| With the advancement of science and technology,wireless communication technology has developed rapidly.Wideband,multi-band and circularly polarized(CP)antennas have become the focus of current antenna research.The CP antenna has unique advantages in resisting rain,fog,ionosphere,etc.It not only resists the interference of multipath effect,but also has large capacity of channel.In modern wireless communication systems,the antenna which just work in single band can no longer meet the actual needs.The dual-band dual-sense CP antenna can improve the channel capacity and enhance the anti-jamming ability of communication system.Compared with the traditional CP antenna,dual-band dual-sense CP antenna has more obvious advantages.In addition,although Radio Frequency Identification(RFID)technology has been applied worldwide,each country has its own frequency allocation for UHF RFID system.Therefore,an antenna across the entire UHF RFID band would be beneficial for RFID system cost reduction.However,the conventional CP RFID antenna is difficult to operate in universal UHF RFID band because of the fact that the working bandwidth of CP antenna is limited by both impedance bandwidth and axial ratio(AR)bandwidth.Based on actual requirements,the dual-band dual-sense CP antenna and UHF RFID wideband CP antenna are studied in this thesis.The main contributions of this thesis are as follows:1.The design of UHF RFID wideband CP antenna.Firstly,the corner truncated square patch antenna with an L-shaped ground plane was initially studied.However,the antenna just obtained impedance bandwidth of 8.6% and CP bandwidth of 4.8%,which were far from the needs of practical projects.Then,in order to improve both the impedance bandwidth and CP bandwidth of the antenna,four triangular slots were etched on the corner truncated square patch antenna.Finally,for the purpose of making the antenna operated in the universal UHF RFID band,two U-shaped slots were etched on the center of the corner truncated square patch antenna.The antenna was fed by simple microstrip structure.The simulated results show that-10 d B reflection coefficient bandwidth of 79.6%(840-1950 MHz)and 3 d B axial ratio bandwidth of 30%(840-1140 MHz)were attained.Stable gain levels variation of 5.2-7.6 d Bic over the UHF RFID frequency 840-960 MHz were achieved,and the peak gain level could reach to 7.6 d Bic.The antenna was fabricated basing on the simulated model and measured.The measured and simulated results were in good agreement.2.The design of dual-band dual-sense CP antenna fed by coplanar waveguide(CPW).A dual-band dual-sense CP antenna was proposed basing on the monopole antenna structure.The antenna was fed by CPW,consisting of a cross-shaped branch,a rectangular parasitic element and three I-shape grounded stubs.Firstly,a monopole antenna was presented,which achieved good impedance matching and CP characteristics over the lower band by loading a cross-shaped branch.Then,in order to achieve the dual-sense CP radiation and improve the impedance matching characteristics over the higher band,a rectangular parasitic element was loaded into the right side of the monopole antenna.Finally,to further attain the designing requirements and obtain good CP bandwidth,three I-shape grounded stubs were added to the ground plane.The simulation results show that,the antenna obtained-10 d B reflection coefficient bandwidths were 71.3%(1860-3920 MHz)and36.0%(4170-6000 MHz).The simulated 3 d B axial ratio bandwidths were 27.5%(2960-3910 MHz)for the lower band(RHCP)and 14.9%(4830-5610 MHz)for the upper band(LHCP),respectively.The antenna gain levels variation was 2.2-7.2 d Bic,and the peak level gain could reach 7.2 d Bic.According to the simulated antenna model,the antenna was fabricated and measured.The measured results were agree well with the simulated ones. |
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