| Radio-frequency identification(RFID)is an automatic identification technology realized by using the radio frequency signals,which has been widely used in internet of things systems such as goods management and animal tracking.Generally,RFID tag antennas are linearly polarized and are arbitrarily placed.In order to improve the reliability of the system,the circularly polarized(CP)RFID reader antenna is an important part of the entire system.At present,in order to apply to different ultra-high frequency(UHF)RFID bands in different regions of the world or be compatible with microwave bands,dual-band and broadband circularly polarized RFID reader antennas have become a research focus.This thesis researches the UHF broadband CP RFID reader antenna and 915 MHz/ 2.45 GHz dual-band CP RFID reader antenna.The specific work mainly includes the following aspects:(1)A single-fed CP RFID reader antenna is designed.The antenna is composed of a singlefed perturbed ring patch and a square patch loaded with circular perturbations at the vertexes.In addition,the shorting pins are loaded between the circular perturbation structures and the ground plate to reduce the overall size of the antenna.The results show that the impedance bandwidth of the antenna is 9.98%(872.1-963.4 MHz),the 3-d B axial ratio(AR)bandwidth is4.95%(892.8-938.1 MHz),and the gain of the antenna is greater than 7.07 d Bic in the 3-d B AR bandwidth,and good right-handed circularly polarized radiation is achieved.(2)A miniaturized feed network applied to a four-fed CP antenna is designed.The feed network consists of a miniaturized hybrid ring and two trans-directional(TRD)couplers.Ttype structures and step-impedance microstrip lines are used for the miniaturization of the hybrid ring.The output ports of the hybrid ring are respectively connected to two identical TRD couplers to provide equal power distribution and 90° phase differences.The results show that,in the band of 840-960 MHz,the insertion loss of each output port of the proposed feed network is 6.3 ± 0.8 d B,and the phase difference between adjacent output ports is 90° ± 7°.Balanced power distribution and phase difference characteristics are obtained.(3)Based on the proposed feed structure,a broadband CP RFID reader antenna with a wide 3-d B AR beamwidth is designed.The working band of the designed antenna covers the840-960 MHz UHF band.Four sequentially-rotated isosceles right triangular patches fed by coplanar capacitance are adopted to achieve broadband characteristics.In addition,loading a row of the shorting pins on the hypotenuse of each triangular patch not only achieves the wide AR beamwidth,but also reduces the size of the patch.The measured results show that the impedance bandwidth of the antenna is 18.4%(820-986 MHz),the AR bandwidth is 20.4%(772-956 MHz,less than 2 d B),and the 3-d B AR beamwidth is more than 177°.(4)A dual-band CP cross-dipole RFID reader antenna is designed.The antenna consists of two pairs of crossed dipoles,and the center operating frequencies are 915 MHz and 2.45 GHz,respectively.A quarter-wavelength ring-shaped microstrip line is used to feed two pairs of crossed dipoles and form a 90° phase shift to achieve CP radiation.In addition,the stepped strip structures are used to reduce the overall size of the antenna,and the AR bandwidth of the antenna is increased by controlling the width ratio of the strips.The measured results show that the impedance bandwidth of the lower and upper bands are 25.8%(876-1112 MHz)and 19.6%(2.216-2.696 GHz),respectively.And the 3-d B AR bandwidths are 6.1%(874-930 MHz)and6.9%(2.385-2.554 GHz),respectively.Good CP characteristics are achieved in both 915 MHz and 2.45 GHz RFID bands. |
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