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This article proposes a novel reflective-mode phase-variation sensor based on a movable step impedance resonator (SIR) implemented in coplanar waveguide (CPW) technology. The sensing principle is the variation of the phase of the reflection coefficient at a specific (operating) frequency generated by SIR motion. The sensor is modeled by a transmission line terminated with a series resonator connected to ground, and the reactive element subjected to motion-related variations is the SIR capacitance. The movable SIR, etched on an independent substrate, can be displaced either vertically or transversally, i.e., orthogonal to the symmetry plane of the CPW. In the first case, the device can be used as a highly sensitive proximity sensor with 10- $\mu $ m resolution. In the second case, the device can operate either as a linear or angular displacement and velocity sensor, provided that a linear or a circular chain of SIRs is etched in a movable substrate. The measurement of the (medium/long) range linear or angular displacement and velocity is based on pulse counting of the phase-modulated signal generated in the reflection coefficient by the motion of the SIR chain (transversely to the axis of the CPW). The presented prototypes, a micrometer-scale proximity sensor and a linear displacement/velocity sensor, validate the approach and point out the versatility of the proposed structure for motion sensing.

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 ${S}_{\max } = 396.7^{\circ }$ per unit of dielectric constant variation, with FoM =85652°/ $\lambda ^{{2}}$ , 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.

This article is focused on the analysis of the effects of losses on the sensitivity of reflective-mode phase-variation liquid sensors implemented by means of an open-ended (either quarter-wavelength or half-wavelength) transmission line distributed resonator (the sensing element) cascaded to a set of high/low impedance inverters (implemented by means of quarter-wavelength line sections). The device operates at a single frequency, and the working principle is the variation of the phase of the reflection coefficient (the output variable) caused by changes in the dielectric properties of the material under test (MUT), a liquid in this study. Since liquids are lossy samples, it is expected that the sensitivity of the sensor is influenced by losses, an aspect demonstrated in this article. It is shown that two operation regimes can be distinguished: 1) the low-loss regime, where the sensitivity is influenced by MUT losses, but the behavior is similar to the one of the lossless case and 2) the high-loss regime, where losses not only play a key role on the sensitivity but also modify the dependence of the output variable with the dielectric constant of the liquid under consideration compared to the lossless case. Nevertheless, contrary to general belief, with an adequate sensor design, losses might be beneficial in order to boost the sensitivity in the limit of small perturbations (this is the main contribution of this article). Validation is carried out by considering different types of oil samples and various sensing devices operating either in the low-loss regime or the high-loss regime.

Microwave sensors are typically characterized using a vector network analyzer (VNA) that measures the scattering parameters of the device under test (DUT) over a wide frequency band with great accuracy. However, these features of the VNA are not always required, and its bulky form factor and high cost can even be a limitation in many practical scenarios, such as microwave sensors, which are generally characterized by low cost, a small footprint, and often planar profiles. This article presents a novel, low-cost portable reader for reflective-mode microwave sensors. The proposed device is based on a hybrid 180° directional coupler that separates the input signal from the reflected one, a voltage-controlled oscillator (VCO) to generate the input signal, and a gain/phase detector board. The performance of the proposed system has been experimentally assessed for reference load cases, showing a good match between the simulated and measured results. The capability and usefulness of the proposed device are demonstrated through the implementation of a reflective-mode distance sensor where the input impedance of the one-port sensing element is the output variable of interest. The results obtained with the proposed reader well match the VNA measurements, with a mean error of about 8%. It is worth noting that the presented solution can be easily adopted as the measurement device by other reflective-mode sensors operating in the frequency band of the coupler. Overall, the proposed novel reader offers a new approach to measuring the output variable of one-port reflective-mode microwave sensors that is battery-powered, portable, versatile, accurate, and cost-effective compared with using a VNA.

