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In this paper, a high sensitivity concentration sensor for aqueous solutions is presented. The device is a cylindrical cavity resonator operating around a frequency of 1.5 gigahertz. The measurement procedure is simple, instantaneous, non-destructive, and obtains information from two different resonance parameters (frequency and amplitude). Samples are introduced via small, standard test tubes, and a very small amount (around four milliliters) is sufficient to perform a measurement. The design is first approached from a theoretical perspective, after which, empirical adjustments are paired with a simulation-based design optimization. The resulting topology is constructed, showing agreement with said simulations. The final sensor can be used, after a regression type calibration, to indirectly measure the concentration of a liquid solution of known components. The experimental validation, employing different mixtures, demonstrates that the proposed device shows an enhanced sensitivity to solution concentration when compared to other resonator-based arrangements found in the literature. [ABSTRACT FROM AUTHOR]

The present research work is the part of the feasibility study to implement non-invasive blood-glucose sensing using microwave-based sensor. The one-port microstrip patch antenna sensor developed to exhibit variation in its electromagnetic response with respect to blood-glucose level. It has been demonstrated through in vitro controlled experiment under laboratory conditions. The novel multi-parameter-based sensitivity factor termed as GlucoSense was also defined to evaluate the sensitivity of sensor towards the change in glucose concentration. It was established that there was a good correlation between sensor's reflection properties and glucose concentration. The findings of the controlled experiment convincingly proved that the sensor has the potential to be effectively used for developing a suitable device for non-invasive blood-glucose sensing. [ABSTRACT FROM AUTHOR]

In this paper, three dual notch microwave sensors are presented based on a microstrip transmission line and complementary metamaterial resonators. The main aim of this paper is to compare the constitutive parameters and sensitivity of all three dual notch sensors which are based on complementary symmetric split ring resonator (CS-SRR), complementary asymmetric split ring resonator (CAS-SRR) and complementary bisymmetric split ring resonator (CBS-SRR). The main motivation beyond the presented work is to use dual notches to estimate the relative permittivity of material under test (MUT). Electromagnetic simulation elucidates the origin of dual mode resonance of all the three resonators. Sensitivity analysis is performed on each sensor by using fifteen MUTs with relative permittivity ranges from 1.006 to 16.5 and constant dimensions 10 mm × 10 mm × 1 mm. To verify the concept, a sensor is fabricated and its response is measured using a vector network analyzer (AV3672). Using curve fitting technique the shift in the resonance frequencies of the fabricated sensor due to interaction with MUT is presented as a function of permittivity. Simulated, measured and formulated results are in good agreement with each other.

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.

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]

The article presents the design of the near-field probe, which is a combined emitter (a combination of a symmetric dipole and an annular frame). The design of the probe allows forming a prolonged zone of the near-field. This effect can be used for in-depth penetration of the field in media with high absorption, without loss of information. Particular attention in this article is given to a detailed study of the interaction of the field created by this probe on plane-layered biological media. A theoretical analysis of the interaction of the electromagnetic field was carried out in a wide frequency band with a model plane-layer biological medium containing blood vessels of shallow depth using the proposed probe design. Conclusions are drawn about the depth of penetration of a useful signal into different media-analogs of biological tissue. This study is necessary to consider the possibility of using this probe for non-invasive measurements of blood glucose concentration. The studies were carried out using numerical simulation in the CST (Computer Simulation Technology) Microwave Studio environment. All biological tissues were simulated over a wide frequency range from 10 MHz to 10 GHz. [ABSTRACT FROM AUTHOR]

This paper reviews methods used for measuring the vibration of dynamic objects, including a promising method for non-contact vibration diagnostics based on Doppler microwave sensors. The method involves actively irradiating an object with electromagnetic waves and applying a two-channel quadrature processing to the reflected signal received. The article also discusses modes of analysis of the subtle resulting signal using spectral or wavelet transformations depending on the nature of the active vibration. The advantages of this non-contact and remote vibration analysis method for studying complex dynamic objects are described. Finally, we evaluate the applicability of this method in machine learning and intelligent systems employed for the non-destructive continuous monitoring of the state of complex dynamic objects under vibration. [ABSTRACT FROM AUTHOR]

This work proposes a novel method for detecting the concentration of water present in liquid‐water binary mixture in real time with a low cost, portable measurement setup. The sensing element in the proposed system is a two‐port microstrip bandstop resonator based on coupled line open‐ended quarter wavelength section. Voltage control oscillator and power detector ICs are used to generate and detect the microwave power at specific bandstop frequency. Water concentrations in commercially available liquids are monitored both in batch and continuous mode of operation using separate 3D printed liquid containing fixtures attached to the open end of the microstrip line. The measured results successfully validate the design theory. [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]

