Dual-Band Dual-Sense Circularly Polarized Stacked Patch Antenna With a Small Frequency Ratio for UHF RFID Reader Applications

A single-feed dual-band dual-sense circularly polarized (CP) stacked patch antenna with a small frequency ratio is proposed for Chinese 842.5/922.5-MHz radio frequency identification (RFID) reader applications. Two elliptical-ring patches are configured orthogonally to operate at different frequency bands with different senses of circular polarization. A π-type dual-band complex impedance transformer is modified with asymmetric open-ended stubs to easily tune the input matching for both bands. Measured impedance bandwidth is 2.99% (824-849 MHz) for the lower band and 2.72% (908-933 MHz) for the upper band, in which the input return loss is greater than 10 dB. The measured 3-dB axial ratio (AR) bandwidths for the lower and upper bands are 1.07% (838-847 MHz) and 1.19% (918-929 MHz), respectively. The measured frequency ratio is 1.10. The measured gain is more than 4.5 dBic over both bands. Therefore, the proposed antenna can be a good candidate for Chinese RFID readers operating in the bands of 840-845 MHz and 920-925 MHz.