A miniaturized circularly polarized (CP) patch antenna is proposed for ultra high frequency (UHF) radio frequency identification (RFID) reader applications. The antenna is composed of a vertical ground, a U-shaped strip, and a square patch with truncated corners. A novel U-shaped strip feed structure is proposed for good impedance matching. Broadband CP operation is achieved by using a thick air-layer substrate (about 0.08
) and selecting an optimal size of the truncated corners. The antenna has an impedance bandwidth (
< -20 dB) of about 141 MHz (861-1002 MHz or 15.1%), a 3-dB axial ratio (AR) bandwidth of about 30 MHz (899-929 MHz or 3.3%), and a gain level of about 7.2 dBi or larger within the 3dB AR bandwidth. The dimension of the antenna is 13 cm x 13 cm x 2.8 cm. The small size and high gain make this element suitable for portable UHF RFID reader applications.