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Altres ajuts: Institució Catalana de Recerca i Estudis Avançats 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.

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.

Altres ajuts: Secreteraria d'Universitats i Recerca (Generalitat de Catatunya), European Social Fund for the FI Grant, Institució Catalana de Recerca i Estudis Avançats This paper presents a single-frequency reflective-mode phase-variation microwave sensor devoted to the dielectric characterization of materials. The device is implemented in coplanar waveguide (CPW) technology and consists of two parts: (i) the sensing region, a step-impedance resonator (SIR) as termination of a CPW transmission line, and (ii) the design region, a cascade of high/low impedance quarter-wavelength inverters, used to boost up the sensitivity. By placing the so-called material under test (MUT) on top of the sensing region, the capacitance of the SIR is altered due to the effects of the dielectric constant of the MUT. This modifies the phase of the reflection coefficient seen from the input port, the output variable. From a circuit analysis, it is demonstrated that the sensitivity for small perturbations in the vicinity of the dielectric constant of a reference (REF) material can be optimized by setting the operation frequency of the sensor to the resonance frequency of the SIR loaded with such REF material. The maximum sensitivity in one of the reported sensors is as high as 66.5°, and the main figure of merit, defined as the ratio between the maximum sensitivity and the area of the sensing region expressed in terms of the squared guided wavelength, is FoM=3643∘/λ2 . Such figure of merit represents a significant improvement as compared to the one of the equivalent sensor implemented by means of an open-ended quarter-wavelength sensing line. Such equivalence between the semi-lumped element (i.e., SIR-based) sensor and the fully distributed counterpart is also analyzed in the paper.

Altres ajuts: the work of Ferran Martín was supported by the Institució Catalana de Recerca i Estudis Avançats 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∘/λ2 and 12690°/ λ , 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.

This paper presents an active microwave sensor for the characterization of dielectric materials. The sensor is consisted of a microstrip complementary split-ring resonator (MCSRR) structure and an active feedback loop. The loop uses an amplifier to generate negative resistance to compensate the resonator’s loss and increase the loaded quality factor. The developed sensor possesses the advantages of high quality factor, ultra-small electrical size, and high sensitivity. A prototype of the sensor is fabricated and measured for validation.

None-invasive blood glucose measurement enables effective diagnosis and treatment of diabetic patients. The existing microwave sensors suffer from low sensitivity and large size. This article presents a highly sensitive and compact size non-invasive microwave sensor for real-time blood glucose measurements. We have developed a new sensing technique using directly connected Branch Line Coupler and Split Ring Resonators. This technique significantly improves the sensitivity by mitigating the limited coupling between transmission lines and resonators. In addition, using Split Ring Resonators reduces the sensor size due to capacitive and inductive effects, which are loaded to the Branch Line Coupler. The proposed sensing function is based on shifting the transmission notch frequency in coupled arm of the Branch Line Coupler, which is caused by loading the Split Ring Resonators with varying glucose concentration. A prototype sensor is fabricated and successfully tested using several glucose concentrations in deionized water. Experimental results demonstrate 0.72 MHz/mgdL−1 measurement sensitivity, which is higher in comparison with available sensors in PCB technology. The prototype sensor size is $3.5\times 3.5\times 0.16$ cm3.

In daily life, people consume many liquids in their diet like water, soda, milk, juices, etc. Due to the growing demand of these liquids, many adulterants are being used in the market, which degrades the quality of the product and may be harmful to the human being’s health. For a human being, it is very difficult task to find out adulteration in the liquids that they are having in their daily diet. In order to resolve this problem, a device that is very easy to use and provides quick and accurate results is designed, where change in resonant frequency is the parameter used for adulteration detection. In this paper, a split ring resonator-based microwave sensor is proposed for the detection of adulteration in different fluids. The resonant frequency of the sensor is 9.24 GHz. The sensor design exhibit defected ground structure to obtain a sharp resonant frequency. The proposed sensor is fabricated and tested by using water and milk with adulteration (sugar, salt and soap). 2 µl drop of each fluid is placed on the sensor and a shift in resonant frequency from 9.24 to 7.3 GHz for water and 8.4 GHz for milk is noticed.

