A circularly polarized radio frequency identification (RFID) tag antenna with a capacitive coupling-feed mounted on a metallic plane is presented. The tag antenna is a compact square-ring patch antenna with four different sizes of square-slot to obtain circular polarization (CP) radiation. A conjugate match to an Alien Higgs-4 chip with an impedance of 8-j149 Ω at 925 MHz is obtained by connecting two short-circuited microstrip lines in series. By tuning the proper lengths for the microstrip lines and its coupling gap distance, the tag antenna impedance is easy matched to the desired chip impedance. Tests of an RFID reader with a linear polarization (LP) antenna showed that the circularly polarized tag antenna has a good CP performance.
