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In this paper, an inductively coupled capacitively loaded ring resonator (IC-CLRR)-based microwave resonant sensor has been proposed for the accurate identification of any unknown liquid sample and its permittivity estimation. The key element of this work is the sensor's wide range capability towards the non-invasive testing of liquids covering a wide dielectric range of liquid samples, i.e., εr = 2 to 80. The proposed microwave sensor is etched over the FR-4 substrate and is excited by the microstrip line through inductive coupling. The placement of an unknown liquid sample in close proximity to the sensor alters its natural resonant frequency due to a change in effective inductance and capacitance as per the dielectric property of the liquid sample. Further, a mathematical formulation using curve fitting has also been derived. The measurement results show a good accuracy in estimating the permittivity and, thus, the unknown liquid identification capability of the designed sensor with a very low error (nearly 5%). This sensor design is simple to fabricate, cost-friendly, and small in size. [ABSTRACT FROM AUTHOR]

Purpose: The purpose of this study is to introduce a new type of sensor which uses microwave metamaterials and direct-coupled split-ring resonators (DC-SRRs) to measure the dielectric properties of solid materials in real time. The sensor uses a transmission line with a bridge-type structure to measure the differential frequency, which can be used to calculate the dielectric constant of the material being tested. The study aims to establish an empirical relationship between the dielectric properties of the material and the frequency measurements obtained from the sensor. Design/methodology/approach: In the proposed design, the opposite arm of the bridge transmission line is loaded by DC-SRRs, and the distance between DC-SRRs is optimized to minimize the mutual coupling between them. The DC-SRRs are loaded with the material under test (MUT) to perform differential permittivity sensing. When identical MUT is placed on both resonators, a single transmission zero (notch) is obtained, but non-identical MUTs exhibit two split notches. For the design of differential sensors and comparators based on symmetry disruption, frequency splitting is highly useful. Findings: The proposed structure is demonstrated using electromagnetic simulation, and a prototype of the proposed sensor is fabricated and experimentally validated to prove the differential sensing principle. Here, the sensor is analyzed for sensitivity by using different MUTs with relative permittivity ranges from 1.006 to 10 and with a fixed dimension of 9 mm × 10 mm ×1.2 mm. It shows a very good average frequency deviation per unit change in permittivity of the MUTs, which is around 743 MHz, and it also exhibits a very high average relative sensitivity and quality factor of around 11.5% and 323, respectively. Originality/value: The proposed sensor can be used for differential characterization of permittivity and also as a comparator to test the purity of solid dielectric samples. This sensor most importantly strengthens robustness to environmental conditions that cause cross-sensitivity or miscalibration. The accuracy of the measurement is enhanced as compared to conventional single- and double-notch metamaterial-based sensors. [ABSTRACT FROM AUTHOR]

In this paper, a microwave sensor with a meandered microstrip structure is introduced to measure the permittivity of the various liquids. The performance of the sensor is based on the change of operating resonance frequency during the absence and presence of materials in the sensing area. In this study, liquids including ethanol, methanol, aqueous glucose solution, and deionized water have been selected as samples under test. The proposed sensor has been implemented on RO4003 substrate with dimensions of 35 × 20 × 0.508 mm3 and operating frequency of 6.21 GHz in a free load state with a Q-factor of 506. When the samples are present in the sensing area, the frequency of the sensor shifts from 5.42 to 3.27 GHz. The sensitivity of 0.64% is obtained for the presented sensor. Compact size, linearity of changes, and low cost are the most important advantages of the sensor. The introduced sensor can be used in various fields of industrial, medical, and diagnostic due to its performance. [ABSTRACT FROM AUTHOR]

Maintaining engine oil quality is critical for industrial machinery and transformers to ensure efficient operation and reduce the risk of failure. This paper presents a novel application of an X-band microwave sensor, using a third-order electroformed iris waveguide filter, for real-time, non-invasive engine oil quality monitoring. By detecting shifts in the dielectric properties of engine oil, the sensor accurately tracks oil degradation as it ages. Unlike traditional methods that require oil extraction and laboratory analysis, this sensor enables continuous, in-situ monitoring, providing immediate feedback without disrupting system operations. Any change in oil quality causes a frequency shift, as variations in its dielectric properties affect the waveguide’s resonance. This shift can be measured in real time, enabling accurate monitoring of oil degradation. Additional measurements were conducted using the coaxial probe technique to analyze the changes in the oil’s electrical behavior during various heating periods. Experimental results demonstrate the sensor’s sensitivity, with measurable frequency shifts of up to 10 MHz observed in the most aged oil samples. These shifts clearly correlate with the oil’s aging process, confirming the sensor’s potential for practical use in predictive maintenance. This system, utilizing an electroformed waveguide, offers a cost-effective and efficient solution for enhancing machinery longevity and optimizing maintenance schedules in industrial applications.

This paper presents a planar microwave sensor for the noninvasive detection of glucose concentration in diabetic patients. The designed sensor operates from the 3.8 to 6.2 GHz frequency band, which covers the 5.8 GHz Industrial Scientific and Medical (ISM) band. The designed sensor shows a percentage bandwidth of 23.8% with a reflection coefficient (S 11) of −50 dB at the resonance frequency of 5.7 GHz. The detection was carried out by varying the relative permittivity of the blood in accordance with the glucose concentration based on the Cole–Cole model. The measured result is calculated in terms of varying resonance frequency with variation in the reflection coefficient | S 11| of the designed sensor. The observed frequency shift and corresponding sensitivity of the sensor are found at 1.7 GHz and 0.089 MHz/mg dL−1, respectively. An experimental validation has also been performed, and the frequency shift is analyzed by interacting the human thumb with the sensor. The simulated and experimental results of the designed sensor suggest that it can be useful for detecting glucose concentration noninvasively for diabetic patients. [ABSTRACT FROM AUTHOR]

This paper introduces a symmetric bar chart-shape (SBCS) microwave sensor for measuring permittivity of fluidic samples. For designing purposes, the introduced sensor was used based on the field changes between the SBCS and rectangular loop microstrip (RLM) structure. When a sample is placed on the sensing location, interaction between SBCS and RLM varies the field intensity. The vinegar samples were combined with water and then they are placed on the sensor. The change in field intensity changes the resonance frequency. However, there is a relationship between the permittivity of samples and the resonance frequencies. The proposed sensor is implemented on the substrate of RO4003C. The relative permittivity of samples changed from 59 to 77 and the resonance frequencies changed from 2.3 to 1.4 GHz. The quality factor is 3544 and the sensitivity is 2.2%. [ABSTRACT FROM AUTHOR]

