Gain Enhancement of Ground Radiation Antenna for RFID Tag Mounted on Metallic Plane

A novel design for a radio-frequency identification (RFID) tag antenna mounted on a metallic plane is presented. The tag antenna has a rectangular radiating plate with a Z-shaped slot, which enables it to function like a half-wavelength slot resonator and a dipole-type ground radiator. A conjugate match to an Alien Higgs-3 chip with an impedance of 8 -j142 Ω at 925 MHz is obtained by etching a half-wavelength slot resonator in the center of the rectangular radiating plate (120 mm × 60 mm). The tag antenna resistance and inductive reactance can be matched to the desired chip impedance by simply tuning the length and width of the Z-slot resonator. The space between the half-wavelength resonant radiating plate (dipole-type ground radiator) and the metallic plane enhances the antenna gain by increasing the reflection from the metallic plane. A horizontal electric dipole placed above the perfectly flat electric metallic plane enables a maximum gain of 8.7 dBi over a short distance, which is fourfold higher than that of an isolated half-wavelength dipole. Tests of an RFID reader with an effective isotropic radiated power of 4.0 W showed that the maximum read coverage of the proposed tag antenna mounted on a metallic plane exceeded 10 m. Measurement data were in good agreement with the simulation results.