LAUSR

ABSTRACT We report a broadband ultra-high frequency (UHF) radio frequency identification (RFID) tag antenna of compact size mounted on a metallic object in a near field communication (NFC) RFID system. One half of the square radiation patch has a semi-annular strip feedline embedded inside a semicircle-shaped slot to improve the antenna bandwidth and gain, and the other half of the radiation patch has an L-shaped slot and a shorting pin to increase the circular polarization (CP) bandwidth and reduce the antenna size. The resonant input impedance of the tag antenna can be tuned easily by varying the length of the semi-annular feedline, and the gap between the feedline and the etched semicircular slot. The proposed tag antenna has a wide measured impedance bandwidth (−10-dB |Γ|) covering the whole operating frequency range of 181 MHz (from 790 to 971 MHz) for a UHF RFID system. The measured 3-dB axis ratio (AR) bandwidth is 43 MHz (from 895 to 938 MHz), which is much larger than those of the CP tag antennas reported in the literature for metallic objects. Further experiment has shown that the proposed tag antenna can provide a stable reading range when mounted on metal plates of various sizes.