A broadband circularly polarized antenna of square-ring patch for UHF RFID reader applications

A circularly polarized, broad bandwidth, square-ring patch antenna for radio-frequency identification (RFID) is proposed. The antenna has a dimension of 100 x 100 x 22.9 mm(3). By using a Wilkinson power divider and a patch-antenna structure, a measured 3-dB axial-ratio bandwidth of approximately 140 MHz (16.47%), an impedance bandwidth of 136 MHz (15.81%), and a measured peak gain of approximately 6.8 dBic are being achieved. The operating band of the proposed antenna is suitable for China (840-846 MHz), Europe (865-868 MHz) and the United States (902-928 MHz) ultra-high frequency (UHF) RFID applications.