Compact RFID Tag Antenna With Circular Polarization and Embedded Feed Network for Metallic Objects

A low-profile radio frequency identification (RFID) tag antenna is proposed with a compact size of 45 ×45 mm² and circular polarization (CP) for metallic surfaces. A star-shaped slot is etched onto the antenna for the CP and antenna size reduction. A terminal-grounded L-shaped feedline is embedded in the etched slot to efficiently excite the wideband CP mode, complexly match the tag-antenna impedance, and further reduce the antenna size. The embedded feed structure also helps achieve a relatively higher gain, resulting in a longer reading range. The proposed antenna can cover the impedance bandwidth of 60 MHz (from 880 to 940 MHz) as well as the 3-dB CP bandwidth of 12 MHz (from 905 to 917 MHz). Further results demonstrate that the proposed tag antenna provides a stable reading range when mounted on metal plates of various sizes.