One of the most important characteristics of planar microwave resonator-based sensors is their sensitivity. Since their measuring parameter is resonance frequency shift, the sensitivity could be described as the shift in the resonance frequency of the microwave resonators versus variation in the dielectric constant of the material under the test. Although label-free microwave resonator-based sensors are capable of providing moderate levels of sensitivity sufficient for enabling numerous applications, detection of material variations in sub-percent or ppm (part per million) range is not easily possible with these structures. This paper investigates the reasons behind the limited sensitivity of planar microwave resonator-based sensors and suggests a measurement structure capable of measuring extremely low concentrations of water contamination in mineral oil samples. Simulation and experimental results verify the capability of the presented sensing structure in the detection of as low water concentrations as 50 ppm in mineral oil samples expressing extremely high sensitivity for the presented sensor with 30 kHz in the frequency shift of the resonator.

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.

This article presents a microwave sensor composed of a cylindrical cavity resonator and thin plastic liquid container for noncontactual detection of the liquid solution concentration. The resonant frequency is tuned to around 11 GHz, resulting in miniaturized dimensions and easy integration with a wireless sensor system. The sensor is with a small volume of 21 cm3 and sensitivity of 0.4 kHz/(mg/L). Simulated results show a good linear relationship between the concentration of the sample solution and resonant frequency shift with the high sensitivity, which is consistent with the experimental result. Compared with other designs, the sensor also has a simple structure, low cost and easy implementation to in-situ and in-vitro detection scenarios.

A novel sensing method that uses the separate design of a sensitivity enhancer and sensing zone is reported. Unlike most resonance-based sensors, the flexible design of a sensing zone is beneficial to the increment of effective material-under-test volume. A novel technique to enhance sensitivity is proposed by engineering the −1/ $x$ -like function microwave transmission to slow the energy-flow velocity. The measured large group delay is up to $\sim 16~\mu \text{s}$ at 2.967 GHz. A lumped model is built to study the condition of the engineered transmission. Compared with a regular coplanar waveguide sensor, the sensitivity is improved up to ~227 times when it is connected to the slow microwave circuit.

Altres ajuts: Universidad Politécnica de Madrid Young Researchers (Grant Number: VJIDOCUPM18MGB) i ICREA 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.

A planar sensor designed using the fourth Hilbert fractal curve iteration for solid material characterization at multiple frequencies is reported in this paper. The fractal curve is self-similar with space filling properties. The Hilbert fractal geometry is used to form a compact resonator, with a multiband frequency response where miniaturization is achieved, since a large transmission line length is effectively confined in a limited area. The sensor provides five resonances in the frequency range from 0 to 5 GHz. The resonant frequencies are 0.56, 1.68, 2.72, 3.69 and 4.72 GHz, all used to measure the real permittivity of known samples with a sensitivity of 7, 20, 27, 43 and 50 MHz/permittivity, respectively. The sensor is used to measure dielectric samples with 20 x 20 mm2 areas with several thicknesses. Simulations and measurements demonstrate that the Hilbert fractal geometry can be used to design a multiband planar sensor for solid dielectric material characterization.

In this study, a loss-compensated microwave (MW) planar sensor is used to characterize fluids at ~1 GHz. The environmental temperature is shown to adversely impact the recorded resonance frequency of the MW sensor, leading to data mixing. This issue is resolved using a feedforward artificial neural network with two hidden layers. Various concentrations of methanol in water (0%-100% with 10% increments) are measured at temperatures ranging between 22 °C and 60 °C. This smart sensor system exhibits a strong ability to discriminate the correct data regardless of erroneous interfering factors up to 92%.

An angular displacement sensor is designed using a corrugated substrate integrated waveguide (CSIW) cavity resonator. Where, in the proposed cavity, instead of copper vias, one of the electric wall...

In this study, a portable prototype of a planar microwave sensor is proposed for intermittent glucose level monitoring. The sensor element basically consists of four distinct hexagonal-shaped complementary split ring resonators (CSRRs) configured in the honey-cell pattern and fabricated on an FR4 dielectric substrate. The CSRRs are coupled via microstrip transmission line to a radar system operating in the ISM band 2.4–2.5 GHz. The integrated sensor shows a remarkable detection sensitivity for the diabetes-spectrum glucose levels dissolved in the blood mimicking aqueous solutions. This is demonstrated by the raw data collected by the radar receiving channel, which shows a clear trend following the variations in glucose levels.

