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

Abstract

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.

Techniques to Improve the Performance of Planar Microwave Sensors: A Review and Recent Developments

“…For CSRR-based sensors, the ratio C /C that appears in (11) can be evaluated by isolating C and C from (7), however, the resulting expression is not as simple as (12). In this case, it is found that C /C is (13) and the dielectric constant of the MUT is inferred by introducing (13) in (11). Another important aspect is the determination of the equivalent dielectric constant of the substrate.…”

“…By measuring, or simulating, the response of the bare sensor, and the one of the sensor loaded with that MUT, ε r,eq , can be isolated from (12). If, in contrast, a CSRR is the sensing element, in this case, (13) and ( 11) must be used. The experimental validation of frequency-variation permittivity sensors based on different resonant elements, such as CSRRs, SRRs or DB-DGS resonators, has been reported in the literature [18,58].…”

“…It is found that, compared to other methods, microwave devices bring about highly sensitive regions to the surrounding medium including solid, liquid, and even gaseous compositions [9,10]. The interaction of EM waves with material, even remotely, is intensified in the desired regions, which makes them useful for applications that require distant sensing [11][12][13]-specifically in applications involving noninvasive near-surface measurements such as soil moisture sensing using time-domain reflectometry [14,15]. Contact sensors reduce their lifespan which adds to their regular maintenance cost while being continuously used, or increases the supplement cost while carrying consumable and disposable parts [16,17].…”