Compact Folded Dipole With Embedded Matching Loop for Universal Tag Applications

A coin-shaped folded dipole is proposed for designing a UHF tag that can be placed in free space and on metal. The proposed tag antenna is compact ($0.095\lambda )$ and low in profile (1.6 mm). It is composed of a matching loop, which is encircled by two patch-shaped radiating arms, forming an embedded structure that has high compactness. Slots are etched in the radiators for fine-tuning the resonant frequency. An equivalent circuit has also been obtained for analyzing the impedance characteristics of the tag antenna across the frequency range, and it is found that the radiating element itself is capacitive and inclusion of the matching loop is essential for making the tag antenna inductive for achieving good matching with the chip. Simulation and experiment have been conducted to study the charateristics of the proposed tag antenna. When tested in free space with an Effective Isotropic Radiated Power (EIRP) input power of 3.28 W, the tag antenna is able to reach a maximum read range of 5.2 m, but it reduces to 1.8 m when placed on metal. Also, the resonant frequency of the proposed tag antenna is found to be stable and not affected much by its backing object.