An Inkjet Printed Chipless RFID Sensor for Wireless Humidity Monitoring

A novel chipless RFID humidity sensor based on a finite artificial impedance surface (AIS) is presented. The unit cell of the AIS is composed of three concentric loops thus obtaining three deep and high-Q nulls in the electromagnetic response of the tag. The wireless sensor is fabricated using the low-cost inkjet printing technology on a thin sheet of commercial coated paper. The patterned surface is placed on a metal-backed cardboard layer. The relative humidity information is encoded in the frequency shift of the resonance peaks. Varying the relative humidity level from 50% to 90%, the frequency shift has proved to be up to 270 MHz. The position of the resonance peaks has been correlated to the relative humidity level of the environment on the basis of a high number of measurements performed in a climatic chamber, specifically designed for RF measurements of the sensor. A very low error probability of the proposed sensor is demonstrated when the device is used with a 10% relative humidity level discrimination.