Study On Circularly Polarized Non-uniform Loop Antenna

Circularly polarized(CP)antennas can be employed to mitigate the multipath interference effects in various wireless systems.Owing to such a feature,a series of CP antennas have been developed and used in satellite communications,radio frequency identifications(RFID),wireless local networks(WLAN),and the global positioning systems(GPS).Design approach to a novel circularly polarized(CP)non-uniform square loop antenna with differential feeder is proposed in this thesis.Both the bi-directional and uni-directional cases are designed and investigated.The main content of this thesis is organized as follows:1.According to the basic partial traveling-wave distribution of the loop antenna,the natural boundary condition is determined as follows: An infinite perfect magnetic wall occurs along the major diagonal,while an infinite perfect electric wall occurs along the other one.The natural boundary condition of a short dipole can be determined in the same way.It is seen the dipole can be employed to excite the loop antenna along one of the diagonals.2.According to the simulation results,the CP performance of the antenna is investigated and the most critical parameter is found.Further,a dipole with a fork-like structure can be introduced for better wideband impedance matching.3.According to the empirical results and preliminary parametric studies,this antenna is numerically analyzed and optimized by employing the commercial Ansoft's HFSS software.A few antenna prototypes are then designed,fabricated and measured.The loop antenna is diagonally fed by a fork-like dipole launcher.Both the bi-directional and uni-directional cases are designed and investigated.By matching the respective natural boundary condition of the loop radiator and the dipole launcher,a bi-directional CP loop antenna with a wide impedance bandwidth of 22.2% and a 3-dB axial ratio(AR)bandwidth of 6.6% is achieved.4.A uni-directional antenna having a planar metallic reflector is further designed.The uni-directional loop antenna exhibits excellent uni-directional CP performance with a high front-to-back ratio of 40 dB and a 3-dB axial ratio(AR)bandwidth of 7.6% in both theory and experiment.The operating frequency band of the antenna is 2.27-2.45 GHz.The proposed antenna has a unidirectional CP radiation characteristic.