Lijuan Su

This article presents a contactless microwave sensor useful for monitoring the purity of liquid products. The device is a one-port reflective-mode microstrip structure terminated with a patch capacitor and a dumbbell defect-ground-structure (DB-DGS) resonator (the sensitive element), transversally etched in the ground plane. A small capillary, placed on top of the DB-DGS resonator, is used to drive the liquid under test (LUT) close to the DB-DGS resonator. The sensor operates at a single (operating) frequency and uses the phase of the reflection coefficient at such frequency as the output variable, highly dependent on the complex permittivity of the LUT. The device is first validated by means of solid samples, where the high sensitivity (1891° per unit of dielectric constant) is applied to detect small defects, and then by detecting the presence of water/detergent in commercial wine. In the latter application, sensor resolution is 0.25%, and the peak sensitivity is 1298°/%, very competitive values.

This article presents a strategy for sensitivity optimization and control in reflective-mode phase-variation microwave sensors that exploits the resonance and antiresonance of an open-ended 50- $\Omega $ line terminated with a complementary split ring resonator (CSRR), the sensing element, etched in the ground plane. This structure can be described by means of a capacitor (that accounts for the electric coupling between the line and the CSRR) series-connected to a grounded parallel resonator (modeling the CSRR) with a capacitance that depends on the dielectric constant of the so-called material under test (MUT), or analyte. The structure exhibits a resonance and an antiresonance frequency. Since the phase of the reflection coefficient, the output variable, should experience a variation of 180° between such frequencies, by designing the sensor with closely spaced resonance and antiresonance frequencies, the derivative of the phase with frequency is expected to be very high between such frequencies, a sufficient condition to boost the sensitivity. In this work, it is shown that the relative separation between the resonance and antiresonance frequencies can be mechanically controlled, thereby providing an approach for sensitivity tuning. Validation is carried out by considering both nontunable and mechanically reconfigurable sensors. The maximum sensitivity in one of the reported nontunable devices is 1270°, corresponding to a figure of merit (FoM), or ratio between the maximum sensitivity and the area of the sensing resonator expressed in squared guided wavelengths, of 135 $848^{\circ } /\lambda ^{2}$ , but such sensitivity is further enhanced (5 times) in the reported tunable sensor.

This paper 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 Smax = −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 = 25917°/λ 2, very competitive values. 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 (VNA). Specifically, a phase detector for that purpose is considered in this paper. Sensor validation is carried out by detecting tiny defects (small holes) in samples and the presence of minuscule perturbations (small strips of Polyethylene terephthalate −PET) in the sensing region.

The paper presents a single-frequency planar microwave sensor designed for characterizing solid and liquid samples. The sensor operates in reflective-mode one-port configuration utilizing a microstrip line with a series inductive strip terminated with a transversely oriented dumbbell defect-ground-structure (DB-DGS) resonator (etched in the ground plane), the sensing element. The output variable is the phase of the reflection coefficient at a specific (operating) frequency. The device is modelled by a lumped circuit consisting of a parallel resonator (accounting for the DB-DGS resonator) series-connected to an inductor and a parasitic capacitor. This configuration enables the generation of a steep phase slope in the reflection coefficient at the operating frequency, which is essential for achieving high sensitivity. The sensor can be applied to the detection of small variations in the permittivity of the environment surrounding the sensing element in the vicinity of a reference (REF) value, where such REF value can be selected at wish by conveniently tuning the operating frequency. To demonstrate the versatility of the proposed device, three different scenarios are considered: (i) dielectric constant of solid samples measurement, (ii) ethanol concentration in deionized (DI) water determination, and (iii) detection of water/detergent in wine. The maximum sensitivity achieved for the dielectric characterization of solids is -382.44° per unit of dielectric constant. For liquid sensing, the sensor exhibits sensitivities of 9°/% for ethanol concentration in water and 0.88°/% for detecting water/detergent mixtures in wine. Despite the similar electromagnetic characteristics of water/detergent and wine, the proposed sensor can resolve a volume fraction (percentage of water/detergent in wine) as little as 2.5%, demonstrating high sensitivity. For the mixture of ethanol and DI water, the sensor is capable of detecting a concentration of 1% of ethanol.

This paper proposes a highly sensitive two-port single-frequency permittivity sensor operating in transmission. The device consists of a shunt-connected short-circuited quarter-wavelength resonator (the sensing element) sandwiched between a symmetric configuration of cascaded impedance inverters with alternating high and low impedance. The output variable of the sensor is the phase of the transmission coefficient at the operating frequency, highly sensitive to variations in the permittivity of the medium surrounding the sensing resonator. In the proposed two-port sensor, the feeding (harmonic) signal and the output signal do not share the same port, which facilitates retrieving the output variable in scenarios where a vector network analyzer (VNA) cannot be used. The maximum sensitivity in the reported prototype is S_max = −63.94° per unit of dielectric constant variation, whereas the figure of merit (FoM), or ratio between the maximum sensitivity and the area of the sensing region expressed in squared wavelengths, is FoM = 3384°/λ², a high value achieved by virtue of the small sensing area. The sensor is applied to the micron-scale measurement of thickness in polytetrafluoroethylene (PTFE) samples, with a sensitivity and resolution of 7074°/mm and 5 μm, respectively.