A microwave device able to detect differential-mode to common-mode conversion (or vice versa) in a four-port balanced circuit is proposed. For mode conversion detection, the balanced circuit should be fed by a differential-mode signal, generated by means of a rat-race balun from a single-ended signal. By connecting the pair of output ports of the balanced circuit under test to one of the two pairs of isolated ports of a second rat-race coupler, conversion to the common-mode (if it exists) can be detected and recorded in the $\Sigma $ -port. Since the signal level at the output (single-ended) port depends on mode conversion efficiency, the complete two-port structure, including the pair of rat-race couplers plus the balanced circuit in between, can be used as a comparator or differential microwave sensor based on mode conversion. In such a sensor, the working principle is symmetry disruption (caused, e.g., by an asymmetric dielectric load), and the output variable is the transmission coefficient, related to the level of asymmetry. A detailed analysis, considering an arbitrary unbalanced four-port test structure (to account for symmetry disruption), is carried out. Then, such analysis is particularized to the case of a pair of unbalanced and uncoupled matched lines, and the conditions for sensitivity optimization are obtained. Finally, a differential sensor and comparator based on a pair of balanced (and meandered) matched lines, designed according to the guidelines for sensitivity optimization, is fabricated and validated.

Ship detection is important for a wide range of applications. In this article, an airborne passive interferometric microwave sensor (PIMS) is proposed as a powerful complementary tool for ship detection. The fundamental of ship detection by the airborne PIMS is introduced. A specification called “the detection factor” is addressed to quantitatively assess the performance of the airborne PIMS for ship detection. The detection probability is discussed and addressed based on the constant false-alarm rate (CFAR). Numerical simulations are performed to demonstrate the feasibility of ship detection by the airborne PIMS. The impacts of the atmosphere and the sea wind on the performance of the airborne PIMS for ship detection are also addressed in terms of theoretical analyses and numerical simulations. The airborne experiments are also carried out by an X-band PIMS to verify the practicability of ship detection by the airborne PIMS in a sunny weather and a cloudy weather. The data processing procedure and a ship detection algorithm in the experiments are introduced. The airborne experimental results reveal that the ships can be effectively detected and tracked by the airborne X-band PIMS in a sunny weather and a cloudy weather. It demonstrates the practicability of ship detection by the airborne PIMS. The ship detection probability is also analyzed, which agrees well with the theoretical detection probability.

Microwave electromagnetic devices have been used for many applications in tropospheric communication, navigation, radar systems, and measurement. The development of the signal preprocessing units including frequency-selective devices (bandpass filters) determines the reliability and usability of such systems. In wireless sensor network nodes, filters with microstrip resonators are widely used to improve the out-of-band suppression and frequency selectivity. Filters based on multimode microstrip resonators have an order that determines their frequency-selective properties, which is a multiple of the number of resonators. That enables us to reduce the size of systems without deteriorating their selective properties. Various microstrip multimode resonator topologies can be used for both filters and microwave sensors, however, the quality criteria for them may differ. The development of every resonator topology is time consuming. We propose a technique for the automatic generation of the resonator topology with required frequency characteristics based on the use of evolutionary algorithms. The topology is encoded into a set of real valued parameters, which are varied to achieve the desired features. The differential evolution algorithm and the genetic algorithm with simulated binary crossover and polynomial mutation are applied to solve the formulated problem using the dynamic penalties method. The experimental results show that our technique enables us to find microstrip resonator topologies with desired amplitude-frequency characteristics automatically, and manufactured devices demonstrate characteristics very close to the results of the algorithm. The proposed algorithmic approach may be used for automatically exploring the new perspective topologies of resonators used in microwave filters, radar antennas or sensors, in accordance with the defined criteria and constraints.

Planar microwave sensors have become increasing developed in recent decades, especially in material characterization (solid/liquid) as they provide regions highly sensitive to the surrounding medium. However, when it comes to deciphering the content of practical biological analytes or chemical components inside a host medium, even higher sensitivities are required due to their minute concentrations. This review article presents a comprehensive outlook on various methodologies to enhance sensitivity (e.g., coupling resonators, channel embedding, analyte immobilization, resonator pattern recognition, use of phase variation, using coupled line section, and intermodulation products), resolution (active sensors, differential measurements), and robustness (using machine learning) of arbitrary sensors of interest. Some of the most practical approaches are presented with prototype examples, and the main applications of incorporating such procedures are reported. Sensors with which the proposed techniques are implemented exhibit higher performance for high-end and real-life use.

