RF Humidity Sensor Implemented with PEI-Coated Compact Microstrip Resonant Cell

This paper presents a novel application of RF Compact Microstrip Resonant Cells (CMRC) coated with polyethylenimine (PEI) to form a high performance RF relative humidity (%RH) sensor. The single-port, short-circuit terminated CMRC sensor cell is coated with 5 uL of 10 w.t.% PEI diluted in methanol, and is coupled to the ultra-micro coaxial connector (UMCC) through a direction transmission line. The PEI coating coming in contact with humid air undergoes changes in both physical volume and dielectric constant, thereby shifting the resonant frequency of the CMRC sensor. The return loss $(\mathrm{S}_{11})$ measured near the resonant frequency of the sensor has seen an average humidity sensitivity of −16.2 mdB / %RH, with great linearity across the full range of relative humidity levels. All measurements are done at atmospheric temperature and pressure (ATM).