A compact wideband circularly polarized square slot antenna for universal UHF RFID reader applications is proposed and tested. An L-shaped radiator at lower surface of the substrate is used to feed the proposed antenna. To achieve a broadband circular polarization (CP) and good performance, two rectangular stubs with different sizes are inserted at opposite corners of the square slot at the upper surface of the substrate. A small rectangular slit is used to improve the impedance matching of the proposed antenna. The antenna's measured <10-dB impedance bandwidth and measured 3-dB axial ratio bandwidth are 70.2% (660-1374 MHz) and 45.8% (796-1269 MHz), respectively. The proposed antenna has a dual circular polarization characteristic, wide impedance bandwidth, wide axial ratio, compact size, and maximum measured gain about 3.9 dBi. The total size of the proposed antenna is 120×120×1.6 mm3. Furthermore, the impedance bandwidth and axial ratio bandwidth of the proposed antenna cover the entire UHF RFID band easily. The proposed antenna is suitable for UHF RFID reader applications.
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