Passive Chipless RFID Tags for Humidity Sensing: A Review

Abstract

This review paper presents comprehensive literature on passive chipless Radio Frequency IDentification (RFID) humidity sensor tags, including working principle, performance measures, and use cases. The RFID humidity sensor tag comprises of RF resonators for identification and separate resonators along with smart material for humidity sensing. The humidity sensor tags are effective permittivity-based sensors, where the smart material’s effective permittivity changes with humidity. The change is recorded with different coding methods, whereas the sensitivity depends on both tag configuration and sensing material. Therefore, this paper emphasizes state-of-the-art tag configurations, approaches to designing RFID humidity sensor tags, and the properties of the humidity-sensitive material. Most of the RFID humidity sensors are designed based on the frequency coding method where coding capacity depends on the number of resonators, hence the overall size of the tag. Therefore, a variety of resonators like patch- and slot-types, along with their orientation with respect to linearly polarized reader antenna are discussed. The advantages of slot-type tags coated with humidity-sensitive material are highlighted in terms of dual-side readability, polarization-independent nature, higher data coding capacity, and better sensitivity. RFID humidity sensors will be beneficial for future Internet-of-things (IoT) applications.