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A modified design topology of planar submersible sensor, to realize the improved sensitivity, using the split ring resonator (SRR) loaded coupled microstrip line is presented. The designed sensor can easily be immersed in the liquid sample under test. The maximum numerical sensitivity of the designed sensor (11.58%) at the design frequency of 6.12 GHz is quite higher than the sensitivity of earlier reported submersible sensor (4.01%). The measured unloaded resonant frequency of the fabricated sensor shows a close match with the corresponding simulated data. The fabricated sensor prototype can distinguish between the test samples having nearly similar dielectric properties due to its improved sensitivity. Two significant aspects of petrochemical industries in the Indian continent, i.e. contamination/blending of petrol with kerosene/ethanol are mainly considered here. As the discrimination between petrol and kerosene using the commonly available microwave sensors is especially quite challenging due to the very similar dielectric behaviour of these two chemicals. The developed prototype can typically measure the minimum 2% level of kerosene adulteration in petrol corresponding to 7.5 MHz shift in the measured resonant frequency. The uncertainty in the measured frequency shift of the liquid samples using the proposed sensor is found to be ∼1 MHz. [ABSTRACT FROM AUTHOR]

This article presents a technique based on a modified complementary split-ring resonator (CSRR) to detect angular displacement and direction of rotation with high resolution and sensitivity over a wide dynamic range. The proposed microwave planar sensor takes advantage of the asymmetry of the sensor geometry and measures the angle of rotation in terms of the change in the relative phase of the reflection coefficients. The sensor consists of a movable modified CSRR (the rotor) and a microstrip line with a circular defect in the ground plane (the stator). By selecting the substrate material and the rotor thickness, the sensor can be designed to work at different operating frequencies. A theoretical model of the sensor is proposed and is followed by a detailed numerical analysis involving equivalent circuit simulations, full-wave computations, and measurement results. Using positioning error estimation and air-gap analysis, a technique based on phase-change measurements is found to be better than those based on magnitude measurements alone. The maximum sensitivity for measuring the angular rotation is found to be a 4.3° change in the relative phase of the reflection coefficient per 1° of rotation. The sensor has an angular measurement range from −90° to +90°. The sensor—a stator fabricated on a 0.5-mm-thick Rogers RT5880 substrate and three rotors fabricated on a 1.5-mm-thick Rogers RT5880, a 1-mm-thick FR4, and a 0.5-mm-thick Rogers RT5880—can effectively detect the direction of rotation, measure the angle of rotation and angular velocity with reasonable accuracy. [ABSTRACT FROM AUTHOR]

A broadband permittivity extraction method for fluids by means of electrically small impedance sensors is reported. It relies on an equivalent circuit model of the sensors, which takes into account the most significant parasitic effects. It assumes the sensor’s intrinsic susceptance and conductance to depend linearly on the real and the imaginary part of the permittivity, respectively. The simplicity of the approach eases calibration. Besides conventional thru-reflect-line standards, which define the reference plane, only air and a single reference liquid are needed. The method is applied to three different devices, namely, a common open-ended coaxial line, a compact printed circuit board design, and an integrated microchip sensor for (milli-)liter, microliter, and picoliter sample volumes, respectively. It is shown how the frequency limits can be determined, and how the sensitivity can be maximized in terms of the model parameters. The method is shown to be robust against production tolerances. [ABSTRACT FROM AUTHOR]

A differential-mode sensor based on a pair of electroinductive wave transmission lines (EIW-TLs) is proposed in this article. The EIW-TLs are implemented by etching a chain of complementary split-ring resonators (CSRRs) in a metallic plate. Such metallic plate acts as the ground plane of the microstrip (access) lines, necessary for feeding each EIW-TL and collecting the transmitted power at the output ports. The working principle of the sensor is mode conversion, caused when the EIW-TLs are asymmetrically loaded. Thus, one of the lines is loaded with the reference (REF) sample, whereas the other one is loaded with the sample under test (SUT). If both samples are identical and the pair of EIW-TLs is fed by a common-mode signal, a pure common-mode signal is collected at the differential output port. Conversely, mode conversion arises when the lines are unequally loaded, due to the different propagation characteristics (particularly the phase velocity) of the lines. Due to the typical dispersion relation of the EIW-TLs, with strong variation in the phase velocity (or phase constant) with frequency, the structure is very sensitive to dielectric constant differences between the REF and SUT samples. Thus, the proposed structure is useful for the accurate measurement of dielectric constants (sensor functionality) and the detection of tiny differences between the REF and SUT samples (comparator functionality). [ABSTRACT FROM AUTHOR]

