Dual Circularly Polarized Non-Uniform Square Loop Antennas Under Even-Mode Resonance

Circularly polarized(CP)antennas have been widely employed to reduce the multipath effect and random polarization mismatching in wireless communications and wearable sensing systems.Thus,CP antennas have been widely applied in wearable human activity recognition,embedded sensing network,home automation,and globalpositioning system(GPS).Although distributed CP diversity systems can be implemented by using a pair of CP antennas with opposite sensing,dual CP antennas would provide a co-location diversity transmitting/receiving functionality,and thus reduce the size,complexity and cost of wireless sensor systems.In this work,conceptual designs of dual CP loop antenna using a pair of resonant even modes for wireless sensor applications are proposed.Both the bidirectional and unidirectional designs are researched in this thesis.The main content is organized as follows:1.According to the past articles and documents,it is found that under the condition of even-mode excitation,the loop antenna can realize CP radiation without introducing two orthogonal LP components with equal-magnitude and ±90o phase-shift.By changing the width ratio of the adjacent sides of the square loop to adjustthe phase condition,the dualCP radiation of the antenna can be realized.2.On the basis of CP radiation,a pair of fork-like stubs is introduced at the left and right corners of the square loop,thus the bidirectional dual CP radiation of the antenna is realized by the inherent circular polarization characteristic of the even-mode,which further expands the radiation range of the antenna.3.After numerically analyzed and optimized the model of dual CP non-uniform loop antennaby the commercial Ansoft's HFSS 13,several antenna prototypes are then designed,fabricated and measured with the same size as the simulated model.The loop antenna uses a coaxial line to connect the fork-like stubs to the loop radiator for balanced feeding.By matching the natural boundary conditions of the square loop and the fork-like stubs,the bidirectional dual CP radiation performance is realized.The antenna has 18.3%impedance bandwidth,14.5%axialratio(AR)bandwidth and 70.3% radiation efficiency.4.By adding a planar metallic reflector to the loop's back,a unidirectional antenna is further producted.Comparing the simulated and measured data results,the directional designed loop antenna still has good dual CP performance with 23.9% impedance bandwidth and 10.8% axial ratio(AR)bandwidth.The gain of the antenna has increased to 7.9dB due to unidirectional design with the impedance bandwidth23.9% and 10.8% axial ratio bandwidth.The antenna radiation efficiency can reach 83.1%.The available operation bandwidths of bi-and uni-directional casesare respectively 10.0% and 8.5%.The non-uniform square loop antenna utilizes the inherent circular polarization characteristic of the even-mode loop and adopts a balanced feeding mode to realize stable dual CP radiation with clear principle and simple structure.