A Low Profile Two‐Sided Anti‐Metal UHF RFID Tag Antenna Based on Square–Ring Embedded Coupling Structure

ABSTRACT A compact ultra‐high frequency (UHF) radio frequency identification (RFID) tag antenna for two‐sided anti‐metal performance is proposed. The proposed tag antenna consists of a lower bottom patch, a middle patch and an upper top patch, separated by two layers of foam substrate. Owing to its three‐layer design, the current paths and field distribution across the three layers of the patch are almost identical, which enables the tag antenna’s two‐sided anti‐metal properties. The middle patch includes an inner meandered microstrip line, an outer open square ring patch, and two embedded stubs, which can miniaturise the antenna and improve the impedance matching between the antenna and the tag chip. The tag antenna has a compact structure with dimensions of 38 mm × 20 mm × 1.15 mm (0.1159 λ × 0.061 λ × 0.0035 λ at 915 MHz). When the proposed tag is placed on the 200 × 200 mm 2 metal plate and the effective isotropic radiated power (EIRP) is fixed at 3.28 W, the test results show that the read distance can reach 5.6 and 5.8 m, when bottom patch and top patch, respectively, are mounted on metal plate. The proposed compact, two‐sided anti‐metal tag antenna is particularly suitable for metal‐mounted applications where the tag is hung freely, eliminating the need to distinguish which side faces the metal surface.