Folded Patch Antenna With Tunable Inductive Slots and Stubs for UHF Tag Design

A miniature folded patch with a dimension of 40 mm × 40 mm × 1.6 mm (0.1156λ × 0.1156λ × 0.0046λ at 867 MHz) is proposed for designing a passive metal-mountable tag. It consists of a square patch which is equally separated into two rectangular patches. Since the patch resonator usually has high resonant frequency, multiple tuning mechanisms are always needed to bring down the frequency so that it can be used for designing a tag antenna in the UHF band. In the proposed design, each patch is incorporated with a C-shaped slot and three thin inductive stubs for performing different levels of frequency tunings. An equivalent circuit has been built for estimating the input impedance of the antenna. Reasonable agreement is found between the measured, modeled, and simulation results. The proposed tag antenna is readable from 5 m with an EIRP of 3.28 W when placed on a 20 cm x 20 cm metal plate.