Planar Compact Square-Ring Tag Antenna With Circular Polarization for UHF RFID Applications

This paper describes a novel compact design of a planar circularly polarized (CP) tag antenna for use in a ultrahigh frequency (UHF) radio frequency identification (RFID) system. Introducing the meander strip into the right-arm of the square-ring structure enables the measured half-power bandwidth of the proposed CP tag antenna to exceed 100 MHz (860-960 MHz), which includes the entire operating bandwidth of the global UHF RFID system. A 3-dB axial-ratio bandwidth of approximately 36 MHz (902-938 MHz) can be obtained, which is suitable for American (902-928 MHz), European (918-926 MHz), and Taiwanese UHF RFID (922-928 MHz) applications. Since the overall antenna dimensions are only 54 x 54 mm(2), the proposed tag antenna can be operated with a size that is 64% smaller than that of the tag antennas attached on the safety glass. With a bidirectional reading pattern, the measured reading distance is about 8.3 m. Favorable tag sensitivity is obtained across the desired frequency band.