Altres ajuts: Secreteraria d'Universitats i Recerca (Generalitat de Catalunya) and European Social Fund FI grant Microstrip transmission lines loaded with dumbbell defect-ground-structure (DB-DGS) resonators transversally oriented have been exhaustively used in microwave circuits and sensors. Typically, these structures have been modelled by means of a parallel LC resonant tank series connected to the host line. However, the inductance and capacitance of such model do not have a physical meaning, since this model is inferred by transformation of a more realistic model, where the DB-DGS resonator, described by means of a resonant tank with inductance and capacitance related to the geometry of the DB-DGS, is magnetically coupled to the host line. From parameter extraction, the circuit parameters of both models are obtained by considering the DB-DGS covered with semi-infinite materials with different dielectric constant. The extracted parameters are coherent and re-veal that the general assumption of considering the simple LC resonant tank series-connected to the line to describe the DB-DGS-loaded line is reasonable with some caution. The implications on the sensitivity, when the structure is devoted to operating as a permittivity sensor, are discussed.

This article presents an optimized microwave sensor for the non-contact measurement of complex permittivity and material thickness. The layout of the proposed sensor comprises the parallel combination of an interdigital capacitor (IDC) loaded at the center of the symmetrical differential bridge-type inductor fabricated on an RF-35 substrate (εr = 3.5 and tanδ = 0.0018). The bridge-type differential inductor is introduced to obtain a maximum inductance value with high quality (Q) factor and low tunable resonant frequency. The central IDC structure is configured as a spur-line structure to create a high-intensity coupled electric field (e-field) zone, which significantly interacts with the materials under test (MUTs), resulting in an increased sensitivity. The proposed sensor prototype with optimized parameters generates a resonant frequency at 1.38 GHz for measuring the complex permittivity and material thickness. The experimental results indicated that the resonant frequency of the designed sensor revealed high sensitivities of 41 MHz/mm for thickness with a linear response (r2 = 0.91567), and 53 MHz/Δεr for permittivity with a linear response (r2 = 0.98903). The maximum error ratio for measuring MUTs with a high gap of 0.3 mm between the testing sample and resonator is 6.52%. The presented performance of the proposed sensor authenticates its application in the non-contact measurement of samples based on complex permittivity and thickness.

An automatic humidity control system for mushroom growing house based on the free-space technique is presented. The novelty of this work is the modified free-space technique by measuring the amplitude only of transmission coefficient |S21| that reflected from mushroom by using beam-switching antenna with artificial neural networks (ANNs) as a humidity sensor to control quantity and time of water misting nozzle. In the proposed system, the antenna is designed to act as the transmitting antenna at the frequency of 2.45 GHz. Its radiation patterns can be switched to 4 directions covering all corners of mushroom growing house. The measured |S21| from each direction are converted to direct current (DC) voltage by a radio frequency (RF) detector; then are trained with ANNs in the humidity range of 60-85%. The optimized ANNs structure consists of 4 input nodes, two layers of 5 hidden nodes, and 3 output nodes. To verify the proposed system, experiments were set up in controlled humidity mushroom growing house at the humidity level of 75-80% for 120 hours. The results showed that there was slightly average standard deviation (S.D.) of humidity level 1.36. Consequently, the performance of sensor system assures that it is able to apply for humidity control in large growing house.

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.

This work reports an alternative technique based on a complementary double split-ring resonator to determine the concentrations of ammonia and iron in water. The conventional methods for measuring ammonia and iron concentration involve expensive and bulky instruments in the laboratory. Furthermore, the current methods involve hazardous chemical reagents and complicated procedures. In this work, a low-cost microwave sensor which is portable and easy to operate is proposed. The water samples that have different ammonia or iron concentration is dropped deposited at the sensing area. Their concentration is determined based on the shifting of the resonance frequency in the measurement. The measurement results indicate that the resonance frequency of the resonator reduced from 2.38 GHz (unloaded condition) to a low-frequency range when the water sample containing different concentrations of ammonia or iron are introduced to the sensing area. Based on the collected results, Fourier models are used to establish the relationship between the concentrations of ammonia and iron (in the range of 0 to 53 mg/L) to the resonance frequency shifting. The validity of these models is proven as they introduced a root mean square error (RMSE) of less than 0.009. The findings of this work demonstrated the feasibility of the microwave sensor in detecting water pollutants.

