Chipless RFID Sensors Based on Multistate Coupled Line Resonators

Abstract A chipless radio frequency identification (RFID) sensor commonly consists of a tag ID and smart sensing materials. The electrical properties of these materials vary with the sensing physical parameters, such as temperature, humidity, and pressure. Novel and compact temperature sensor integrated with a chipless RFID tag without either increasing the tag size or decreasing the tag capacity is presented. For sensing purposes, a new microstrip coupled line resonator is suggested to support four state ID data and extra sensing information simultaneously. The resonator consists of a coupled line section and three lines with different-length elements (arms). The longest arm, connected at all times, is particularly designed for temperature sensing, and has resonance frequency f1′. The other two arms are used for encoding four different codes. The resonator can be structured for any of the following possible states — 100, 101, 110, and 111 — demonstrating resonance at f1′, resonance at f1′ and f2, resonance at f1′ and f1, and resonance at f1′, f1, and f2, respectively. The change in f1′ can be directly converted object temperature. A multi-resonator structure with six resonators was designed and implemented for RFID sensor applications. Two of these resonators were designed based on the suggested structure, to include temperature sensing and reference frequency. Experimental tests were performed within the temperature range 22 °C to 60 °C. The sensor exhibits a sensitivity of 742 KHz/°C. Experimental and simulation results confirmed the proposed sensing RFID integration concept.