In this work, a microwave resonator sensor with a unique configuration consisting of three resonators and two feedlines is proposed. This novel design aims to improve the performance and functionality of microwave resonator sensors for various applications. The frequency response of the sensor to materials with different dielectric constants is simulated. The results show that the most sensitive region of the sensor is located on the first interdigital structure, and placing the materials in other regions would enhance the linear correlation of its frequency response. The sensor also exhibits the ability to distinguish whether the same material has defects and the ability to qualitatively detect subtle changes in dielectric constant. Finally, the proposed sensor is fabricated and measured under the condition consistent with the simulation environment. The measured results are basically consistent with the simulation results, which confirms the potential of this sensor in detecting dielectric constants and resolving materials with defects, and the response of the sensor to the materials under test demonstrates its potential in measuring different thicknesses and loss tangents. [ABSTRACT FROM AUTHOR]

In this letter, a new microwave sensor of the coaxial resonant cavity with a single-open-ended circuit loaded capacitor is proposed and used to detect the liquids' complex permittivity. The improved cavity has a higher internal electric field and smaller size when compared with the cylindrical cavity, which gives the sensor a high sensitivity in measuring liquids' complex permittivity. A coaxial resonant cavity operating in 1.9 GHz was designed in this work. The silicone hose used for the test is inserted vertically from the center of the cavity, and the liquids under test (LUTs) are guaranteed to be 2 ml each time. The dielectric properties of LUTs will cause perturbation to the internal electric field of the cavity. By analyzing the measured data, the sensitivity of the cavity is 0.12%, and the relative errors of the real part of the measured value and the reference value are 3.67%. which shows the measured value has a good agreement with the reference value. [ABSTRACT FROM AUTHOR]

Simple Summary: Glycerin is an organic substance used as an ingredient for many industries, including pharmaceuticals and cosmetics, but also, glycerin is an important product during biodiesel refining. Accurate and real-time sensors are needed to improve the industrial process; therefore, we proposed a workflow to measure concentrations of glycerin using a microwave sensor enhanced by machine learning models. We tested this methodology with complex electronic instrumentation and a designed low-cost portable electronic reader. As a result, we found that both devices achieved excellent and similar performance. These findings are valuable since monitoring the glycerin concentration may help to increase efficiency and reduce costs in the industry. In addition, the methodology proposed in this study could be applied to any sensor, making it a valuable contribution to liquid analysis with microwave sensors. Glycerin is a versatile organic molecule widely used in the pharmaceutical, food, and cosmetic industries, but it also has a central role in biodiesel refining. This research proposes a dielectric resonator (DR) sensor with a small cavity to classify glycerin solutions. A commercial VNA and a novel low-cost portable electronic reader were tested and compared to evaluate the sensor performance. Within a relative permittivity range of 1 to 78.3, measurements of air and nine distinct glycerin concentrations were taken. Both devices achieved excellent accuracy (98–100%) using Principal Component Analysis (PCA) and Support Vector Machine (SVM). In addition, permittivity estimation using Support Vector Regressor (SVR) achieved low RMSE values, around 0.6 for the VNA dataset and between 1.2 for the electronic reader. These findings prove that low-cost electronics can match the results of commercial instrumentation using machine learning techniques. [ABSTRACT FROM AUTHOR]

In this work, a real-time label-free microwave sensing mechanism for glucose concentration monitoring using a planar biosensor configured with an inset fed microstrip patch antenna has been demonstrated. A microstrip patch antenna with the resonating frequency of 1.45 GHz has been designed and is fabricated on the Flame Retardant (FR-4) substrate. Due to the intense electromagnetic field at the edges of the patch antenna, edge length has been used as the detecting area where the sample under test (SUT) interacts with the electromagnetic field. The Poly-Dimethyl-Siloxane (PDMS) with the trench in the centre has been employed as the sample holder. Here, the SUT is the glucose dissolved in DI (de-ionized) water with the concentration range of 0.2 to 0.6 g/mL. The dielectric constant dependency on the glucose concentration has been used as the distinguishing factor which results in a shift in the S-parameter. The experimentally measured RF parameters were observed closely which showed the shift in S11 magnitude from -40 to -15 dB and resonant frequency from 1.27 to 1.3 GHz w.r.t the SUT solution of 0.2 to 0.6 g/mL with linear regression coefficient of 0.881, and 0.983 respectively. [ABSTRACT FROM AUTHOR]

Summary: In this paper, a microwave sensor is designed and built to measure the permittivity of aqueous solutions. The samples used in this experiment are different purities of ethanol mixed with water. Ethanol solution with different purities is a widely used material in industry. Structures of meander, ladder, and T‐structure were used to design the sensor. Then the designed sensor is simulated in CST software in range of relative dielectric from 10 to 80. After obtaining the desired answer in the simulation, the proposed sensor is built on the Rogers 4003 substrate, the built‐in sensor measured ethanol solution with a purity of 5% to 35% (equivalent to permittivity of 76 to 58), and is showed a sensitivity of 4% and a Q‐factor of 3500. The results of simulation and measurement are consistent with each other, and they indicated that the proposed sensor beyond the fluids can be supported and determine their relative dielectrics. [ABSTRACT FROM AUTHOR]

A non-contact method for detecting salt concentration in water using a microstrip patch sensor is presented in this work. The microstrip patch sensor, which has a low cost and simple build process, consists of a circular split ring resonator (SRR) with a hole drilled through the substrate in the middle area, and a microstrip patch antenna. The sensor was designed and fabricated using a printed circuit board (PCB) technique based on a negative dry film photoresist and photolithography method. It was built on an Arlon DiClad 880 substrate with a thickness of 1.6 mm and a relative permittivity of 2.2. The resonant frequencies (Fr) and reflection coefficients (S11) in the frequency range from 0.5 GHz to 0.8 GHz were recorded for analysis, both through simulation and experiment. The concentration of chloride and carbonate salts was varied from 0 mg/mL to 20 mg/mL in the tests using the sensor. The statistical analyses of S11 and Fr data obtained from measurements of five different salts at seven different concentrations (using the Shapiro–Wilk test, Bartlett test, and Kruskal–Wallis H test) were conducted using R version 4.2.0 to determine the relationship between the individual salts. The experimental results showed that the frequency response and resonance amplitude are functions of the concentration of each salt. The proposed method has the potential to be used for the non-contact measurement of industrial products, food quality, and health in the future. [ABSTRACT FROM AUTHOR]