The use of microwave technology is currently under investigation for non-invasive estimation of glycemia in patients with diabetes. Due to their construction, metamaterial (MTM)-based sensors have the potential to provide higher sensitivity of the phase shift of the S21 parameter (∠S21) to changes in glucose concentration compared to standard microstrip transmission line (MSTL)-based sensors. In this study, a MSTL sensor and three MTM sensors with 5, 7, and 9 MTM unit cells are exposed to liquid phantoms with different dielectric properties mimicking a change in blood glucose concentration from 0 to 14 mmol/L. Numerical models were created for the individual experiments, and the calculated S-parameters show good agreement with experimental results, expressed by the maximum relative error of 8.89% and 0.96% at a frequency of 1.99 GHz for MSTL and MTM sensor with nine unit cells, respectively. MTM sensors with an increasing number of cells show higher sensitivity of 0.62° per mmol/L and unit cell to blood glucose concentration as measured by changes in ∠S21. In accordance with the numerical simulations, the MTM sensor with nine unit cells showed the highest sensitivity of the sensors proposed by us, with an average of 3.66° per mmol/L at a frequency of 1.99 GHz, compared to only 0.48° per mmol/L for the MSTL sensor. The multi-cell MTM sensor has the potential to proceed with evaluation of human blood samples.

This paper presents a microwave sensor based on a two-ports network for liquid characterizations. The proposed sensor is constructed as a miniaturized microwave resonator based on Moore fractal geometry of the 4th iteration. The T-resonator is combined with the proposed structure to increase the sensor quality factor. The proposed sensor occupies an area of 50 × 50 × 1.6 mm3 printed on an FR4 substrate. Analytically, a theoretical study is conducted to explain the proposed sensor operation. The proposed sensor was fabricated and experimentally tested for validation. Later, two pans were printed on the sensor to hold the Sample Under Test (SUT) of crude oil. The frequency resonance of the proposed structure before loading SUT was found to be 0.8 GHz. After printing the pans, a 150 MHz frequency shift was accrued to the first resonance. The sensing part was accomplished by monitoring the S-parameters in terms of S12 regarding the water concentration change in the crude oil samples. Therefore, 10 different samples with different water percentages were introduced to the proposed sensor to be tested for detecting the water content. Finally, the measurements of the proposed process were found to agree very well with their relative simulated results.

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 (27.8×18.4×0.508 mm3) is smaller than those reported in the literature.

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 ...

In this paper, a microwave microfluidic sensor based on spoof surface plasmon polaritons (SSPPs) was proposed for ultrasensitive detection of dielectric constant. A novel unit cell for the SSPP structure is proposed and its behaviour and sensing potential analysed in detail. Based on the proposed cell, the SSPP microwave structure with a microfluidic reservoir is designed as a multilayer configuration to serve as a sensing platform for liquid analytes. The sensor is realized using a combination of rapid, cost-effective technologies of xurography, laser micromachining, and cold lamination bonding, and its potential is validated in the experiments with edible oil samples. The results demonstrate high sensitivity (850 MHz/epsilon unit) and excellent linearity (R2 = 0.9802) of the sensor, which, together with its low-cost and simple fabrication, make the proposed sensor an excellent candidate for the detection of small changes in the dielectric constant of edible oils and other liquid analytes.

Microwave sensors have recently been introduced as high-temporal resolution sensors, which could be used in the contactless monitoring of artery pulsation and breathing. However, accurate and efficient signal processing methods are still required. In this paper, the matrix pencil method (MPM), as an efficient method with good frequency resolution, is applied to back-reflected microwave signals to extract vital signs. It is shown that decomposing of the signal to its damping exponentials fulfilled by MPM gives the opportunity to separate signals, e.g., breathing and heartbeat, with high precision. A publicly online dataset (GUARDIAN), obtained by a continuous wave microwave sensor, is applied to evaluate the performance of MPM. Two methods of bandpass filtering (BPF) and variational mode decomposition (VMD) are also implemented. In addition to the GUARDIAN dataset, these methods are also applied to signals acquired by an ultra-wideband (UWB) sensor. It is concluded that when the vital sign is sufficiently strong and pure, all methods, e.g., MPM, VMD, and BPF, are appropriate for vital sign monitoring. However, in noisy cases, MPM has better performance. Therefore, for non-contact microwave vital sign monitoring, which is usually subject to noisy situations, MPM is a powerful method.

