Printed Microwave Frequency Humidity Sensor Operating With Phase Shifting Scheme

This paper investigates a shifting sensing scheme combining slots, transmission lines, and printing technologies. This sensing scheme translates the electrical sensitivity of a transmission line conductor to the insertion phase as a measurement variable. A coplanar waveguide (CPW) based structure was designed, screen-printed, and tested on relative humidity (RH) conditions ranging from 22.8 - 75.3 %RH. For the first time, a composite material made of poly-pyrrole and TEMPO Oxidized Cellulose Nanofibers (TOCN/PPy) was integrated to the structure and studied as a humidity sensitive conductor in microwave frequencies. The measured sensitivity was 0.154°/%RH at 5.870 GHz, while insertion losses decreased by 1.26 dB. The effects of sensing layers thickness as well as trade-off considerations between phase sensitivity and signal attenuation were analyzed by simulation.