The novel application of microwave directional couplers to develop angular‐displacement microwave sensors is reported. The proposed sensor approach employs as stator a branch‐line‐type coupler arranged in transversal mode by loading its direct and coupled ports with two distinct‐length open‐ended stubs. Thus, by taking the isolated port of the coupler as the stator output node, a bandpass filtering transfer function with transmission zeros (TZs) is created. Then, a rotor made up of an angularly‐moveable open‐ended stub is attached to a curved section of the longest loading stub of the stator through physical contact, so that their interconnection point varies with the angular‐displacement of the rotor. In this manner, the sensor transfer function is altered with the stub rotation through TZ reallocation, angular‐displacement sensing capabilities are achieved. The theoretical operational foundations of the conceived branch‐line‐coupler‐based microwave angular‐displacement sensor, which features single/multi‐band sensing properties in terms of inter‐TZ spacing and stop band attenuation levels, along with design examples and curves are provided. The extrapolation of this sensor principle to other classes of power‐distribution circuits, such as the rat‐race‐type directional coupler, is also demonstrated. Finally, for experimental‐validation purposes, two 920 MHz microstrip prototypes of the conceived branch‐line‐coupler‐based angular‐displacement microwave‐sensor approach are built and measured. [ABSTRACT FROM AUTHOR]

A novel rotational angle microwave sensor with a high dynamic range and sensitivity for detecting angular displacement is presented. It exploits the spectral separation between two notch frequencies or transmission zeros that are generated in its transfer function, and whose value varies as a function of the rotation angle to be measured. The sensor is based on a bi-path transversal-signal-interference (TSI) circuit configuration that is implemented on microstrip technology, where a rotor comprised of an open-circuit-ended stub is loaded onto a stator of this transversal circuit with equal-length electrical paths. The mechanical rotation of the rotor changes the contact point of the open-circuit-ended line to the stator, thus effectively modifying one of the overall paths of the stator and, hence, the inter-transmission-zero separation. Design guidelines for the synthesis of the proposed rotation sensor with a specified dynamic range and sensitivity are provided. Furthermore, a proof-of-concept microstrip-substrate-based prototype with an angular rotation range of 180° is developed at 920 MHz and tested to validate this approach experimentally. [ABSTRACT FROM AUTHOR]

We propose a low-cost, easy-to-fabricate, contactless microwave sensor for dielectric characterization of liquids. The design of the proposed sensor is based on a multiple complementary split-ring resonator (MCSRR) fabricated on a low-cost FR-4 substrate. A glass capillary tube having an inner diameter of $0.008\lambda _{{0}}$ is inserted in the high electric field region of the MCSRR to carry the liquid under test. The sensor is designed to operate at a resonant frequency of 2.45GHz for an empty tube and shifted resonant peaks are utilized for the dielectric characterization of different liquids. The maximum observed shifts in resonant frequency and Q factor are up to 400MHz and 31, respectively. The numerically established relations are experimentally verified through fabricated sensor for various binary mixtures of water and ethanol. The percentage errors between the calculated and reference permittivity of different samples are noticed to be less than 5%. The proposed device promises to be a cost-effective and convenient solution for accurate dielectric characterization of liquids and their binary aqueous solutions. [ABSTRACT FROM AUTHOR]

A microwave pressure sensor based on variations in the transmission zero of a microstrip circuit is proposed in this article. The structure of the sensor is consisting of a microstrip line connected in parallel to a single stub, which the stub is terminated in a reactive load. When the length of stub is around quarter‐wavelength, the frequency of the transmission zero of microstrip line can be controlled by the reactance value. Therefore, by using a simple parallel plate capacitor as a transducer in which its capacitance is changed by the pressure, a microwave pressure sensor is realized. A prototype pressure sensor by using a circular‐shape parallel plate capacitor is designed and fabricated that the measurement results show a good linear variation of transmission zero with pressure with sensitivity of −0.57 MHz/kPa when the pressure changes from 0 to 466 kPa. [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]