In this paper, we describe a low-cost microwave microfluidic system of ultrahigh sensitivity for detecting small changes in the concentration of polar solutions (liquid dielectrics) in the 2.4 GHz ISM band. Its principle of operation is based on microwave interferometry, which is implemented using planar microstrip lines and integrated microwave components. The key features of this system include small solution intake (&lt, 200 µL per measurement), short time of measurement (ca. 20 ms), ultrahigh sensitivity of concentration changes (up to 55 dB/%), and low error of measurement (below 0.1%). The ultrahigh sensitivity was proven experimentally by measurements of the fat content of milk. In addition, it is a user-friendly system due to an effortless and fast calibration procedure. Moreover, it can be made relatively compact (&lt, 20 cm2) and features low power consumption (200 mW). Thus, the proposed system is perfect for industrial applications, especially for highly integrated lab-on-chip devices.

The article presents the results of experimental studies of the dc and high-frequency electrical characteristics of planar microwave diodes that are fabricated on the base of the n-AlxGa1-xAs layer (x = 0, 0.15 or 0.3), epitaxially grown on a semi-insulating GaAs substrate. The diodes can serve as reliable and inexpensive sensors of microwave radiation in the millimeter wavelength range, they sense electromagnetic radiation directly, without any external bias voltage at room temperature. The investigation revealed a strong dependence of the detection properties of the microwave diodes on AlAs mole fraction x: in the Ka microwave frequency range, the median value of voltage responsivity is several volts per watt in the case of GaAs-based diodes (x = 0), and it substantially increases, reaching hundreds of volts per watt at higher x values. Also, a model enabling us to forecast the responsivity of the sensor in other frequency ranges is proposed.

Microwave planar sensors employ conventional passive complementary split ring resonators (CSRR) as their sensitive region. In this work, a novel planar reflective sensor is introduced that deploys CSRRs as the front-end sensing element at fres=6 GHz with an extra loss-compensating negative resistance that restores the dissipated power in the sensor that is used in dielectric material characterization. It is shown that the S11 notch of −15 dB can be improved down to −40 dB without loss of sensitivity. An application of this design is shown in discriminating different states of vanadium redox solutions with highly lossy conditions of fully charged V5+ and fully discharged V4+ electrolytes.

A complex permittivity characterization method for liquid samples has been proposed. The measurement is carried out based on a self-designed microwave sensor with a split ring resonator (SRR), the unload resonant frequency of which is 5.05 GHz. The liquid samples in capillary are placed in the resonant zone of the fabricated senor for high sensitivity measurement. The frequency shift of 58.7 MHz is achieved when the capillary is filled with ethanol, corresponding a sensitivity of 97.46 MHz/μL. The complex permittivity of methanol, ethanol, isopropanol (IPA) and deionized water at the resonant frequency are measured and calibrated by the first order Debye model. Then, the complex permittivity of different concentrations of aqueous solutions of these materials are measured by using the calibrated sensor system. The results show that the proposed sensor has high sensitivity and accuracy in measuring the complex permittivity of liquid samples with volumes as small as 0.13 μL. It provides a useful reference for the complex permittivity characterization of small amount of liquid chemical samples. In addition, the characterization of an important biological sample (inositol) is carried out by using the proposed sensor.

Microwave sensors are principally sensitive to effective permittivity, and hence not selective to a specific material under test (MUT). In this work, a highly compact microwave planar sensor based on zeroth-order resonance is designed to operate at three distant frequencies of 3.5, 4.3, and 5 GHz, with the size of only λg−min/8 per resonator. This resonator is deployed to characterize liquid mixtures with one desired MUT (here water) combined with an interfering material (e.g., methanol, ethanol, or acetone) with various concentrations (0%:10%:100%). To achieve a sensor with selectivity to water, a convolutional neural network (CNN) is used to recognize different concentrations of water regardless of the host medium. To obtain a high accuracy of this classification, Style-GAN is utilized to generate a reliable sensor response for concentrations between water and the host medium (methanol, ethanol, and acetone). A high accuracy of 90.7% is achieved using CNN for selectively discriminating water concentrations.

