Omni-directional circularly polarized antennas combining dipole and loop radiators

Abstract

This thesis presents a series of novel omni-directional circularly polarized antennas combining a dipole and a loop radiator. Based on the fact that the far-fields of a short dipole and a small loop excited by currents with the same amplitude and phase, are orthogonal to each other and 90° difference in phase, an omni-directional left-handed circularly polarized (LHCP)/ right-handed circularly polarized (RHCP) radiation with low axial ratio at the azimuthal plane can be achieved. In addition, the polarization status, i.e., LHCP or RHCP, of the antenna depends on the direction of the current along the loop at the initial time. According to these, four novel omni-directional circularly polarized antennas working at GPS L1 band (1.575GHz) are proposed in this thesis. Firstly, the theoretical analysis of omni-directional circularly polarized (CP) radiation, which is the foundation of the whole thesis, is provided. To make the realization of the antenna possible, a design combining a monopole and a loop radiator is studied. The design can be realized by adopting a coaxial probe as a monopole, and printed spoke-like metal strips fabricated on two substrates working as a loop. Measured results show that the proposed antenna can achieve a -10dB impedance bandwidth of 13.8% and 3-dB axial ratio bandwidth of 17.5% in the azimuthal plane. The average axial ratio and RHCP gain are 1.91dB and 0.86dBi, respectively, in the azimuthal plane. Secondly, based on the previous omni-directional circularly polarized antenna, a polarization reconfigurable omni-directional antenna is designed. The direction of the current along the loop is controlled by PIN switch diodes, and a bias tee has also been made to apply DC bias current/voltage between diodes. Polarization reconfigurability is realized by using diodes to switch the direction of RF current along the loop. The measured average axial ratio of the RHCP and LHCP waves are 2.45dB and 2.33dB, respectively. Previous two antennas are fed by coaxial cable which need baluns to suppress the currents along the outer conductors of the cable. This causes the antenna to be elongated. In order to lower the antenna height, an omni-directional circularly polarized antenna that is fed by microstrip line instead of coaxial line is designed. The feeding substrate is inserted between the two substrates which realize the loop. In this way, the metal sleeve, which assists in building the ground, can be removed. Finally, the polarization reconfigurable omni-directional antenna fed by microstrip line is presented. The antenna adopts two metal probes acting as a dipole and printed spoke-like metal strips fabricated on two substrates acting as a loop. The antenna is fed by microstrip line, and the feeding substrate, in addition, includes DC bias network in it. As a result of these changes in the new feeding method, the total height of the antenna can be reduced from 123mm to merely 15mm compared with the second antenna. The -10dB impedance bandwidth is about 18% and 14%, and the average axial ratio in the azimuthal plane is 1.7dB and 1.5dB for the RHCP and LHCP radiation, respectively.

Date of Award	15 Jul 2014
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Kim Fung MAN (Supervisor)