A hybrid plasmonic resonator is proposed via electromagnetic modal interference. The hybrid mode exhibits a 3.4-fold enhancement to quality factor ($Q$ -factor) and a 79.8-fold enhancement to figure of merit (FoM) over the original mode, breaking the common tradeoff between the ${Q}$ -factor and resonance intensity. Electromagnetic simulation and temporal coupled-mode theory are used to interpret the mode evolution in the hybrid resonator structure. More importantly, this high-Q and high-FoM resonance can be excited in a single resonator unit, reducing the footprint and showing potential applications in integrated sensor chips. The sensing feasibility is demonstrated with a 4.1-fold enhancement of sensitivity and 12.8-fold enhancement of sensing FoM. [ABSTRACT FROM AUTHOR]

In this study, prepared nanocomposite materials made of polyester were prepared with a fixation hardener ratio of 0.1 g per 10 ml of polyester. This percentage was constant in all samples and addition of both chlorophyll(Cll) and lead oxide (PbO<100nm) as well as lead oxide with chlorophyll by weight percentage (0.2,0.5, 0.2 (Cll) with 0.5(PbO)wt.% receptivity, then study coefficient of loss of reflectivity of all samples where the results indicated that most prepared materials correct for use attenuation materials of microwaves where is reflection coefficient of all samples larger than (10dB) this show that absorption of composite materials consistence larger than 90%, then study reflection coefficient and also definition complex permittivity in the range frequencies (3-5)GHz where observed that value of the complex permittivity larger than one (μR >1). [ABSTRACT FROM AUTHOR]

This letter presents a practical application of a differential microstrip microwave sensor for label-free and real-time monitoring of Escherichia coli (E. coli) growth under different nutritional conditions on solid agar media. Two planar microstrip ring resonators coupled with a one-port power divider form the core of the sensing system. The sensing resonator operates at 1.509 GHz with an amplitude of −10.21 dB and the reference resonator operates at 1.798 GHz with an amplitude of −11.75 dB. The growth profile of E. coli at each glucose concentration in Luria-Bertani (LB) agar was derived by measuring the resonant amplitude variations ($\Delta {A}$) and is further supported by microscopic images. A complete inhibition of growth was observed at 10% glucose concentration. This research expands the application of microwave sensors to investigate the effect of nutritional parameters on bacterial growth. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [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]

This work proposes the development of a novel sensor comprising a rectangular microstrip antenna enclosed within a small cubic container made of acrylonitrile-butadiene-styrene (ABS). A complementary split ring resonator (CSRR) has been incorporated into the antenna, endowing it with two resonance frequencies within the desired range. The sensor is designed to determine the soil water content (SWC) of various soil types. The operational principle relies on the modulation of two resonance frequencies of the antenna due to changes in the relative permittivity of the material under test (MUT). The sensor's sensitivity was thoroughly examined and compared with various sensors documented in the literature. Additionally, an electric field analysis was conducted, revealing that optimizing the dimensions of the CSRR can enhance its sensitivity. Consequently, the proposed sensor exhibited superior sensitivity compared to other sensors reported in the literature. [Display omitted] • Sensitivity analysis of the proposed Compact CSRR-Based sensor. • The optimization of the sensor based in the electromagnetic fields' analysis. • The analysis of dielectric properties of two different types of soil. • Determination of soil water content with the proposed sensor and fitted equations. • Improvement of the sensitivity compared with the other systems presented. [ABSTRACT FROM AUTHOR]

Molecularly imprinted sol–gel silica (MIS) coupled to a microwave sensor was designed and used to detect phenylacetaldehyde (PAA), a chemical tracer of wine oxidation. The developed method is fast, cheap and could replace the classical chromatographic methods, which require a tedious sample preparation and are expensive. To reach our objective, five MIS and their control non-imprinted silica (NIS) were synthesized and their extraction capacity toward PAA was studied in hydro alcoholic medium. The selected polymers, based on this first step, were subjected to a selectivity study in the presence of PAA and three other competing molecules. The best polymer was integrated in a microwave sensor and was used to assess PAA in red wine. The developed sensor was able to detect PAA at the µg·L−1 level, which is below the off-flavour threshold.

Robust calibration and validation (Cal and Val) should guarantee the accuracy of the retrieved information, make the remote sensing data consistent and traceable, and maintain the sensor performance during the operational phase. The DRAGON program has set up many remote sensing research topics on various application domains. In order to promote the effectiveness of data modeling and interpretation, it is necessary to solve various challenges in Cal and Val for quantitative RS applications. This project in the DRAGON 4 program aims to promote the cooperation of the Cal and Val experts from European and Chinese institutes in Cal and Val activities, and several achievements have been obtained in the advanced on-orbit optical sensor calibration, as well as microwave remote sensor calibration and product generation. The outcomes of the project have benefited the related remote sensing modeling and product retrieval, and promoted the radiometric calibration network (RadCalNet) as an international operational network for calibration, intercalibration, and validation. Moreover, this project provided local governments with a more accurate OMI NO2 data in China, which were used to study the air quality control during APEC period, Parade period and G20 period. This will be of ongoing be value for monitoring atmospheric environmental quality and formulating pollution control strategies.

