Highly Sensitive Phase-Variation Microwave Sensor Operating in Transmission Based on an Open Split Ring Resonator (OSRR)

This article presents a sensor able to detect small changes in the dielectric constant of its surrounding with regard to a reference (REF) value. The sensor operates in transmission, is fed by a single-frequency (harmonic) signal, and uses the phase of the transmission coefficient at that frequency as the output variable. The sensitive element is an open split ring resonator (OSRR), an electrically small particle ideal for applications that require a high figure of merit (FoM), or ratio between the maximum sensitivity and the area of the sensing region expressed in terms of the squared guided wavelength. It is shown in this article that the sensitivity and the FoM can be significantly boosted up by merely cascading inverter stages with alternating high and low impedance at both sides of the OSRR. In one of the fabricated prototypes, with two inverter stages at both sides of the OSRR, the maximum sensitivity is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S}_{\max } = 396.7^{\circ }$ </tex-math></inline-formula> per unit of dielectric constant variation, with FoM =85652°/<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda ^{{2}}$ </tex-math></inline-formula>, very competitive values. Such an unprecedented FoM is in part achieved by virtue of the extremely small size of the OSRR. The sensor can be of interest in applications requiring the measurement of small changes in variables related to the dielectric constant, such as solute concentration in liquids, proximity, and defect detection.