Multi-polarization Reconfigurable Omnidirectional Antennas

Reconfigurable antenna technology is one of the key technologies to break through the choke point for the development of wireless communications,thus is a hot research topic in the field of antenna theory and design.Polarization reconfigurable(PR)antennas have the ability of switching the polarization according to the real-time status of the environment,which is quite different from other reconfigurable antennas,such as frequency reconfigurable antennas and pattern reconfigurable antennas.Omnidirectional antennas uniformly radiate in the horizontal plane,which is suitable for use in point-to-multipoint communications.Therefore,omnidirectional antennas have found wide applications in radio frequency identification(RFID),position tracking,and television broadcasting.In this thesis,three broadband lowprofile multi-polarization reconfigurable omnidirectional antennas have been investigated.The contributions of this thesis include:1.A horizontal/vertical(H/V)polarization reconfigurable omnidirectional antenna is proposed.The antenna is made up of a horizontal loop and a top-loaded monopole.The monopole is located at the center of the horizontal loop.The horizontal loop is composed of four pairs of printed arc dipoles,which form a ring.An in-phase current is uniformly distributed on the loop;thus,horizontal polarization is realized.Due to the introduction of a circular patch and four shorted pins,the top-loaded monopole has a low profile structure.Four microtrip baluns are used to feed the horizontal loop and the impedance matching of the top-loaded monopole is improved with the help of a microstrip transmission line with an open stub.By switching the states of the 2 PIN diodes that are inserted in the feeding lines,HP and VP are achieved.The experimental results show that the bandwidth of the H/V reconfigurable omnidirectional antenna for |S11|<-10 d B is about 25.6%(1.7–2.2 GHz)with good omnidirectional patterns.2.A horizontal/vertical/circular polarization reconfigurable antenna is proposed,which is based on the loop-monopole structure mentioned above.By introducing a new feedline to excite the loop and monopole simultaneously,circular polarization is generated.Microstrip transmission lines are used to make 90° phase difference and an open stub is introduced to improve impedance performance for the circular polarization.7 PIN diodes are employed in the polarization reconfigurable antenna.The experimental results show that the |S11|<-10 d B covers the band 1.67-2.21 GHz(27.8%)for all polarized radiations and the axial ratio<3 d B for circularly polarized radiation covers the band 1.7-2.2 GHz(25.6%).The antenna has omnidirectional radiation patterns in all three states.3.A horizontal/vertical/left-hand circular/right-hand circular polarization reconfigurable omnidirectional antenna is proposed,which is still based on the loop-monopole structure.By introducing a pair of ?/2 phase shifters to the feedline of the loop,left-hand and righthand circular polarization and are realized respectively: when the ?/2 phase shifters are not connected to the feedline,the horizontal component has a 90° phase delayed to the vertical component,thus the antenna radiates left-hand circularly polarized wave;when the ?/2 phase shifters are connected to the feedline,the horizontal component has a 90° phase leading to the vertical component,the antenna radiates right-hand circularly polarized wave.12 PIN diodes are employed in the polarization reconfigurable antenna.Experimental results show that the bandwidth for |S11|<-10 d B covers the band 1.65-2.25 GHz(30.8%)for all polarization states and circularly polarized radiation patterns for axial ratio<3 d B cover the band 1.6-2.2 GHz(31.6%).The antenna has an omnidirectional radiation pattern for all four polarizations.This thesis systematically investigates the procedure for the development of multipolarization reconfigurable omnidirectional antennas,especially the method for impedance matching between linear polarization and circular polarization,thus provides a novel solution for multi-polarization reconfigurable omnidirectional antennas.