Phase-Oriented sensing by means of loaded UHF RFID tags

Radio frequency identification (RFID) tags are currently evolving from an augmented version of barcodes to passive and distributed sensors in the emerging Internet of Things. Most of the RFID sensing devices that have been experienced till now are based on power metrics. The change in the environment is converted into a variation of the antenna performance remotely sensed by a reader through a modulation of turn- on or backscattered power. This paper investigates a new sensing paradigm derived from the measurement of the phase of the electromagnetic signals that are backscattered from the tag in the UHF-RFID band. The main goal is to introduce a model of the phase response of antennas that are loaded by lumped sensitive materials (SMs). General properties, data-processing issues, the achievable dynamic ranges, sensitivities, and the measurement reproducibility of phase-oriented sensing are investigated by means of both numerical analysis and experimentations concerning an RFID humidity sensor as case study.