Transmission-Mode Phase-Variation Planar Microwave Sensor Based on a Step-Impedance Shunt Stub for High Sensitivity Defect Detection, Dielectric Constant, and Proximity Measurements

Abstract

This article proposes a highly sensitive planar microwave sensor able to detect tiny changes in the dielectric constant of the medium surrounding the sensing element or other variables related to it. The sensor is a two-port structure consisting of a step-impedance shunt stub made of a cascade of high/low-admittance inverters (implemented by means of quarter-wavelength transmission line sections) terminated with an open-ended quarter- or half-wavelength resonator (the sensitive element). The sensor operates in transmission, is fed by a harmonic (single-frequency) signal, and uses the phase of the transmission coefficient at the frequency of the feeding signal as the output variable. The achieved sensitivity in the fabricated prototype, in the limit of small perturbations, is $S {_{\max }} = - 227.9 {^{\circ}}$ per unit of dielectric constant variation, and the figure of merit (FoM), defined as the ratio between the maximum sensitivity and the area of the sensing region expressed in guided squared wavelengths, is FoM = 7912°/ $\lambda ^{2}$ . The sensor is the transmission-mode counterpart of previously reported phase-variation sensors based on one-port reflective-mode structures made of a cascade of inverters terminated with a sensing resonator. The advantage of the proposed sensor over those reflective-mode sensors is the fact that the feeding (input) harmonic signal and the (output) signal containing the relevant (phase) information are present at different ports. This fact eases the measurements in applications where, rather than a vector network analyzer (VNA), a phase detector must be used.