In this study, an RFID tag inspired microwave sensor design is proposed for the dielectric parameter characterization of the water-methanol binary mixture through the RFID tag operating principle with RSSI magnitude and phase output values based on the input variables of operating frequency, RFID reader power strength and sample location in machine learning assisted manner. The proposed microwave sensor design operates at ETSI frequencies of UHF band reserved for RFID. In the characterization of the water-methanol binary mixture by processing the RSSI data received from the RFID reader with machine learning Gaussian Process Regression, the mixing ratios of the liquid components and real and imaginary parts of the complex dielectric constant of the mixture can be conveniently obtained. For the machine learning study, eleven mixtures with 10 % differences, three different power levels, four different frequencies, and four different locations have been combined and carried out with a total of 528 data obtained. Three different machine learning algorithms have been developed using the same input data for three different characterization outputs. R2 values of Gaussian Process Regression method have been obtained as 0.99, 0.99 and 0.98 for the volumetric mixing ratios, real part, and imaginary part of the dielectric constant, respectively. [Display omitted] • Microwave sensor offers cheaper solution for determination of dielectric parameters and volumetric content from RFID system data. • Sensing capability of microwave liquid sensor is improved through machine learning techniques with result of higher accuracy. • Multiple RSSI measurements can be conducted simultaneously for various binary solutions with different volumetric concentrations at once. • Same microwave liquid sensor circuitry can be utilized multiple times due to disposable liquid container inserted inside. [ABSTRACT FROM AUTHOR]

A microwave Mach–Zehnder Interferometer (MZI) sensor for the permittivity characterization of liquids is presented in this paper. Two spoof surface plasmon polariton (SSPP) waveguide transmission lines (TLs) composed of compact spiral units is utilized in a differential architecture. Loading a dielectric material on the sensing arm induces a phase difference between the two arms, leading to periodic interference dips on the transmission curve. The spectral positions and magnitudes of these dips change with different dielectric materials. Due to the nonlinear dispersion characteristics of the SSPP TL, the proposed SSPP-based MZI sensor achieves improved sensitivity compared with microstrip-based MZI sensor. To validate the sensor's performance, six different oil samples and a set of adulterated oils were measured, incorporating a polydimethylsiloxane (PDMS) channel on the sensing arm. The variations in the spectral position and magnitude of the interference dip centered at 3.2 GHz are employed to determine the dielectric constants and loss tangents of the tested oils. The sensitivity in measuring the dielectric constant is 3.61 %. The maximum errors for measuring the dielectric constant and the loss tangent are 4.3 % and 6.7 %, respectively. Compared to other sensors for liquid oil detection, the proposed sensor provides a relatively high sensitivity while maintaining a low liquid volume, making it a promising candidate for permittivity measurements of liquid analytes and solid materials with similar dielectric constants. [Display omitted] • A microwave Mach–Zehnder Interferometer (MZI) sensor. • Two spoof surface plasmon polariton waveguides used in a differential architecture. • Compact spiral unit with nonlinear dispersion curve. • Characterization of the dielectric constants and loss tangents of liquids. • Sensitivity outperforms that of microstrip-based MZI sensor. [ABSTRACT FROM AUTHOR]

This paper presents the design, optimization, and calibration of multivariable resonators for microwave dielectric sensors. An optimization technique for the circular complementary split ring resonator (CC-SRR) and square complementary split ring resonator (SC-SRR) is presented to achieve the required transmission response in a precise manner. The optimized resonators are manufactured using a standard photolithographic technique and measured for fabrication tolerance. The fabricated sensor is presented for the high-resolution characterization of dielectric substrates and oil samples. A three-dimensional dielectric container is attached to the sensor and acts as a pool for the sample under test (SUT). In the presented technique, the dielectric substrates and oil samples can interact directly with the electromagnetic (EM) field emitted from the resonator. For the sake of sensor calibration, a relation between the relative permittivity of the dielectric samples and the resonant frequency of the sensor is established in the form of an inverse regression model. Comparisons with state-of-the-art sensors indicate the superiority of the presented design in terms of oil characterization reliability. The significant technical contributions of this work include the employment of the rigorous optimization of geometry parameters of the sensor, leading to its superior performance, and the development and application of the inverse-model-based calibration procedure. [ABSTRACT FROM AUTHOR]

Accurate monitoring of contaminants, such as wear particles in lubricating oil, is critical for the efficient maintenance of engines. Here, a TE011-mode cylindrical cavity resonator-based sensor system is presented. In the measurement, a glass test tube was used to contain the oil sample under test and placed in the center of the cavity. Particles in oil change the dielectric properties and perturb the resonance parameters, thereby achieving sensing. Sensitivity to both the presence of conductive and nonconductive powders is demonstrated. Linear relationships between the powder weight and the quality factor are established. Hence, the degree of contamination can be better evaluated compared with existing microwave sensors. Powder differentiation can be achieved using the resonance frequency and quality factor. In addition, analytical modeling of the sensing system incorporating field analysis is proposed, and the effective permittivity of the mixture can be determined. The sensor developed is compact, portable, easy to implement, complementary to other approaches for liquid evaluation, and has the potential for high-throughput online measurement. [ABSTRACT FROM AUTHOR]

