A Compact Chipless RFID Tag Sensor for Humidity Monitoring

A chipless humidity sensor based on frequency domain is designed in this paper. The sensor consists of an resonator based on interdigital capacitor (IDC) structure and three multi-branched U-shaped coding resonators, which coupled to a microstrip line in a cascade. The IDC structure resonance unit based on microstrip coupling is used for humidity change in the sensing environment, and the other three resonance units are used to characterize the information encoding of the tag. In this paper, the RF transfer characteristics of the humidity-sensitive polymer Kapton at microwave frequencies are explored. By simulating an IDC resonator coated with a humidity-sensitive polymer film in the Ansys simulation software, changing the dielectric constant can simulate changes in ambient relative humidity and the change in the dielectric constant of the polymer film is transmitted to the resonator resulting in a frequency shift. This paper provides a feasible solution for developing a high sensitivity, low cost, compact humidity sensor RFID tag.