Design Of Wideband Circularly Polarized Antennas

With the rapid development of modern wireless communication technology,people have put forward higher requirements for the broadband communication equipment.Circularly polarized(CP)antennas have been paid much attention than linearly polarized(LP)antennas because it has the advantages of combating polarization mismatch and multi-path interference.Therefore,broadband CP antenna has important significance for the future development of wireless communication.In this master thesis,some research on broadband CP antenna is studied.Three broadband CP antennas are designed and completed.Prototypes of the proposed antennas are fabricated and measured.Details are as follows: 1.A tilted monopole printed patch antenna with wide CP bandwidth is proposed.The broadband circular polarization property of the antenna can be realized by adjusting the ground structure and tilting the rectangular monopole based on the broadband characteristic of the monopole.The proposed antenna has a good bidirectional radiation performance of left-hand circular polarization(LHCP)radiation in the –z direction and right-hand circular polarization(RHCP)radiation in the +z direction.The simulated impedance bandwidth is 81.7%(1.89-4.50GHz)and the axial-ratio bandwidth is 70.68 %(2.15-4.5GHz).2.A novel compact dual-band CP slot antenna with simple structure is proposed.The dual-band circular polarization property of the antenna can be realized by utilizing the microstrip feedline to excite the asymmetrical U-shaped slot.The impedance matching and resonance at high band are further improved by loading two L-shaped slits with the size of 1/4 wavelengths at the high frequency on the edge of the U-shaped slot.The dual-band circular polarization at the desired bands can be achieved finally.The measured impedance bandwidths are 43.11%(2.42-3.75GHz)and 30.31%(5.01-6.80GHz)while the axial-ratio bandwidths are 43.9%(2.40-3.75GHz)and 16.22%(5.10-6.0GHz)at the lower and upper bands,respectively.The operating bands of the antenna can cover 2.4/5.2/5.8-GHz WLAN bands and 2.5/3.5/5.5-GHz WiMAX bands.3.Firstly,a novel back cavity broadband CP antenna with unidirectional radiation based on crossed-dipole is proposed.The antenna impedance bandwidth and axial ratio bandwidth are improved by optimizing the parameters of the radiation patch and introducing the rectangular slots in the cavity wal1.The measured impedance bandwidth is 70.0 %(1.17-2.43GHz),and the axial-ratio bandwidth is 47.83%(1.4-2.28GHz).The maximum 3dB axial ratio beamwidth is over 170° in the operating bandwidth.Secondly,in order to further improve the impedance bandwidth and axial ratio bandwidth of the antenna,its array antennas of 1×2 and 2×2 arrangements are designed based on the proposed antenna unit.The feed networks of the antenna arrays are realized by utilizing power divider for 90° phase shifter and 180° phase shifter.The axial ratio bandwidths of 1×2 and 2×2 arrays are 90.3% and 92.6%,respectively.