Design optimization of multivariable resonators is a challenging topic in the area of microwave sensors for industrial applications. This article proposes a novel methodology for rapid redesign and parameter tuning of complementary split-ring resonators (CSRRs). Our approach involves inverse surrogate models established using preoptimized resonator data as well as analytical correction techniques to enable rapid adjustment of geometry parameters and CSRR optimization over broad ranges of operating frequencies. The tuning process is arranged to precisely allocate the operating frequency while maximizing the quality factor of the circuit. The procedure is generic and characterized by an extremely low computational cost of up to two electromagnetic (EM) analyses of the circuit at hand (not counting the inverse model setup). The presented technique is demonstrated using a circular CSRR coupled to a microstrip transmission line (MTL) and optimized to operate between 5 and 20 GHz. The design optimized for 15 GHz is fabricated and experimentally validated using a vector network analyzer. The sensor works in the transmission mode and senses the shift in resonance frequency determined by the properties of the material under test (MUT). Furthermore, an inverse regression model is developed that allows for directly finding the unknown permittivity of the MUT based on the measured resonant frequencies of the sensor. The obtained results corroborate the design utility of the proposed optimization method, as well as the practical usefulness of the specific CSRR structure developed with the aid thereof. [ABSTRACT FROM AUTHOR]

In the field of microwave measurement, how to miniaturize devices and improve measurement sensitivity is a major challenge. In this article, a planar microwave sensor loaded with a complementary curved ring resonator (CCRR) is proposed to measure the permittivity of substrate materials. The current forms a magnetic field along the path of the curved ring, which acts on the capacitance between the rings of the curved ring and makes the circuit resonate. At this time, the curved ring structure stores a huge amount of energy. When the substrate materials with different permittivity are loaded into the measurement area of the sensor, the resonant frequency of the sensor will change. By establishing a mathematical model between the permittivity and the resonant frequency and conducting a full-wave electromagnetic simulation, it can be seen that the CCRR combined with the substrate integrated waveguide (SIW) structure can effectively enhance the electric field intensity in the measured region. Compared with the conventional planar microwave resonator. The sensitivity, resolution, and size of the proposed sensor are 3.51%, 133 MHz, and 52 $\times $ 34 $\times $ 0.508 mm3, respectively. After optimizing the structural parameters of the sensor, the actual sensor was manufactured. Four kinds of substrate materials commonly used in microwave circuits are used as samples to test the sensor. The results show that the sensor has the characteristics of high sensitivity, high resolution, and high measurement accuracy, which confirms the promising application of the sensor in the field of permittivity measurement. [ABSTRACT FROM AUTHOR]

In this paper, the human milk dielectric constant by a microwave sensor has been evaluated. The proposed sensor is result of coupling between a simple microstrip line and a split ring resonator and its resonance frequency is 6 GHz. When up to 30 μl of milk are placed on the sensing area on the sensor, it causes the resonance frequency to shift. Six samples of human milk were measured by a network analyzer, then the dielectric constant of the samples were determined, and three similar samples were measured by the built‐in sensor. The proposed microwave sensor has Q‐factor of 38 and its sensitivity is 0.17%. The implemented sensor has acceptable performance compared to previous sensors and it can be used to determine the normal behavior of human milk with a simple method and for this work, it requires to small value of sample for measurement. Key Points: The proposed sensor is able to measure of the milk sample up to a volume of 30 μlTypes of milk including human and animal can be measured by the proposed sensorThe Q‐factor and sensitivity of the sensor is acceptable compared to previous works [ABSTRACT FROM AUTHOR]

Simultaneous characterization of permittivity and permeability of magnetodielectric materials using a single resonator-based microwave sensor is challenging due to their similar impacts on the sensor’s response. This work presents an interdigital structure-based planar microwave resonator sensor featuring high-intensity electric (E)-field zones and magnetic (M)-field zones to characterize permittivity and permeability. The proposed sensor’s sensing E-field and M-field zones were implemented using the interdigital fingers configured as a spur line structure and interdigital electrodes, respectively, generating a 5.9-GHz resonance frequency with a high Q-factor (1700). The resonance frequency of the designed sensor exhibited high sensitivities (324 MHz/ $\Delta \varepsilon _{r}$ for permittivity and 825 MHz/ $\Delta \mu _{r}$ for permeability) for characterizing single/multilayers magnetodielectric materials with excellent resolutions (0.324 for permittivity and 0.825 for permeability). The demonstrated high sensitivity and resolution results validate the performance of the interdigital structure-based microwave resonator sensor for simultaneous characterization of multilayer magnetodielectric materials. [ABSTRACT FROM AUTHOR]

In this work, a compact, inexpensive, and efficient dual band microwave sensor is proposed. The sensor is based on two Complementary Symmetric Split-Ring Resonators (CSSRRs) and possesses a high $Q$ factor and wide sensing range. These CSSRRs are coupled electrically with two inductive patches to the Microstrip Transmission Line (MTL). This combination provides two dual bands, first at 5.35 GHz with a notch depth of -55.20 dB and second at 7.99 GHz with a notch depth of -22.54 dB. The sensor works in transmission mode and senses shift in frequency. Some commonly available dielectric substrates with relative permittivity ranges between 1 and 12 are considered Material Under Test (MUT), and detailed sensitivity analysis is being performed for each band. The dual band sensor is fabricated on a low-cost, widely available FR4 substrate and measured by CEYEAR AV3672D vector network analyzer. Additionally, the least square curve fitting method is used to develop a mathematical model for the measured results. An excellent agreement is observed between simulated, measured, and formulated results.

In the natural gas industry, gas-water flow endures complex transfer process in the transmission pipeline, resulting in a variety of flow regimes. The research on the sensor with abilities of flow regime identification and water content measurement is of significance, an orthogonal electrodes microwave sensor (OEMS) is thus designed in present work. Based on the microwave transmission technology, two orthogonal electrodes are cross assembled constructing a 2-D electromagnetic detection plane inside OEMS. By virtue of the COMSOL Multiphysics software, the measurement characteristics of water content of OEMS are investigated, and the optimum working frequency band (0.36– 0.40 GHz) is determined. The coupling simulation results of flow field and electromagnetic field manifest that the OEMS can identify the flow regimes of stratified, wavy, slug and annular flow. The measurement results of the simulation, vector network analyzer (VNA) and self-developed circuit coincide, suggesting the rationality of OEMS. The recommended working frequency is effective for flow regime identification and water content measurement in the range of 0–100% in gas-water flow systems. [ABSTRACT FROM AUTHOR]

