Theory And Study Of Dual-band Circularly Polarized Loop Antenna Under Even-mode Resonance

This thesis is devoted to the study of the properties of the eigenmode in the even-mode loop antenna.The first-order even-mode(i.e.,fundamental even-mode)and third-order even-mode(i.e.,high-order even-mode)in the even-mode loop antenna are mathematically analyzed.The loop antenna under even-mode resonance has the characteristics of radiating dual-band inherently circularly polarized waves,and the design method of dual-band circularly polarized loop antenna under even-mode resonance is analyzed.The main content of this thesis includes:1.Based on the classical loop antenna theory,the thesis analyzes the mode of the even-mode loop antenna and establishes the equivalent source model of the square loop antenna.According to its current distribution and its natural boundary conditions,combined with the theoretical knowledge of classical antennas,the total field distribution formulas of fundamental and higher order modes are derived and the radiation characteristics and circular polarization characteristics of fundamental order-mode and higher order-mode are preliminarily verified.2.The structure of the feed is determined by the boundary conditions of the loop structure.In order to achieve the pattern matching of the boundary conditions,the dipole is placed on the diagonal of the loop structure while exciting the fundamental order-mode and the higher order-mode;and the tuning stub is added to the loop antenna.The tuning stub flexibly disturbs the resonant frequency of the higher order mode.The HFSS is used to establish an antenna model for numerical calculation,and the main performances such as axial ratio and radiation pattern are analyzed,and compared with theoretical derivation results.The key parameters affecting the circular polarization performance and the dual frequency ratio are extracted,and the optimal dual-band circularly polarized loop antenna is optimized.3.Based on the above-designed even-mode resonant dual-band circularly polarized loop antenna,the design of circularly polarized antenna with directional radiation is further discussed.The main design scheme is to add a metal reflector on the back of the antenna,then use the simulation software HFSS for simulation analysis,optimize the antenna parameters,and make the antenna object.Finally,the radiation performance of the antenna was measured by microwave darkroom and vector network analyzer.The measured results proved the correctness of the theory of even-mode dual-band circularly polarized antenna.The fundamental even-mode has an impedance bandwidth of 40% and a 3dB axial ratio bandwidth of 5.5%.The high-order even-mode has an impedance bandwidth of 14% and a 3dB axial ratio bandwidth of 1.4%.Both frequency bands of the proposed antenna have the characteristic of directional radiation circular polarization.