We present a chipless RFID for relative humidity sensing. It consists of three spiral resonators coupled to a 50 Ω microstrip line. One resonator is used for humidity sensing, and the other two are used for encoding ID. The sensing resonator is coated with a humidity sensitive polymer film. As the relative humidity changes the permittivity of the polymer film varies changing the resonant frequency of the sensing resonator. Results show that a dedicated resonator can sense relative humidity over 21–53% range with 2.5 MHz/%RH sensitivity whereas the other two resonators can represent the ID. The sensor does not require any IC and is amenable to low-cost production using printed electronic technology.
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