Multipolarized Reader Antenna With Periodic Units Based on Electric Field Coupling for UHF RFID Near-Field Applications

Abstract

This paper presents a multipolarized reader antenna with periodic units based on electric field coupling for near-field applications of ultra-high frequency (UHF) radio frequency identification (RFID). The antenna units are composed of two branches that are mirror-symmetric with respect to the ${x}$ -axis and produce ±45° linear polarization. A 90° phase difference is produced between the upper and lower branches by introducing a phase shifter into the upper branch, so that the near-field electric fields excited in the horizontal plane exhibit a quasi-circular polarization distribution. The proposed design is validated by simulations and experiments using prototype 4-unit and 6-unit antennas. The proposed antennas can generate strong and uniform electric fields and provide reading volumes of 400 mm $\times \,\,320$ mm $\times \,\,300$ mm and 560 mm $\times \,\,320$ mm $\times \,\,300$ mm for the 4-unit and 6-unit antennas, respectively, regardless of the orientation of the tags in the horizontal plane. In addition, the far-field gain is low with a maximum of −6 dBi, thus the misreading of tags outside the reading region is avoided. The proposed antenna achieves impedance matching over the Chinese UHF RFID band (920–925 MHz) and therefore meets the application requirements of near-field RFID reader antennas.