Transmission-Mode Phase-Variation Sensor Based on Weakly Coupled Distributed Resonators and Phase Detector for High-Sensitivity Applications

This letter presents a highly sensitive single-frequency permittivity sensor based on a pair of weakly coupled quarter-wavelength resonators. The sensor is a two-port structure operating in transmission, with the pair of coupled resonators shunt connected to the host line, and the output variable is the phase of the transmission coefficient at the operating frequency, i.e., the pole frequency. The maximum sensitivity of the reported prototype is <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> = −593.5° per dielectric constant, and the figure of merit, or ratio between the maximum sensitivity and the area of the sensing region expressed in terms of the squared wavelength, is FoM = 25 917°/<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ</i><sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup>, which is a very competitive value. By operating in transmission, the main advantage of the proposed sensor over reflective-mode phase-variation sensors based on coupled resonators is the separation of the input (feeding) signal and the output (transmitted) signal at different ports. This facilitates retrieving the phase information in a real scenario, i.e., without the use of a vector network analyzer. Specifically, a phase detector for that purpose is considered in this letter. Sensor validation is carried out by detecting tiny defects (small holes) in samples and the presence of minuscule perturbations (small strips of polyethylene terephthalate) in the sensing region.