Passive RFID Sensor Tag Concept and Prototype Exploiting a Full Control of Amplitude and Phase of the Tag Signal

This paper presents a passive radio-frequency identification (RFID) sensor transponder (tag) concept that uses the antenna as the sensing device and exploits a full control of amplitude and phase of the tag signal by optimizing the tag antenna impedances in the different sensing states. This optimization is based on a sensor tag efficiency that evaluates the quality of the tag signal in terms of its amplitude for a reliable power transfer toward the passive tag chip and in terms of its phase for a transmission of the sensing states toward the RFID reader. An antenna transducer prototype for an RFID sensor tag is presented that verifies the concept by detecting three different water filling levels. The prototype achieves a high sensor tag efficiency of 92% at 915 MHz. A reliable power transfer to the passive tag chip is ensured by constant power transmission coefficients of about 90% in all three sensing states. In addition, the phase modulation quality is high and thus shows a high robustness for the detection of the sensing states at the reader. Furthermore, reasonably high tag modulation efficiencies are achieved with a minimum of 11% that ensure a reliable tag ID transmission in each sensing state.