A planar microwave resonator with odd resonance for calibration in permanent moisture sensing applications

Applied Physics Letters, Volume 118, Issue 14, April 2021. A cylindrical dual-mode planar microwave ring resonator for a permanent moisture sensor is presented. The ring is design for TMn10 fundamental (odd-mode) resonance, and optimized to sustain stability (suitable for calibration reference against environmental interference) using Ansys high-frequency structure simulator. An even-mode resonance is perturbed by etching a step-impedance stub on the upper 0.5λ ring. The equations for the TMn10 mode resonance and its mode splitting are stated. The equivalent circuits of the ring, odd-, and even-modes are derived. The even-mode is set to capture the response of the permanent moisture withholding hydrophilic polyvinyl alcohol (PVA)–polyaniline (PANI) 0.26 mm-thick films. The in-lab made films are produced by infusing certain concentrations of PANI into pure PVA. On-spot measurements using a vector network analyzer (VNA) and time-based continuous data acquisitions using LabVIEW on NI PXIe–1075 VNA are done to confirm repeatability of the obtained results. A measured permanent moisture response of 180 MHz is confirmed for 50% PANI with a thickness (hfilm) of 0.55 mm. The sources of permanent moisture withholding capability of the films are investigated. Structural observation of the residue films, its permittivity changes, and morphological structural imagery confirmed the sources of permanent response.