An UHF RFID Circularly Polarized Tag Antenna With Long Read Distance for Metal Objects

In this letter,a circularly-polarized (CP) metal-tolerant tag antenna with a long reading distance (RD) is proposed for the ultra-high frequency (UHF) radio-frequency identification (RFID) systems. In order to realize CP radiation and high gain, a cross slot is etched on top patch to provide two orthogonal polarizations, whereas the meandered strip on the central copper layer is used to obtain a 90° phase difference between adjacent ports. An equivalent circuit model is established to better understand the operating mechanism and impedance characteristic. A prototype mounted on a 200 mm × 200 mm metal plate has been manufactured and measured to verify the design and shows an impedance bandwidth of 8 MHz (918–926 MHz) and axial ratio bandwidth of 1.3 MHz (921.6–922.9 MHz). Also, a maximum gain of 2 dBi at 922 MHz and a maximum RD of about 22.4 m have been demonstrated.