Research And Design On Passive UHF RFID Tag Antenna

RFID technology plays an important role among mainline identification technology because of its' excellent performance.On account of the RFID technology applies to more and more fields and it's very closed to virtual circumstances,many questions have been raised.The tag antenna acts as a substantial part in RFID system and always be the research hotpot according to different application environment.In terms of this dissertation,two types of tag antenna developed research based on passive UHF RFID system.First is the design of tag antenna when put it on the metallic environment;Secondly,how to design the tag antenna to meet RFID system with remote reading and writing function.Given the aforementioned situations,two tag antennas have been researched and designed,concrete content is summarized as follows:(1)A tag antenna with fully flat surface and circular polarization can adhere to the metallic objects has been proposed.It realized effectively conjugate matching to tag chip's input impedance with high capacitance by exploiting orthogonal feeding microstrips and controlling the coupling strength between microstrips and quasi-square radiation patch.And to a large extent,the proposed theory that resistance and reactance of tag antenna can be designed respectively provides an effective solution to impedance-matching's problem that is sensitive to metallic environment.Otherwise the orthogonal feeding and quasi-square radiation patch contains perpendicular and crossed slot are the necessary condition to develop the two adjacent harmonic modes of circularly polarized tag antenna,consequently broadened the 3dB AR bandwidth.Finally,tag antenna absences of shorting pin and wall has been reached to 3dB impedance bandwidth with 36MHz(892MHz-928MHz)and 3dB AR bandwidth with 20.85MHz(906.15MHz-927MHz)for metallic environment(150mm?150mm)respectively,which covers the defined UHF RFID bandwidth to mainland china.At the same time,combining with own experience and summarizing the design cycle for integrated design of anti-metal tag antenna provides a clear reference idea.(2)To satisfy the tag with remote reading and writing distance in some applied scenario,a broadband high-gain and circularly polarized annular tag antenna has been proposed.As for impedance-matching,taking advantage of “T” matching loop that is characterized of affecting the input impedance of tag antenna obviously and its' impedance can be controlled easily with change of dimensions makes 3dB impedance bandwidth covered entire UHF RFID bandwidth(840MHz-960MHz)allocated for different countries and regions in the world.And circularly polarized tag antenna is realized by means of using mutual imbedding outer and inner quasi-squares with gap.The outer loop is encouraged to a closed and harmonic circularly polarized mode because of introducing inner loop,and thus broadened the 3dB AR bandwidth(93.34 MHz,840MHz-933.34MHz).Besides,the appropriate location of gap on the loop drives its' current distribution to the same phase,it's the basis of obtaining high-gain to the tag antenna.The final result demonstrates the furthest reading and writing distance can keep 10ft-13.5ft among the full part of UHF RFID bandwidth(840MHz-960MHz),so the purpose of remote reading and writing performance has been achieved.At the same time,annular tag antenna with broadband feature heightens tremendously compatibility in different countries and regions.