On-Metal UHF Tag Antenna Design Using Concentric Step-Impedance Rings

A compact planar tag antenna, which is designed using two closely placed concentric step-impedance rings, is presented for metal-mountable applications in the UHF RFID passband. The tag antenna can be easily constructed using a flexible polyimide and a foam substrate with a dimension of 40 mm × 40 mm × 3.32 mm (0.122λ × 0.122λ × 0.010λ). Narrowing the line width of the rings are found to be useful for increasing the antenna resistance and reactance, and a conjugate matching condition is achievable between the antenna and the microchip. The proposed tag antenna can read a maximum read distance of ~15 meters with an effective isotropic radiated power of 4 W. In this design, the two intercoupled stepped-impedance rings are cascaded concentrically. An inductive stub is utilized for bringing down the tag resonant frequency. A transmission line model has been constructed to design the tag antenna, and a detailed analysis has been performed to understand the resonance properties. It is important to note that the proposed tag antenna's operating frequency is stable, and it is not easily altered by the backing metallic object.