  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
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
proven to be up to 270MHz. 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% RH humidity
level discrimination.