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 reveal 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.

In this paper, a high-sensitive microwave dielectric sensor targeted for material permittivity characterization is proposed. A reflective voltage controlled oscillator is devised by virtue of the compact microstrip resonant cell (CMRC) and embedded into a fractional-N phase-locked loop frequency synthesizer. The CMRC structure, covered with material under test, changes its resonating characteristics and, hence, the initial oscillating frequency. Due to the presence of the frequency synthesizer, this frequency shift is finally translated into the controlling voltage variation. In this paper, the material permittivity is extracted by measuring the voltage deviation. Several different samples with known permittivity values are tested to verify the effectiveness of the design sensing platform.

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 |S21| as a function of the water concentration in ppm. The best sensitivity Δ|S21|Δρ=1.42 mdB/ppm is obtained at a frequency of f=9.76 GHz, with a coefficient of determination R2=0.94. The sensor is low-cost, low profile and ensures a good sensitivity for constant and real-time monitoring.

In this paper, a very high sensitivity microwave-based planar microfluidic sensor is presented. Sensitivity enhancement is achieved and described theoretically and experimentally by eliminating any extra parasitic capacitance not contributing to the sensing mechanism. The sensor consists of a microstrip transmission line loaded with a series connected shunt LC resonator. A microfluidic channel is attached to the area of the highest electric field concentration. The electric field distribution and, therefore, the resonance characteristics are modified by applying microfluidic dielectric samples to the sensing area. The sensor performance and working principle are described through a circuit model analysis. A device prototype is fabricated, and experimental measurements using water/ethanol and water/methanol solutions are presented for validation of the sensing mathematical model.

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.

This paper presents the design and fabrication of proximity coupled feed disk resonator coated with Multi Walled Carbon Nanotubes (MWCNTs) and Polypyrrole-Chitosan (PPy-CHI) layers as a napropamide sensor. Computer Simulation Technology (CST) microwave studio was used to obtain the best design of disk resonator and feed line position in 5 GHz resonant frequency. Also, MWCNTs - PPy-CHI layers were coated on the disk resonator using electric field deposition and chemical interaction between sensing layer and napropamide was investigated by Fourier Transform Infrared Spectroscopy (FT-IR). The evaluation of the system was performed using different concentrations of commercial napropamide and pure napropamide at room temperature (25 0C). Experimental results prove that proximity coupled feed disk resonator coated with MWCNTs-PPy-CHI layers is a simple, fast (Measurement- time=5 seconds), accurate (as low as 0.1 ppm), low cost and it has the potential of fabrication as a portable instrumentation system for detecting pesticides in water and soil.

The construction of ammonia gas sensors with wide detection ranges is important for exhalation diagnosis and environmental pollution monitoring. To achieve a wide detection range, sensitive materials must possess excellent spatial confinement and large active surfaces to enhance gas adsorption. In this study, an ammonia microwave gas sensor with a wide detection range of 10 ppb-0.55 v/v% at room temperature was fabricated by incorporating hollow multishelled-structured BaTiO 3 (HoMS-BaTiO 3 ). The effect of the number of shells and the quantity of the sensitive material on the gas-sensing performance was investigated, and two-layered HoMS-BaTiO 3 demonstrated the best response at high concentrations (0.15-0.55 v/v%). Conversely, single-layered HoMS-BaTiO 3 displayed outstanding performance at low concentrations (10 ppb-0.15 v/v%). The lower the quantity of the sensitive material, the higher the response. This study offers a method for preparing room-temperature ammonia sensors with a wide detection range and reveals the link between the structure and quantity of sensitive materials and gas-sensing performance.

This paper presents high-sensitivity sensors based on an open complementary square split-ring resonator and a modified open complementary split-ring resonator operating at 4.5 GHz and 3.4 GHz, respectively. The sensors are designed for the detection of multiple liquid materials, including distilled water, methanol, and ethanol. The liquid under test is filled in a glass container loaded using a pipette. Compared to the conventional OCSSRR, the modified OCSSRR with multiple rings exhibits a higher frequency shift of 1200 MHz, 1270 MHz, and 1520 MHz for ethanol, methanol, and distilled water, respectively. The modified sensor also demonstrates a high sensitivity of 308 MHz/RIU for ethanol concentration which is the highest among the existing microwave sensors. The sensors in this manuscript are suitable for multiple liquid-material-sensing applications.

