Material Sensing Using Arrays of Open Dipole RAIN RFID Tags

Abstract

This work demonstrates how open dipole (OD) RAIN RFID tags and tag-arrays can be used for material characterization in a standard compliant way. Unlike T-matched antennas, OD designs exhibit reduced resonance complexity, with a single minimum in the power-on-tag-forward (POTF) and a single maximum in the power-on-tag-reverse (POTR) response. We introduce a setup-invariant metric based on the transmitted and received reader powers at tag threshold, which preserves resonance information while eliminating the need for calibration. In addition, we demonstrate that auto-tune (AT) code changes in self-tuning ICs of OD tags provide clear and narrowband min-max transitions that remain stable over a wide range of transmit powers, providing high tuning flexibility and enabling sensing at fixed reader power. Building on this, we propose a novel approach for material sensing, leveraging the AT-code transitions within the narrow UHF RFID band using arrays of individually tuned OD tags. We demonstrate the repeatability of open dipole-based sensing using 6 commercial OD RAIN RFID tags to estimate the effective dielectric permittivity of 7 materials, obtaining lower variance than T-matched antennas for high $\varepsilon _{r}$ dielectrics. Furthermore, we propose a custom three OD tag array capable of discriminating between four specific materials and air within the UHF band, demonstrating the sensing capability of this new method in compliance with regulatory standards.