In this work, a novel method for non-invasive, label-free, multi-variable volumetric concentration analysis of mixtures is presented based on wide-band microwave resonators. The proposed microwave structure is a resonator with multiple splits for improving the overall sensitivity. The wide-band analysis of the sensor’s spectrum provides many resonances which shift in any of them could be translated to the dielectric permittivity at that frequency. Therefore, by considering frequency shifts in as many as 30 resonance frequencies over a high-frequency span from 100 MHz to 6.5 GHz, the sensor provides invaluable information of the mixture components. This is because of the non-linear and unique variations of the dielectric permittivity of different materials resulting in a unique frequency-dependent shift in the resonance frequencies. Seven liquids soluble in each other are mixed with different percentages with a fixed overall mixture volume in this work. According to the extreme complexity of the problem, an artificial neural network is employed with the shifts in the mentioned resonance frequencies due to introducing each sample to the sensor are considered as the input features and the volumetric concentration of each of the seven components are calculated as the outputs. The network is trained with 750 samples and tested with the different 150 other samples with the overall average MSE of as small as 0.057%. [ABSTRACT FROM AUTHOR]

In this work, we have designed and fabricated low-cost microwave sensor for the determination of the dielectric constant of liquid samples with high accuracy. The biggest challenge with the microwave planar sensor is its low sensitivity and limited sensing range. A triple Complementary Split Ring Resonator (CSRR) with highly optimized dimensions has been proposed. Two sets of sensors are designed to operate at two different frequencies, viz. 1.2 GHz and 2.4 GHz. Since the resonant frequency of the microwave sensors changes with the variation of the dielectric constant of the liquid in contact, eight different liquids are used to record the shift in the resonant frequency of the sensors. For this, we have measured the $\text{S}_{{21}}$ parameter of the sensors using Vector Network Analyzer (VNA). Sensitivity as high as 0.87% has been achieved, which is much higher than most of the reported sensors. Fit equations have been developed for both the real and imaginary part of the permittivity from which the complex permittivity of unknown samples are obtained with less than 4% of error. Due to low-cost materials and ease of fabrication, the sensor may be a good alternative to expensive commercial sensors for dielectric characterization and impurity measurement of liquids. [ABSTRACT FROM AUTHOR]

A water-in-diesel microwave sensor based on a substrate integrated waveguide (SIW) microwave applicator is designed and characterized in this study. The interaction between the microwave electromagnetic field and the diesel fuel contaminated with small concentrations of water is obtained via suitable radiating slots placed on the top of an SIW waveguiding structure. The SIW applicator working frequency is chosen by observing the behavior of the complex dielectric permittivity of the fuel–water blend based on a preliminary wide band investigation. The performances of the SIW microwave sensor are evaluated in terms of scattering parameter modulus | S 21 | as a function of the water concentration in ppm. The best sensitivity Δ | S 21 | Δ ρ = 1.42   mdB / ppm is obtained at a frequency of f = 9.76   GHz , with a coefficient of determination R 2 = 0.94 . The sensor is low-cost, low profile and ensures a good sensitivity for constant and real-time monitoring. [ABSTRACT FROM AUTHOR]

A novel dual band, highly sensitive Substrate Integrated Waveguide (SIW) sensor for permittivity measurements is presented. A pair of modified Complementary Split Ring Resonators (CSRRs) is etched on SIW surface. CSRRs are located in the center of SIW, where the electric field distribution is high so that the coupling be maximized. The coupling between the SIW and the CSRRs as well as the adjacent CSRRs results in two notches in transmission coefficient. These notches vary with the dielectric loading on the sensor. The ratio of a notch variation to the load permittivity variation determines the sensitivity of proposed sensor. Two sensitivities proportional to two notches are provided. Normalized sensitivities from both notches show identical values. Therefore, any environmental effect have the same variation on the TZs. This demonstrates the potential of the proposed sensor for differential operation that can mitigate the effect of environmental condition. The size of the proposed sensor is small as the inductive and the capacitive effects of CSRRs forced the SIW to operate below the cut off frequency at negative-order-resonance mode. All design steps including SIW design, CSRRs design and modified CSRRs effects are presented in details. The sensor operation principle is described through an equivalent circuit model as well as simulation results. The experimental results indicates that the normalized sensitivity is 3.4% which is much higher than similar sensors. The prototype sensor size ($27.8\times 18.4\times 0.508$ mm3) is smaller than those reported in the literature. [ABSTRACT FROM AUTHOR]

A metamaterial-based complementary split ring resonator (CSRR) structure is used to develop a sensor for dielectric characterization of fluids. The fluid present in the vertical column interacts with the fields around the CSRR causing a shift in the transmission coefficient curve ( S 21 ). An empirical relationship can be established between the dielectric properties and the resonance frequency and Q-factor. This relationship is used for the dielectric characterization of the fluid. A second-order polynomial function is employed for a better curve fitting of the data to achieve higher accuracy in the prediction of complex permittivity ( ε ′ and ε ′ ′ ). Multi-variate polynomial regression is used to determine the coefficients of the polynomial function. The proposed sensor predicts the permittivity of the sample with high accuracy. The design is very simple and the sample can be easily changed by replacing the glass tube. The sensor has very high sensitivity and requires a very little volume of the sample. [ABSTRACT FROM AUTHOR]

This paper presents a parametric analysis relative to the effects of the dielectric constant of the substrate, substrate thickness and slot width on the edge capacitance of a slot-based resonator. The interest is to find the conditions (ranges of the previously cited parameters) compatible with the presence of a quasi-magnetic wall in the plane of the slot (or plane of the metallization). If such magnetic wall is present (or roughly present), the electric field in the plane of the slot is tangential (or quasi-tangential) to it and the edge capacitance can be considered to be the parallel combination of the capacitances at both sides of the slot. Moreover, variations in one of such capacitances, e.g., caused by a change in the material on top of the slot, or by a modification of the dielectric constant of the substrate do not affect the opposite capacitance. Under the magnetic wall approximation, the capacitance of certain electrically small slot-based resonators can be easily linked to the dielectric constant of the material present on top of it. The consequence is that such resonators can be used as sensing elements in a permittivity sensor and the dielectric constant of the so-called material under test (MUT) can be determined from the measured resonance frequency and a simple analytical expression. In this paper, the results of this parametric analysis are validated by considering several sensing structures based on dumbbell defect ground structure (DB-DGS) resonators of different dimensions. [ABSTRACT FROM AUTHOR]