This article presents a small-sized and highly sensitive planar microwave permittivity sensor based on a pair of weakly coupled step-impedance resonators (SIRs), the sensing elements. The sensor, a one-port structure, is implemented in coplanar waveguide (CPW) technology, and it operates in the reflection mode. The sensing principle is the variation in the phase of the reflection coefficient at the operating frequency, the output variable, caused by the changes in the dielectric constant of the material under test (MUT). This article demonstrates that the maximum sensitivity is inversely proportional to the square of the electric coupling coefficient between the sensing SIRs. Thus, the sensitivity can be unprecedentedly enhanced by merely separating enough the coupled SIRs (thereby reducing their mutual coupling). Since the sensing area is circumscribed to the region occupied by the square (capacitive) patches of the SIRs, it follows that the proposed sensor exhibits also a very high figure of merit ( ${\text {FoM} = 26\,551^{\circ} /\lambda ^{2}}$ ), or ratio between the maximum sensitivity and the size of the sensing area expressed in terms of the squared guided wavelength, $\lambda ^{2}$ .

A one-port reflective-mode microwave submersible sensor based on an open-ended step-impedance microstrip line configuration, operating at a single frequency, useful for the dielectric characterization of liquids and the measurement of liquid composition in binary mixtures, is proposed in this article. The sensor constitutes two parts, namely, the microwave module, including the sensitive region, and a 3-D-printed case (made of polylactic acid—PLA), necessary for sealing, thus preventing from any liquid leakage to the nonsensing regions of the microwave module. To enhance the sensitivity, the open-ended line Section of the microwave module, i.e., the sensing region in contact with the liquid under test (LUT), is a quarter wavelength microstrip line with high impedance. Thus, the phase of the reflection coefficient is the output variable, highly dependent on the input variable—the dielectric constant of the LUT placed directly above the sensing region. In order to validate the sensing structure, a prototype device has been designed and fabricated. The average sensitivity to variations in the concentration of ethanol in solutions of deionized (DI) water is 1.22°/%, and sensor resolution is 2%. The fabricated sensor is resistant to liquid leakage, and therefore, it can be completely submerged for measuring purposes, acting as an effective measuring probe.

This article presents a low-cost and portable phase-variation planar microwave sensor operating in reflective mode, where the output variable, the phase of the reflection coefficient of the one-port sensing element, is retrieved by means of a distributed circuit based on a pair of rat-race hybrid couplers and the associated electronics. This includes a microcontroller, a voltage controlled oscillator (VCO), a digital-to-analog converter (DAC), and an RF/microwave gain/phase detector. By this means, vector network analyzers (VNAs), i.e., high-cost instruments typically used for obtaining the reflection coefficient in one-port structures or circulators (microwave components able to separate the injected signal from the reflected signal in any one-port network) are avoided. Indeed, the presented architecture (including the microwave rat-race hybrid configuration and the associated electronics) can be adapted to other one-port reflective-mode phase-variation microwave sensors. A prototype example, where the sensing element is a one-port step-impedance open-ended transmission line, is reported. The device provides the phase of the reflection coefficient of the sensing element as a function of the dielectric constant of the material under test (MUT) with a maximum sensitivity of −65.7° and a relative error of less than 8% in all the measurements.

Highly sensitive permittivity sensors operating in reflection and exploiting phase variation are presented in this article. The sensors are one-port structures implemented by means of a pair of coupled lines, the sensitive region, with an appropriate termination. In particular, it is demonstrated that by either short-circuiting the crossed port to the input port of the device (opening the remaining two ports), or opening the crossed port to the input port (terminating the other two ports with a short-circuit), the sensitivity can be driven to very high values, controlled by the coupling factor. For that purpose, the electrical length of the pair of coupled lines must be set to 90° when such lines are loaded with the reference (REF) material. Thus, the sensitivity is optimized for dielectric constants (the input variable) in the vicinity of the dielectric constant of the REF material, where the indicated phase condition is fulfilled. The output variable is the phase of the reflection coefficient, an easily measurable quantity. For validation purposes, three prototype sensors are designed and fabricated. The achieved sensitivities in two of the fabricated sensors are as high as 659.6° and 736.0°, with figures of merit (FoMs), or ratio between the maximum sensitivity and the area of the sensing region expressed in terms of the squared-wavelength, of FoM $= 9401^{\circ }/\lambda ^{2}$ and 12690°/ $\lambda $ , respectively, i.e., very competitive values. Moreover, the indicated high sensitivities and FoMs have been achieved without the need to add further circuit stages to the sensing region, contrarily to other highly sensitive phase-variation sensors, where sensitivity optimization is achieved at the expense of an increase in the overall sensor size.