E-Shaped Folded-Patch Antenna With Multiple Tuning Parameters for On-Metal UHF RFID Tag

A semiflexible E-shaped folded patch with a compact footprint of 30 mm × 30 mm x 3 mm (0.091λ × 0.091λ × 0.0091λ at 912 MHz) is proposed for designing a miniature ultrahigh frequency tag for on-metal applications. The proposed tag antenna is simple in structure, and it has eight design parameters to achieve different levels of tuning. By changing the parameters, coarse and fine tunings can be easily performed to vary the tag's resonant frequency. In spite of its compactness, the proposed tag antenna is able to provide sufficient resistance and inductive reactance to enable conjugate match with the chip. Due to conjugate match, the proposed tag antenna has achieved a power transmission coefficient of ~1 and a maximum read distance of 14.5 m (with 4 W equivalent isotropic radiated power) on metal even it is electrically small. It has a radiation efficiency of 53%.