Research Of The Circularly Polarized Antenna For RFID

Based on the theory about circularly polarized wave, three models of doubly-feeding point-feed, microstrip-feed and probe-feed antenna have been established. Performance parameters are optimized, and the key sizes of antenna are analyzed, effects of the critical sizes, that are sizes of parasitic element, corner and radiating surface, on gain, axial ratio and impedance are described clearly by simulation graphics and tables. According to this the advantages and disadvantages of three feed methods are analyzed and summarized, and conclusions are obtained: bandwidth of doubly-feeding point-feed antenna is good, but the way of design is complex; matching network of microstrip-feed antenna is simple, but miniaturization is not easy to be realized; performances of probe-feed antenna are too sensitive. The universal methods of designing circularly polarized RFID antenna are presented systematically.There are inherent contradictions among bandwidth, gain and miniaturization, on the basis of study on key technologies such as miniaturization and wide-band, the structure of circularly polarized antenna is innovated by digging a hole on medium board which solves contradictions among them. Through adjusting the diameter of the hole on medium board equivalent dielectric constant of medium board is changed, and quality factor is regulated. The structure capacitance is used in impedance-match, which parallels with the feed microstripe. In this way problem of loading match on the RFID reader antenna is solved primly.According to theory about circularly polarized antenna array, a novel dipole antenna array and eight-unit microstrip antenna array for ETC system are designed. The feed network, in which forms of series and parallel are used, and 2×4 array form is adopted, thus the impedance of ETC antenna array is matched to 50 ohm successfully , and requirements of ETC system for HPBW is met.