A six-band microstrip sensor, based on a pair of electrically coupled resonators (ECRs), is proposed to perform multifrequency dielectric characterization of liquids. A pocket is milled through the substrate bottom of the sensor to contain the fluid sample, avoiding the need for a sample holder. The pocket has been designed to achieve a higher electric field magnitude in the sensing region and a mechanism to separate the resonant frequencies, which helps to ensure a high span between the first and the last resonant frequency. The microstrip sensor is implemented on a 1.905-mm-thick substrate with $\varepsilon ^{\prime }_{r} = 12.2$ and $\tan \delta = 0.0019$. The sensor’s six resonance frequencies span the range between 1 and 3.5 GHz. Experimental tests show that the sensor can measure a wide range of $\varepsilon ^{\prime }_{r}$ values between 2 and 79 resulting in close similarities to the dielectric constants published in previous works. [ABSTRACT FROM AUTHOR]

Water pollution and scarcity of pure drinking water is a current challenging problem. In this situation, quality monitoring of water is of vital importance in order to avoid any health issues caused due to intake of contaminated water. Hence, in this paper, a non-destructive technique using multiple ring CSRR based microwave sensor has been proposed for accurate estimation of relative permittivity of unknown water of varying TDS values. Further, a closed form numerical expression using the curve fitting technique has been developed in order to accurately estimate the quality of any unknown water sample. Less than 1% of error has been achieved in the prediction of relative permittivity and hence, TDS of unknown water sample. The proposed planar microwave sensor will be of great importance towards the real-time water quality monitoring systems development. [ABSTRACT FROM AUTHOR]

This article presents an exhaustive study of the sensitivity in reflective-mode phase-variation sensors based on an open-ended transmission line with a step-impedance discontinuity. Such discontinuity delimits the sensing region (which extends up to the open end of the so-called sensing line), from the transmission line section connected to the input port (design line), which is used to enhance the sensitivity. The theoretical analysis provides the design guidelines to achieve a sensor with high sensitivity compared with the one based on an ordinary (uniform) line with a similar length. In particular, it is shown that for sensitivity optimization, the electrical length of the design line must be set to 90° (or an odd multiple), whereas either a 90° (or an odd multiple) or a 180° (or an even or odd multiple) sensing line can be alternatively used in order to maximize the sensitivity. It is shown that the impedance contrast, defined as the ratio between the characteristic impedances of the design and sensing line, is a key parameter for sensitivity enhancement, and it must be as low or as high as possible for the 90° or 180° sensing lines, respectively. For validation purposes, two prototype devices (one with a 90° and the other one with a 180° sensing line) have been designed and fabricated following the design guidelines. Such devices have been tested by loading the sensing region with several materials with different dielectric constants. Compared with the ordinary line-based sensors, it is found that the maximum sensitivity is enhanced by a factor of 19.7 and 11.4 in the phase-variation sensor based on a 90° and 180° sensing line, respectively. Finally, the sensor concept is generalized to a multisection step-impedance transmission line as a means of further increasing the sensitivity, and a prototype device exhibiting 528.7° maximum sensitivity is implemented. [ABSTRACT FROM AUTHOR]

A feasibility study of microplastic detection and quantification in soil and water using resonance microwave reflectometry is carried out using artificially created samples with a high volumetric concentration of microplastic with $50~\mu \text{m}$ -0.5mm particles size. A mathematical model expressing microplastic concentration in soil and water as a linear function of the measured S11 resonance frequency shift and relative permittivity contrast is developed and is found to be in an excellent agreement with the experimental data based on synthetic contaminated material samples. Next, this model is applied to find the best achievable theoretical resolution of microplastic concentration in the natural environment using microwave sensing technology, which is shown to be at around 100ppm (parts-per-million) level in the linear signal detection regime. It is demonstrated that the best achievable level of microplastic contaminant resolution depends on the sensor probe Q-factor and sensitivity of the microwave receiver. The bound for the achievable contaminant concentration resolution is found in the analytical form for high-Q resonance microwave sensors of arbitrary geometry. Even though several well-established protocols based on optical, infrared, and X-ray spectroscopy are currently being used for microplastic detection in the natural environment, microwave spectroscopy could offer additional benefits, especially for low-cost, real-time in-situ microplastic detection in diverse environmental conditions outside of the laboratory space. [ABSTRACT FROM AUTHOR]

Metamaterial‐based double‐slit complementary split rectangular resonator sensor is proposed for the characterisation of dielectric properties of the materials under test (MUTs). The proposed sensor is designed and simulated on the CST microwave studio software using a low‐cost substrate FR4. An array of three identical resonators is etched in the ground plane of the sensor to achieve a single and deep notch of −58.7 dB in the transmission coefficient (S21) at the resonant frequency of 7.01 GHz, which is the novelty of the proposed sensor. A deep and single resonant frequency band has a significant role in the precise measurement of the dielectric properties of the MUTs. The effective constitutive parameters are extracted from the S‐parameters. An equivalent circuit model is suggested that describes the overall behaviour of the sensor. The sensor is fabricated on the FR4 substrate and measured through the vector network analyser (N5224B) by placing the standard materials. The parabolic equation for the proposed sensor is formulated to approximate the permittivity of the MUTs. A very small percentage of error, 0.77, is found which shows high accuracy of the sensor. This methodology is efficient, simple in fabrication, and reduces cost and computational time also. [ABSTRACT FROM AUTHOR]

This article proposes a novel multibeam dual-frequency sensor system to assess the quality of thick rind fruits based on phase difference between lower- and higher-frequency reflection coefficients ($\Phi _{12}$). The proposed sensor system consists of five dual-band antennas and a customized circuit that approximates lower- and higher-frequency phase difference. The main components of the customized circuit are down-conversion mixers and a phase detector module. For comparison, simulations were initially carried out using single- (1 GHz) and dual-frequency (1 GHz/2.3 GHz) sensor systems, and results indicated lower accuracy in classification of thick rind fruits of the single-frequency scheme. As a result, a dual-frequency sensor prototype was fabricated and experimented with normal and granulated plastic pomelo models and real pomelo fruits. The experimental results revealed that the average $\Phi _{12}$ of the normal and granulated pomelos are almost identical, while the standard deviation (SD) of $\Phi _{12}$ of the granulated pomelo is significantly larger than the normal pomelo. SD is thus used as the determinant of pomelo quality. The multibeam 1-GHz/2.3-GHz dual-frequency sensor system efficiently differentiates between normal and granulated pomelo fruits. As such, the proposed sensor system is operationally appropriate for quality control of thick rind fruits. [ABSTRACT FROM AUTHOR]

