Compact Circularly Polarized RFID Tag Antenna with an Embedded U-shaped Feedline for Metallic Surfaces

A passive radio frequency identification (RFID) tag antenna with circular polarization (CP) radiation is proposed to operate at 902-928MHz and mount on metallic surfaces. A U-shaped terminal short-circuited feedline welded to a tag-chip is embedded in a rectangular slot to achieve a coupled feeding and a good impedance matching. By employing two truncated corners and two L-shaped slots with unequal length on the patch, a good left-hand circular polarization (LHCP) and compact size are obtained. The measured impedance bandwidth of the antenna is 85MHz (875-960 MHz), while the simulated 3-dB CP bandwidth is 6MHz (912-918 MHz). The proposed tag antenna shows reasonable read ranges when mounted on the metal plate in the following experiment.