WIDEBAND CIRCULARLY POLARIZED UHF RFID READER ANTENNA WITH HIGH GAIN AND WIDE AXIAL RATIO BEAMWIDTHS

A broadband circularly polarized patch antenna with high gain and wide axial ratio beamwidths is proposed for ultra-high-frequency (UHF) RF identification (RFID) applications in this paper. The antenna is composed of a square patch, a feed network printed on the bottom side of the substrates and an antenna radome. The CP radiation of the proposed antenna is excited by four cylinder probes which transmit four signals that have equal amplitude with quadrature phase difference (0˚, 90˚, 180˚, and 270˚) generated from the feed network. To obtain an optimum peak gain and a broad CP bandwidth, 100 Ω isolation resistor is omitted in the feed network for obtaining high radiation efficiency, and the effects of varying the feed positions and dimensions of the various parameters on the antenna performances are respectively investigated. Simulation results are compared with the measurements, and a good agreement is obtained. The measured results show that the proposed antenna can provide broad impendence bandwidth of 19.7% (815-993 MHz) (reflection coefficient less than -15 dB), a maximum gain of 9.65 dBi, and a 3-dB axial ratio (AR) bandwidth of about 11% (860-960 MHz). The results indicate that the proposed antenna is an excellent candidate for UHF RFID reader system. At last, read performance of the proposed antenna array in RFID systems is presented, which verify the superior features of the proposed antenna in practical RFID system applications.