Miniature Dipolar Patch Antenna with Nonresonating Ring for Metal-Insensitive UHF RFID Tag Design

Abstract

A compact tag antenna, which has a geometrical dimension of 20 mm $\times18$ mm $\times1.7$ mm ( $0.0615\lambda \times 0.0553\lambda \times 0.0052\lambda$ ), is proposed for metal-insensitive applications in the UHF radio frequency identification (RFID) band. The antenna structure comprises of a pair of dipolar patches, which are loaded with a non-resonating ring underneath. Changing the ring size is found to be able to tune the tag antenna’s resonant frequency and its impedance effectively. Capacitive and inductive effects introduced by the loading ring are useful for lowering the tag’s resonant frequency down to the useful UHF RFID band. The proposed tag is able to achieve more than 80% of power transmission coefficient, where the antenna impedance can match to the chip impedance quite well. Measurements show that the proposed tag antenna is readable from 6.62 m on metal with an effective isotropic radiation power (EIRP) power of 4 W. The tag’s resonant frequency is insensitive to changes in the backing object, which is advantageous for designing a stable RFID tag.