Resonant coaxial SMA microwave permittivity sensor is introduced. It is constructed using only commercially available SMA connectors. The sensor is tested in two different frequency ranges for two different dielectric constant ranges, from 1 to 10 and from 10 to 80. Presented sensor is designed, fabricated, simulated, and tested. Good agreement between simulations and measurements is shown. The sensor is also applicable for differential permittivity measurements. [ABSTRACT FROM AUTHOR]

Microwave sensors based on electrically small planar resonant elements are reviewed in this paper. By virtue of the high sensitivity of such resonators to the properties of their surrounding medium, particularly the dielectric constant and the loss factor, these sensors are of special interest (although not exclusive) for dielectric characterization of solids and liquids, and for the measurement of material composition. Several sensing strategies are presented, with special emphasis on differential-mode sensors. The main advantages and limitations of such techniques are discussed, and several prototype examples are reported, mainly including sensors for measuring the dielectric properties of solids, and sensors based on microfluidics (useful for liquid characterization and liquid composition). The proposed sensors have high potential for application in real scenarios (including industrial processes and characterization of biosamples). [ABSTRACT FROM AUTHOR]

This academic paper introduces a novel sensor known as the Planar Microstrip-based Triple Ring Bridge Complementary Split Ring Resonator (TRB-CSRR) sensor. The main objective of this sensor is to determine the permittivity and thickness of solid dielectric substances. The TRB-CSRR sensor is meticulously designed to resonate precisely at a frequency of 4.86 GHz, achieving a significant notch depth of −33.5 dB. This configuration displays an enhanced average relative sensitivity of 20.2%. The study undertakes numerical assessments across various scenarios, encompassing situations where the sensor interacts with diverse dielectric materials. These assessments yield insights into alterations in resonant frequencies. By meticulously refining the design, the sensor's capability to confine electric fields precisely at the resonant frequency is amplified, ultimately resulting in heightened sensitivity towards dielectric characteristics. The sensor's efficacy is tested using materials featuring relative permittivity values spanning from 1.006 to 12.9, while consistently adhering to dimensions of 5 mm×5 mm×1.6 mm. To validate the conceptual frame-work, a physical sensor is fabricated, and its response is gauged through the utilization of a vector network analyzer (VNA-AV3672D). Employing curve fitting methodologies, alterations in resonance frequencies due to interactions with the tested materials are presented, underscoring the impact of permittivity and thickness. The outcomes derived from simulations, empirical measurements, and calculations display a robust alignment. [Display omitted] • A novel sensor, the Planar Microstrip Triple Ring Bridge CSRR (TRB-CSRR), has been designed and fabricated. • The primary goal of the TRB-CSRR sensor is to ascertain the permittivity and thickness of solid dielectric substances. • The designed sensor exhibits higher sensitivity compared to state-of-the-art designs in the literature. [ABSTRACT FROM AUTHOR]

A conventional cavity-based planar cylindrical resonant sensor for the RF characterization of dielectric materials is proposed in this work. The designed planar cylindrical sensor is about 22% more compact providing 25% higher sensitivity as compared to its rectangular counterpart working at the same frequency. The proposed sensor is designed for the fundamental TM010 mode at 1.5 GHz resonant frequency and is excited using the coaxial probe instead of conventional microstrip feeding, which aids in obtaining further compactness. The sensor is designed on low-cost FR4 substrate and experimentally validated using various dielectric samples like Teflon, polyethylene, PVC and plexiglass yielding a good match between the retrieved dielectric constants and data available in the literature. For characterization of dielectrics, a slot is drilled at the center of the sensor having the maximum electric field for inserting the test sample and later optimized for higher sensitivity. For determining the loss tangent in terms of the transmission coefficient at the resonant frequency, a numerical model is derived using the curve fitting technique. [ABSTRACT FROM AUTHOR]

• Microwave temperature sensing as new application with ETFE materials. • Original differential scheme for temperature measurement in frequency domain. • First temperature measurement with transmission phase shift. • High sensitive performance owing to coplanar waveguide technology (CPW). This paper investigates temperature-dependent dielectric properties of fluoropolymers for the development of original low-profile and cost-effective microwave temperature sensors. Here, ethylene tetrafluoroethylene (ETFE) was chosen for its superior resistance to common solvents, excellent thermal stability and high selectivity compared to other fluoropolymers. The temperature-dependent dielectric constant of the commercially available ETFE sheet named "Fluon" was equated and analyzed. A dual-resonant circuit in coplanar waveguide technology (CPW) was designed and prototyped to achieve temperature sensing. To integrate temperature sensing capability within the device, one resonator was covered by the Fluon sheet while the other one remained uncovered. From there, specific schemes achievable with either electromagnetic (EM) reflected and transmitted through the CPW line were studied and validated. A differential approach comparing the variable resonance with Fluon to the static resonance without Fluon was implemented for reflected signals. At the same time, S21 phase shifting reflecting slowdown/acceleration of the EM field was fulfilled for transmitted signals. Experimental tests were performed within the 23 °C–80 °C temperature range. The sensor exhibits a sensitivity of 4.07 MHz/°C which is significantly better than the results obtained with other sensing approaches and technologies in the literature. The overall phase shift is 13.35°, leading to a sensitivity of 0.235°/°C. [ABSTRACT FROM AUTHOR]

A moisture sensor based on a composite resonator is used to measure different honey samples, which include imitation honey. The sensor changes its frequency response in accordance with the dielectric permittivity that it detects in the measured samples. Although reflectometry sensors have been used to measure the percentage of moisture in honey for almost a century, counterfeiters have achieved that their apocryphal product is capable of deceiving these kinds of sensors. Metamaterial features of the composite resonators are expected to improve their response when detecting lossy samples such as organic samples. It is also sought that these sensors manage to detect small differences not only in the real parts of the dielectric permitivities of samples but also in their imaginary parts, and, thus, the sensors are able to discern between real honey and slightly altered honey. Effectively, not only was it possible to improve the response of the sensors by using lossy samples but it was also possible to identify counterfeit honey.