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 (2) and features low power consumption (200 mW). Thus, the proposed system is perfect for industrial applications, especially for highly integrated lab-on-chip devices.

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.

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.

A class of angular-displacement microwave sensor based on a microwave signal-interference transversal filtering section (TFS) with unequal-electrical-length paths is reported for the first time. It consists of a bi-path TFS composed of two in-parallel transmission-line segments with distinct lengths, which is modified by adding a rotational open-circuit-ended stub as the rotor to a curved section as the stator of one of its line segments. In this manner, the spectral positions of the transmission zeros, which are produced in its filtering transfer function through destructive signal-energy interactions, are modified with the rotation of this additional stub. This allows single/multi-band angular-displacement microwave-sensing capabilities in terms of transmission-zero inter-spacing variation for passbands/stopbands and stopband-peak-level modification to be attained by means of the mechanical rotation of the rotor. The theoretical operational principles of this type of bi-path-TFS-based angular-displacement microwave sensor and design curves for specific values of its electrical parameters are provided. Moreover, for experimental validation purposes, a proof-of-concept prototype of the devised TFS-based microwave-sensor approach operating at 961 MHz with 180° dynamic range is prototyped in microstrip technology and measured. A comparative analysis of the developed angular-displacement microwave sensor with the state-of-the-art is also presented. [ABSTRACT FROM AUTHOR]

In this paper, a new non-contact method for detection of impact damage in carbon fibre-reinforced polymer composites with a complementary split-ring resonator sensor is proposed. The resonance frequency is evaluated as an indicator of the presence of damage. The resonator is made on a printed circuit board, and in the experimental setup it is positioned close to the area of interest. Electromagnetic models are built, and from the resonant responses the appropriate frequency range used for the test is determined. The active sensing element in the resonator is found from the analysis of the magnetic field distribution. The parametric study performed shows that a larger frequency change occurs for a wider impacted region, which is of great use for practical applications. The proposed method is validated by the experimental results, where a frequency shift of 65 MHz was observed for a 0.36 mm deep dent. [ABSTRACT FROM AUTHOR]

The conventional resonant-type microwave microfluidic sensors made of planar resonators suffer from limited sensitivities. This is due to the existence of several distributed capacitors in their structure, where just one of them acts as a sensing element. This article proposes a very high-sensitivity microwave sensor made of a microstrip transmission line loaded with a shunt-connected series LC resonator. A large sensitivity for dielectric loadings is achieved by incorporating just one capacitor in the resonator structure. Applying sample liquids to the microfluidic channel implemented in the capacitive gap area of the sensor modifies the capacitor value. This is translated to a resonance frequency shift from which the liquid sample is characterized. The sensor performance and working principle are described through a circuit model analysis. Finally, a device prototype is fabricated, and experimental measurements using water/ethanol solutions are presented for verification of the sensing principle. [ABSTRACT FROM AUTHOR]

This research study reports on a new coupling structure to demonstrate a constant coupling strength for longer distances in planar microwave sensors. It is shown that there is a positive correlation between the gap size in a pair of gap-coupled transmission lines (GCTLs) and the effective distance of reader and sensing resonators in a planar microwave sensor. On the other hand, the coupling strength in a pair of GCTLs is negatively correlated to the gap size between the two lines, where the larger gap sizes result in weaker coupling strength or vice versa. A technique is introduced to make the coupling strength partially independent of the gap size in GCTLs. The technique enables a sensor designer to increase the gap size, and subsequently, the distance between the reader and sensing resonators of a sensor. Strong electromagnetic coupling is locally demonstrated between a pair of GCTLs using a new artificial perfect electromagnetic conductor (PEMC) boundary, which does not allow electromagnetic power reflect toward the source over larger gap sizes in GCTLs. The concept is experimentally verified with a gap size up to 10 mm between GCTLs with a constantly measured insertion loss of 0.9 dB. Moreover, the structure is employed to design distant microwave resonator for humidity sensing applications. The design has demonstrated the ability of sensing, while the sensing resonator is placed at 35-mm distance from the readout circuitry. The structure is reported to increase the sensing distance with a ratio of about 12 (36:3) compared to the conventional microwave sensors. [ABSTRACT FROM AUTHOR]