Abstract A low-profile ring antenna (39 mm × 39 mm × 1.38 mm), which is loaded with a circular patch at the central region for frequency tuning purposes, has been proposed for anti-metal applications that operate in the Lower European (LEu) and the North American (NA) UHF-RFID passbands. The antenna itself consists of a square ring as well as a hexagonal ring, all of which can be fully made on a single-layer substrate, and they are concentrically stacked for generating a stable broadside radiation pattern on metal surface. The dimensions of the rings can be adjusted to achieve efficient frequency tuning capability, with conjugate impedance matching over the two operating frequencies. The proposed tag antenna is simple and it does not require the use of other complex tuning mechanisms. Independent frequency tuning is possible in the two passbands. With 4W Effective Isotropic Radiated Power (EIRP), it can achieve long read ranges of 8 m and 9 m, respectively, over the LEu and NA UHF-RFID passbands. A detailed analysis was conducted to understand the resonance characteristics, and a Transmission Line (TL) model was constructed to facilitate the design of the tag antenna. It is important to highlight that the two tag resonant frequencies are stable and not much affected by the presence of the backing metallic object.
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Murugesh et al. (2024) studied this question.
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