microstrip antenna are discussed. A novel broadband circularly polarized CPW-fed microstrip slot antenna is presented. The proposed antenna has a simple uniplanar geometry and is fed by a CPW with widened L-type strip protruding in the slot. A prototype antenna is fabricated and measured. The results show that the input return loss less than -10dB is about 48%, and the 3dB axial-ratio (AR) is about 17%.5. A novel polarization reconfigurable microstrip antenna with switchable slot is presented. A novel circularly polarized microstrip antenna with slot is proposed. The antenna can achieve circular polarization by cutting a slot on the patch corresponding to the position of the feed point. Left hand circular polarization (LHCP) or right hand circular polarization (RHCP) can be obtained depending on the postion of the slot according to the feed point. In the light of above antenna design, a polarization reconfigurable mirostrip antenna is proposed. Two orthogonal slots are cutting on the patch and two pairs of PIN diodes are mounted on them. By controlling the two pairs of PIN diodes on and off, the antenna can radiate with LHCP or RHCP. The DC biase voltage for the PIN diodes can be applied through the feed port, which has little impact on the antenna operation. A prototype antenna is fabrictated and measured, the results show that the 3dB AR is about 1.0% in LHCP and RHCP state.6. Based on a known design of broadband microstrip slot antenna, a novel polarization reconfigurable microstrip slot antenna is presented. The proposed antenna has two T-shaped metallic strips orthogonal to the feed line, while the original antenna structure only has one. Two T-shaped strips are connected to ground plane with two PIN diodes. By controlling the PIN diodes on or off, the antenna can switch between LHCP and RHCP. A prototype antenna is constructed and measured. The measured results demonstrate that the proposed antenna realizes the broadband circular polarization, and the 3dB AR bandwidth is about 11% in two states.
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