Development of Multi-Resonance Chipless RFID Tag for Moisture Sensing

Most chipless RFID sensors rely on detecting a change in a single peak frequency, making them susceptible to interference such as frequency-selective noise and fading. This paper presents a multi-resonance chipless RFID moisture sensor that tolerates selective frequency interference by distributing sensing across multiple frequencies within the operating band. Electric-LC (ELC) resonators and interdigital capacitor (IDC) loaded ELC resonators are investigated as the sensing elements of a 4-bit chipless RFID sensor. Both sensors with ELC and IDC structures are tested by analyzing their responses to the presence of water in the test subject, ranging from 0 to 0.5 ml. The results show that, for a single bit response, the ELC structure is highly sensitive to water, but the magnitudes are sharply diminished, whereas the less sensitive response of the IDC loaded structure shows better linearity. The shift in the resonant response of the 4-bit sensors can be observed up to the presence of 0.1 ml of water. Beyond this point, the responses fluctuate, making these sensors best suited for early moisture detection applications.