Microwave Sensing Schemes of CPW Resonators Fully Printed on Humidity Sensitive Substrates

This letter investigates the humidity sensing schemes of the transmission line-based resonators printed on the hygroscopic sheets. A serial resonator in the coplanar waveguide technology was designed, simulated, screen-printed with only one silver layer on a Kapton HN foil, and tested under variable humidity conditions. Sensing schemes in frequency and phase paradigm for integration within resonant or transmitting microwave systems were proposed and experimentally validated. The resonant frequency and S
₂₁
phase were used as the measurement variables. The sensor was proven highly sensitive and wide range. In the 60-100 %RH range, the frequency sensitivity of the sensor reached 3.88 MHz/%RH, and its phase sensitivity reached 1.25°/%RH.