A compact wideband circularly polarized (CP) square slot antenna for universal ultra high frequency (UHF) RF identification (RFID) handheld reader applications is proposed, fabricated, and tested. The antenna is coplanar waveguide (CPW) fed by an inverted Z-shaped feeding line. By inserting four stubs in diagonal directions and two inverted T-shaped strips inside the square slot, broadband CP operation, wide axial ratio bandwidth, and good impedance matching are achieved. The measured < -10 dB impedance bandwidth is from 706 MHz to 1007 MHz (301 MHz, 35.1%). The measured 3-dB axial ratio (AR) is 427 MHz (745-1172 MHz, 44.5%). The maximum measured gain of the proposed antenna is 4.8 dBi. The proposed antenna has wide impedance bandwidth, wide axial ratio bandwidth, and small size. The dimensions of the antenna are only 120×120×1.6 mm3. The impedance bandwidth and AR bandwidth performances of the proposed antenna can easily cover the UHF RFID band as whole.
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