Design of a Three-Dimensional Uniform UHF Near-Field RFID Reader Antenna

This paper proposes a three-dimensional uniform ultra-high frequency (UHF) near-field radio frequency identification (RFID) reader antenna. The antenna achieves a uniform electric field in the x and y directions by placing a single branch microstrip line along the x-axis and y-axis directions, respectively. It reaches a uniform electric field in the z-direction by a centrosymmetric four-branch microstrip line. The proposed antenna achieves three-dimensional direction uniformity through a reconfigurable method. The impedance matching bandwidth range of S-11 <-10 dB for simulation and measurement includes 0.66 to 0.98 GHz, which can meet the near-field RFID operation frequency band demand. The isolation degrees between ports are less than -24.6 dB within the UHF RFID frequency band (0.86 to 0.96 GHz). In addition, the antenna also has the characteristic of low gain in the far field, and the maximum gain in the far field is less than -27 dBi when operating at different ports. The test results show that the proposed antenna three-dimensional uniform volume of dipole tags above the antenna is 99 mm x 99 mm x 20 mm, and the reading volume of the near-field tags is 40 mm x 40 mm x 5 mm. When the tags are placed on a book, there will be a slight variation in the reading range of the tags.