A novel relative humidity sensor based on microwave resonators and a customized polymeric film

This paper reports a new relative humidity (RH) sensor for monitoring operating environment of a radio frequency (RF) system. The proposed RH sensor is based on two different microwave resonators in its structure, a coplanar waveguide (CPW)-to-slotline ring resonator and a microstrip patch antenna, and a customized polyimide film for humidity sensing material. To integrate a humidity sensor without making a RF system complex, first harmonic resonant frequency of the resonators are designed to be 3.375 and 5.0 GHz for the CPW-to-slotline ring resonator and the microstrip patch antenna, respectively. For high sensitivity of the polyimide film to RH change, it was synthesized from oxidianiline (ODA), m-pyromellitic dianhydride (m-PMDA), and N-methly-2-pyrrolidone (NMP) in which any hydrophobic elements are not included. This results in enhancement of water sorption characteristic of the polyimide film which serves as a dielectric layer of the microwave resonators. Calibration of the microwave RH sensors was carried out in terms of sensitivity, hysteresis and stability. These characteristics were analyzed through change in first harmonic resonant frequency of the resonators against various RH levels. Sensitivity of the RH sensor using the CPW-to-slotline ring resonator was −181 kHz/%RH and 4.95 mdB/%RH in terms of its resonant frequency and insertion loss, respectively. Hysteresis was 0.0013%, and average-percent-deviation from an average resonant frequency was 0.002%. Sensitivity of the antenna RH sensor was −108 kHz/%RH and 5.50 mdB/%RH in terms of resonant frequency and return loss, respectively.