Exploitation of chipless RFID technology for humidity monitoring

A novel ink-jet printed chipless RFID humidity sensor is presented. The sensor, based on a finite Artificial Impedance Surface (AIS), is ink-jet printed on a thin sheet of Mitsubishi paper, which is placed on a metal-backed cardboard layer. The frequency response of the AIS provides three deep nulls thanks to the three nested loops that constitute the unit cell of the periodic surface. The interaction between the humidity and the sensor results in a variation of the electric permittivity of the Mitsubishi paper and, consequently, in a frequency shift of the resonance peaks. The humidity sensor exhibits a frequency shift of the resonance peaks of 270MHz with a humidity level variation from 50% to 90%. The reliability of the sensor is confirmed by the high number of measurements performed in an ad-hoc climatic chamber.