Abstract
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 $\Pi $ -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.
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