Design Of Omnidirectional Circularly Polarized Microstrip Antenna

As a front-end equipment of wireless communication system,antenna has received more and more attention.In many types of antennas,omnidirectional circularly polarized antennas have been found in broad applications due to its great advantage of omnidirectional radiation pattern and circular polarization.This thesis mainly focus on the design,analysis and realization technique of omnidirectional circularly polarized antenna in wireless communication.Two omnidirectional circularly polarized antennas with different structures are presented here.The major contributions of this thesis are summarized as follows.(1)After the analysis of background and current status of omnidirectional circularly polarized antenna,a study about microstrip antenna is carried out on principle,basic parameters and feeding mode.The basic methods to realize omnidirectional circularly polarized antenna are introduced.What's more,some techniques for miniaturization and broadening the band are also summarized.(2)On the basis of theoretical research,an omnidirectional circularly polarized patch antenna with center feeding is proposed.By symmetrically loading four L-shaped slots and shorting vias on the patch,omnidirectional circularly polarized radiation is obtained.Four parasitic strips are sequentially rotated around the patch to improve the impedance matching.Measured results show that the proposed antenna has a 10dB impedance bandwidth of 8.2%and a 3dB axial ratio bandwidth of 9.2%.(3)An omnidirectional circularly polarized patch antenna with a dual-layer structure is proposed.The antenna is composed of a disk-loaded coaxial probe,four shorting vias and two pairs of bow-tie patches with curved branches.By symmetrically loading four shorting vias and curved branches on the coupling-fed bow-tie patches to obtain the horizontal and vertical polarization,omnidirectional circularly polarized radiation is realized.A wide bandwidth is achieved by merging the TM01 and TM02 modes.Parasitic strips are sequentially rotated around the patch to improve the impedance matching.For demonstration,a prototype operating at 2.4 GHz-WLAN band is fabricated.Measured results show that the proposed antenna has a lOdB impedance bandwidth of 16.5%and a 3dB axial-ratio bandwidth of 20.5%.