A Broadband Circularly Polarized Patch Antenna Based on Bowknot-shaped Slot-coupled Feeding for UHF RFID Application

In this paper, a broadband circularly polarized patch antenna based on the bowknot-shaped slot-coupled feeding is proposed for ultra-high-frequency (UHF) radio-frequency identification (RFID) application. The proposed antenna comprises a circular radiation patch, four bowknot-shaped slots, and a compact feed network. The bowknot-shaped slot is responsible for enhancing the coupling with the radiation patch, thereby achieving wider bandwidth. To achieve a wide circularly polarized bandwidth within a compact size, a sequentially rotating feed network, consisting of a modified T-shaped power divider and two compact quadrature couplers, is introduced. To validate the proposed method, using the FR4 substrates, an antenna prototype with the size of 160 mm×160 mm×18.5 mm is designed, simulated, manufactured, and measured. The measurement results demonstrate that the impedance bandwidth for |S
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| < –10 dB is 61.1% (598–1124 MHz), and the 3-dB axial-ratio bandwidth covers 25.5% (770–995 MHz) with a gain of about 5 dBic.