This paper presents and experimentally validates various topologies of one-port reflective-mode phase-variation permittivity sensors based on weakly coupled step impedance resonators (SIRs), the sensing elements, implemented in microstrip technology. The differences between such topologies concern the specific configuration, either series or parallel, of the SIR driven by the feeding (harmonic) signal, and the type of coupling, either electric or magnetic, between the SIRs. Thus, four different sensor topologies result. These sensors are modeled by lumped element circuits that describe their behavior to a very good approximation and are useful to predict the sensitivity in the limit of small perturbations. It is shown that the models of one of the pairs of these four sensors are circuit duals, and such circuit duality applies also to the models of the other pair. In all cases, a high (maximum) sensitivity is achievable provided the coupling (either electric or magnetic) between the SIRs pair is weak. Since the sensing regions in the reported sensors are also small, the resulting figures of merit (FoM), or ratio between the maximum sensitivity and the area of the sensing region expressed in squared guided wavelengths, are very competitive.

A new planar structure for the implementation of highly sensitive single-frequency microwave sensors is proposed. The key aspect to obtaining high sensitivity is to generate closely spaced resonance and antiresonance frequencies, achieved by means of a microstrip line with a series gap etched on top of a transversely oriented dumbbell defect-ground-structure (DB-DGS) resonator, the sensitive element. By this means, both the magnitude and the phase of the transmission coefficient exhibit a high phase slope between such frequencies. The result is a high sensitivity of either the magnitude or the phase of the transmission coefficient at the operating frequency with the dielectric constant of the environment surrounding the DB-DGS resonator. Advantages of the proposed device are (i) size and simplicity and (ii) isolation between the analyte (or material under test -MUT) and the line strip. Two sensors with different DB-DGS geometry are reported to validate the proposed approach. Using the phase as the main output variable, the maximum sensitivity in one of the prototypes is found to be 88.7$^{\circ }$ per unit of dielectric constant variation. This sensor has been applied to relative humidity (RH) measurements by using a functional film of polyvinyl alcohol (PVA) deposited on top of the sensing DB-DGS resonator. The average sensitivity in this case is 0.33$^{\circ }$/% (note that RH is given in %).

This paper presents a microwave sensor system (including the electromagnetic module and the associated electronics for signal generation and processing) useful for unattendedly monitoring the corrosion level in urban metallic infrastructures, particularly, streetlights and traffic lights. The electromagnetic module consists of a microstrip line loaded with a slot resonator, the sensitive element, transversely etched in the ground plane. To make the electromagnetic module conformal, a necessity for the intended application, the slot-loaded line has been implemented in a narrow (and hence flexible) low-loss microwave substrate. To adapt it to the circular shape of the metallic infrastructure, streetlights with different curvature shapes in the reported example cases, a conformal 3D-printed piece of polylactic acid (PLA) has been fabricated. By sandwiching the electromagnetic module between such PLA piece and the surface of the streetlight subjected to corrosion control, perfect contact of it with the sensing element is achieved. The output variable of the sensor is the magnitude of the transmission coefficient of the slot-loaded line at a specific frequency (correlated with the level of corrosion of the surface) converted to a voltage by means of an envelope detector. The functionality of the proposed sensor is validated by means of a complete system, including the associated electronics.

In recent years, the focus of microwave engineering has been on minimizing losses to improve system performance. However, recent research has demonstrated that engineered losses can enhance the performance of microwave components. This paper explores the application of loss engineering to reflective-mode phase-variation microwave sensors, showing how resistive losses can be utilized to achieve unprecedented sensitivity. These sensors, designed to detect small environmental variations, hold great promise for applications in sectors such as agriculture and food processing, healthcare, and civil infrastructure monitoring. By leveraging loss as a deliberate design parameter, this approach repositions losses from a limitation to a key element in enhancing sensor functionality and achieving new levels of measurement precision.

Typically, the operating frequency in single-frequency reflective-mode phase-variation permittivity sensors based on semi-lumped resonators (e.g., step-impedance resonators—SIRs) is set to the resonance frequency of the sensing resonator when it is loaded with the so-called reference (REF) material, f_0,REF. For the case of an SIR-based sensor, if the ratio between the inductance and the capacitance is high (corresponding to a high-Q resonator), the sensitivity in the limit of small perturbations of the dielectric constant (in the vicinity of that of the REF material) is also high. However, the optimum frequency for sensitivity optimization in the limit of small perturbations neither corresponds to the resonance frequency nor coincides with the frequency of maximum phase slope. Such frequencies are calculated in this paper, and it is shown that the optimum frequency for sensitivity optimization is located between the frequency of maximum phase slope and the resonance frequency, although such frequencies tend to coincide for high-Q sensing resonators. This aspect is validated in this paper from electromagnetic simulation and experiment.