Bowtie-Shaped Folded Patch Antenna With Split Ring Resonators for UHF RFID Tag Design

A compact UHF tag antenna, which consists of a bowtieshaped folded patch antenna and two split ring resonators (SRRs), is proposed for on-metal applications. This antenna is made by using a flexible substrate and it is electrically small, having a circuit size of 40 mm × 40 mm × 3.18 mm (0.122λ × 0.122λ × 0.010λ). The SRRs introduce additional capacitance and inductance to the tag antenna's impedance for achieving a conjugate match with the chip's impedance. It is also found that the SRRs improve the radiation efficiency of the proposed tag antenna. The proposed tag antenna is able to achieve a far read distance of 9.5 m on metal when tested at EIRP power of 4 W. Most importantly, the resonant frequency of the proposed tag is stable and it is not affected much by the size and curvature of the backing metallic object.