Compact Ring Antennas With High-Impedance Line Loaded With Distributed Inductors for On-Metal Tag Design

Two compact ultrahigh-frequency (UHF) tag antennas, which consist of a square ring loaded with two distributed inductors, are proposed for on-metal applications. Miniaturization of the antenna footprint is realized through the incorporation of two distributed inductors. It will be shown that the required inductances are much dependent on their locations on the ring resonator due to the variation of the current intensity. Due to the high-inductive nature of the distributed inductor, the antenna resistance of the ring resonator can be significantly enhanced so that it can achieve a conjugate impedance match with the microchip. A detailed analysis has been performed to understand the resonance characteristics, and transmission line models have been constructed for designing the tag antennas. Due to the significant improvement in the impedance matching, the proposed miniature tag antennas can achieve a far read distance of greater than 9 m on metal when they are tested with an effective isotropic radiated power (EIRP) of 4 W. Important to mention is that the tag resonant frequencies of the proposed tag antennas are stable and they are not affected much by the backing metallic objects.