In this paper, an extremely sensitive microwave sensor is designed based on a complementary symmetric S shaped resonator (CSSSR) to evaluate dielectric characteristics of low-permittivity material. CSSSR is an artificial structure with strong and enhanced electromagnetic fields, which provides high sensitivity and a new degree of freedom in sensing. Electromagnetic simulation elucidates the effect of real relative permittivity, real relative permeability, dielectric and magnetic loss tangents of the material under test (MUT) on the resonance frequency and notch depth of the sensor. Experiments are performed at room temperature using low-permittivity materials to verify the concept. The proposed design provides differential sensitivity between 102% to 95% as the relative permittivity of MUT varies from 2.1 to 3. The percentage error between simulated and measured results is less than 0.5%. The transcendental equation has been established by measuring the change in the resonance frequency of the fabricated sensor due to interaction with the MUT.

A microwave sensor is presented in this letter for the detection of salinity and sugar concentration. The microwave sensor is used for the transmission and reception of electromagnetic wave. The sensor has a circular shaped radiating patch with semi‐circular shaped slots. The antenna sensor is prototyped on a low‐cost FR4 substrate which is 1.60 mm thick. The sensor has a small size of 26 mm × 24 mm. The high‐frequency structural simulator (HFSS) is utilized to model the microwave sensor. The sensor detects the salinity and sugar concentration by means of the reflection coefficient. The dielectric constant changes with the increased percentages of the salt and sugar which reduces the reflection coefficient. The microwave sensing device shows considerable sensitivity to determine the salinity and sugar concentration. [ABSTRACT FROM AUTHOR]

Highlights • The tunable microwave sensor for permittivity determination of industrial liquids have been considered. • The Substrate Integrate Waveguide (SIW) is used for making the cavity for improve sensing. • The Photonic Band Gap method is utilized for improve the electric field in the hot spots. • The cavity perturbation technique in order to calculate the permittivity is employed. Abstract In this paper, we present a novel tunable microwave sensor for permittivity determination of industrial liquids. The proposed sensor is cavity based which is developed on a Substrate Integrated Waveguide (SIW). To enhance the characterization accuracy, the reconfigurable sensor is equipped with a Photonic Band Gap method and variable capacitors. Moreover, we employ the cavity perturbation technique in order to calculate the permittivity. In the characterization process, we obtain the permittivity of an unknown material by considering a resonant frequency shift. In fact, a capacitance is the main parameter for controlling the sensor resonance. We herein change this capacitance via reconfigurable SIW cavity and applying different materials. The proposed tunable architecture lets us study the material characteristic in the wider frequency range. The structure is designed in 5–6 GHz in order to determine the electromagnetic behavior of a brand new and used transformer oil samples. The results present a highly accurate permittivity of these oil samples. Hence, the proposed method and setup is not only suitable for oil ageing programs, but also applicable for other industrial liquid applications. [ABSTRACT FROM AUTHOR]

This paper proposes a flexible double-sided attached microwave sensor film for real-time measurement of the liquid dielectric constant, and proposes a nested-complementary split ring resonator（NCSRR）structure with high sensitivity and quality factor values that is based on the structure evolution of a complementary split ring resonator（CSRR）. The sensor films are composed of a ground film with an NCSRR structure and a U-shaped microstrip line film. By taking a borosilicate glass beaker and a polypropylene beaker as examples, two types of microwave sensors are formed by attaching films to both sides of the beakers. Lubricating oil with 0%− 25% water content is added to the beakers separately, and the dielectric constants of the mixture are characterized nondestructively by the transmission coefficient (S21) of the device. According to the mathematical model established from the measurement results, the resonant frequencies of both sensors are linearly and negatively correlated with the dielectric constant of the liquid to be measured, which proves that the sensor film can be attached to a substrate with arbitrary curvature to form a microwave resonant sensor. [Display omitted] • The sensor films are flexible and can be attached to a curved substrate to form a microwave sensor. • The sensor is loaded with an NCSRR-based resonant ring that can be used to detect the dielectric constant of lubricant-water mixtures. • The sensor films are attached to both sides of the beaker to form the sensor, eliminating the need to prepare a chamber to carry the liquids. • The designed sensor offers a high sensitivity in comparison with the state-of-art designs in the literature. [ABSTRACT FROM AUTHOR]

Differential techniques are widely used in communication and sensor systems, as these techniques have been shown to improve the performance. This paper shows how differential sensing of permittivity can be conducted in a simple way. For that purpose, a microstrip line loaded with a pair of stepped-impedance resonators is used in two different resonator connections: parallel and cascade. Each resonator is individually perturbed dielectrically so that: 1) when the two individual permittivities are identical, the structure exhibits a single resonance frequency and 2) when the permittivities are different, resonance frequency splitting occurs, giving rise to two resonances (all these resonances are seen in the form of transmission zeroes). The two sensing approaches are successfully validated through electromagnetic simulations and experiments. By virtue of a differential measurement, robustness against changing ambient factors that may produce sensor miscalibration is expected. [ABSTRACT FROM PUBLISHER]

This study proposed a microwave sensor system to monitor single and two phase flow systems. The microwave sensing technology in this study utilises the resonant frequencies that occur in a cylindrical cavity and monitor the changes in the permittivity of the measured phases to differentiate between the volume fractions of air, water and oil. The sensor system used two port configuration S 21 (acted as transmitter and receiver) to detect the fluids inside the pipe. In principle, the strong polarity of water molecules results in higher permittivity in comparison to other materials. A tiny change of water fraction will cause a significant frequency shift. Electromagnetic waves in the range of 5–5.7 GHz have been used to analyse a two phase air-water and oil-water stratified flow in a pipeline. The results demonstrated the potential of a microwave sensing technique to be used for the two phase systems monitoring. A significant shift in the frequency and change in the amplitude clearly shows the percentage fraction change of water in the pipe. The temperature study of water also demonstrated the independence of microwave analysis technique to the temperature change. This is accounted to overlapping modes negating the affect. Statistical analysis of the amplitude data for two phase systems shows a linear relationship of the change in water percentage to the amplitude. The electromagnetic wave cavity sensor successfully detected the change in the water fraction inside the pipe between 0 and 100%. The results show that the technique can be developed further to reduce the anomalies in the existing microwave sensor. [ABSTRACT FROM AUTHOR]