A Novel Anti-Interference Encapsulated 3-Bit Coded RFID Humidity Sensor Based on Bent Microstrip

This article proposed a 3-bit coded chipless radio frequency identification (RFID) humidity sensor that is anti-interference encapsulated. The proposed sensor is fed through a bent microstrip, and the sensing tag includes three split ring resonators (SRRs) as encoding bits and an interdigital-capacitor (IDC) structure covered with Kapton film as the humidity sensor, fabricated in a Rogers RO3003 dielectric substrate. A planar ultra-wideband (UWB) antenna with an operating frequency range of 1.88–6.04 GHz is proposed to be integrated with sensors as receiving and transmitting antennas. The sensor’s humidity detection capability is attributed to the enhanced design of the IDC-sensing element, which is covered with a Kapton film whose dielectric constant varies with humidity, causing a change in the electromagnetic field, so that the sensor is capable of detecting small variations in the electromagnetic properties that characterize different humidities. In addition, humidity-insensitive polydimethylsiloxane (PDMS) is used as the encapsulating layer to minimize the interference of humidity on the coding bits and improve the life cycle. The performance of the fabricated sensor was verified using a vector network analyzer (VNA). The VNA recorded a clear frequency shift response at 23%–90% relative humidity (RH), with a humidity sensitivity of 396 kHz/%RH. Experiments were conducted to validate the repeatability, response time, and recovery time of the sensor measurement. The response time is about 60 s, and the recovery time is about 80 s. Through multiple experiments, it has been confirmed that the measurement accuracy can be maintained within the 30%RH–80%RH range. In addition to having an anti-interference encapsulated, the proposed humidity sensor also has several other advantages, including small size, simple fabrication, and low cost. The use of chipless RFID technology to achieve long-term humidity monitoring is of great significance in structural health monitoring (SHM).