Defect detectors and comparators, able to detect tiny differences between a reference (REF) sample and a material under test (MUT), are presented in this paper. In one version, the comparator is implemented by means of a rat-race hybrid coupler and a single step-impedance open-ended transmission line connected to one of the ports of the coupler. The comparator exploits the imbalance generated in one of the two pairs of isolated ports of the coupler when the MUT sample differs from the REF sample. The other pair of isolated ports is used for comparator feeding (Δ-port) and for recording the output signal (Σ-port), and feeding is achieved by means of a harmonic signal tuned to the operating frequency of the coupler. The resulting device is therefore a two-port structure, and the considered output variable is the magnitude of the transmission coefficient, an easily measurable quantity. By virtue of the step-impedance transmission line, used as the sensitive element of the comparator, the device exhibits very high sensitivity to variations in the dielectric constant between the REF and MUT samples, this being the input variable. A prototype device example, with a maximum sensitivity of 0.72 is reported, and applied to the detection of defects in the REF sample, a dielectric slab, generated by drilling holes of different densities across it. In the second prototype, a true differential-mode sensor, two identical sensing elements (step impedance lines) are connected to one of the pairs of isolated ports of the coupler, and roughly perfect balance is obtained when the MUT is identical to the reference sample. The maximum sensitivity in this case is 1.20, and the device discriminates also small perturbations generated in the REF material. The main relevant aspect of the reported prototypes, phase-variation sensing devices by nature, is the phase-to-magnitude conversion achieved by means of the hybrid coupler. This transforms the reflective-mode phase-variation sensing element to a two-port transmission-mode device where the output variable is the magnitude of the transmission coefficient, an easily measurable quantity.

Recent developments in the field of microwave planar sensors have led to a renewed interest in industrial, chemical, biological and medical applications that are capable of performing real-time and non-invasive measurement of material properties. Among the plausible advantages of microwave planar sensors is that they have a compact size, a low cost and the ease of fabrication and integration compared to prevailing sensors. However, some of their main drawbacks can be considered that restrict their usage and limit the range of applications such as their sensitivity and selectivity. The development of high-sensitivity microwave planar sensors is required for highly accurate complex permittivity measurements to monitor the small variations among different material samples. Therefore, the purpose of this paper is to review recent research on the development of microwave planar sensors and further challenges of their sensitivity and selectivity. Furthermore, the techniques of the complex permittivity extraction (real and imaginary parts) are discussed based on the different approaches of mathematical models. The outcomes of this review may facilitate improvements of and an alternative solution for the enhancement of microwave planar sensors’ normalized sensitivity for material characterization, especially in biochemical and beverage industry applications.

A new microwave sensor is proposed to characterize the complex relative permittivity of building non-magnetic materials and used in the characterization of three concrete samples. The proposed sensor structure consists of a log-periodic planar antenna with microstrip elements tilted forward by an angle &beta, and printed, alternately, on the top and bottom sides of a dielectric layer. The operation principle is based on the measurement of the scattering parameters S11 and S21 in a free space propagation transmitter-receiver setup, for both cases with the material under test (MUT) sample (non-line-of-sight, NLOS) and without it (line-of-sight, LOS). A prototype is fabricated and measured to determine the scattering parameters of concrete samples. After measurements, the obtained results are used in the efficient and accurate Nicolson&ndash, Ross&ndash, Weir (NRW) method, making it possible to estimate the values of the complex relative permittivity of the concrete blocks. The sensor design is demonstrated from initial simulations to measurements for validation of the developed prototype. The obtained results for the complex relative permittivity of concrete are in agreement with those available in the literature and the difference between the simulated and measurement results for the sensor antenna resonant frequency is 4.71%. The used measurement setup can be applied to characterize different types of solid or liquid dielectric materials.

In a numerical study, we propose a metamaterial structure mimicking electromagnetically induced reflectance at microwave frequencies. The structure contains imprinted metallic elements, including two long and two short wires on a dielectric substrate. In analyzing different loss mechanisms affecting the resonance behavior of the structure, we study effects of temperature and metallic thickness in detail, and see an enhanced resonance behavior as the temperature is reduced or the metallic thickness is increased. In applications, we first show that the proposed structure can be used as a sensitive microwave sensor. Furthermore, the group index of the structure is calculated to be 563.8 at 15.14 GHz, which can also be further increased by reducing temperature and increasing metallic thickness. Even this value is high compared to the values reported in the literature at microwave frequencies, and makes the structure a good candidate for slow light applications.

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.

This paper proposed a single port rectangular microwave resonator sensor. This sensor operates at the resonance frequency of 4GHz. The sensor consists of micro-strip transmission line and applied the enhancement method. The enhancement method is able to improve the return loss of the sensor, respectively. Plus, the proposed sensor is designed and fabricated on Roger 5880 substrate. Based on the results, the percentage of error for the proposed rectangular sensor is 0.2% to 8%. The Q-factor of the sensor is 174.