In this paper, we report an embroidered rectangular split-ring resonator (SRR) operating at S band for material characterization based on the differences in dielectric parameters. We designed, fabricated and characterized SRR sensors on a conventional fabric that can be conformally attached over the surface of samples under investigation. The structures are made of conductive threads and can be embroidered on any dielectric fabric at low cost using conventional embroidery methods. We have demonstrated material characterization capability of the sensors using a specific design with a length of 60 mm and a width of 30 mm. We wrapped the sensors on low-density polyethylene (LDPE) bottles filled with deionized (DI) water and common solvents (ethanol, methanol, isopropanol and acetone) in our experiments. We measured the nominal resonant frequency of a specific sensor wrapped around an empty bottle as 2.07 GHz. The shifts in resonant frequencies when the bottle was filled with the solvents follow the dielectric constants of the solvents. [ABSTRACT FROM AUTHOR]

In this paper, we present the design of a compact and highly sensitive microwave sensor based on a metamaterial complementary split-ring resonator (CSRR), for liquid characterization at microwave frequencies. The design consists of a two-port microstrip-fed rectangular patch resonating structure printed on a 20 × 28 mm2 Roger RO3035 substrate with a thickness of 0.75 mm, a relative permittivity of 3.5, and a loss tangent of 0.0015. A CSRR is etched on the ground plane for the purpose of sensor miniaturization. The investigated liquid sample is put in a capillary glass tube lying parallel to the surface of the sensor. The parallel placement of the liquid test tube makes the design twice as efficient as a normal one in terms of sensitivity and Q factor. By bending the proposed structure, further enhancements of the sensor design can be obtained. These changes result in a shift in the resonant frequency and Q factor of the sensor. Hence, we could improve the sensitivity 10-fold compared to the flat structure. Subsequently, two configurations of sensors were designed and tested using CST simulation software, validated using HFSS simulation software, and compared to structures available in the literature, obtaining good agreement. A prototype of the flat configuration was fabricated and experimentally tested. Simulation results were found to be in good agreement with the experiments. The proposed devices exhibit the advantage of exploring multiple rapid and easy measurements using different test tubes, making the measurement faster, easier, and more cost-effective; therefore, the proposed high-sensitivity sensors are ideal candidates for various sensing applications.

Differential permittivity sensors based on a pair of uncoupled microstrip lines, each one loaded with an open complementary split ring resonator (OCSRR), are proposed in this paper. The sensing principle is based on the measurement of the cross-mode insertion loss, very sensitive to asymmetric loading. Thus, by loading one of the OCSRRs with the reference sample, and the other one with the sample under test (SUT), the difference in the complex permittivity between both samples generates an asymmetry that gives rise to mode conversion. From the measurement of the cross-mode transmission coefficient, the dielectric properties of the SUT can be determined, provided those of the reference sample are well known. It is shown that by adding fluidic channels on top of the OCSRRs, the proposed sensor is useful for the measurement of the complex dielectric constant of liquids, and experimental results in mixtures of ethanol and deionized (DI) water and methanol in DI water, as a function of the ethanol/methanol content, are provided. Due to the high sensitivity of the proposed differential sensor to detect small perturbations (asymmetries), the structure is also of interest for the accurate measurement of solute concentrations in liquid solutions. In this paper, the structure is applied to monitor sodium content in aqueous solutions, and it is found that sodium concentrations as small as 0.25 g/L can be resolved.

Electromagnetically induced absorption (EIA) is a quantum phenomenon which occurs when detuned resonant laser fields interfere via atomic transition pathways. The transmission spectrum of a material experiencing EIA consists of an enhanced narrowband absorption line in between the two laser resonances. In this paper, we implement near-field interference of two microstrip radiators to produce a similar absorption mechanism. We propose a practical sensing application to detect foliage moisture by detecting the resonance shifts when sample leaves are made to perturb the near-field radiations. For the sensing, we exploit the anomalous phase signature that accompanies the EIA effect, instead of the amplitude signatures traditionally used in contemporary microwave sensors. The sensing using phase spectrum performs better than the amplitude-based sensing in harsh environments affected by noise and external interferences. Since the proposed EIA-based detector exploits multiple antenna interference in the near field, resonant sensing over distance is also possible. We demonstrate practical moisture detection using actual foliage samples with different moisture levels. We also develop a numerical dielectric model to estimate foliage moisture using full-wave electromagnetic simulations. We anticipate, from this paper, a way to produce low-cost and non-invasive microwave sensors that have reasonable sensitivity and which can be used in remote areas subjected to extreme weather environments.

Linear discriminant analysis (LDA) is a mathematically robust multivariate data analysis approach that is sometimes used for surface oil slick signature classification. Our goal is to rank the effectiveness of LDAs to differentiate oil spills from look-alike slicks. We explored multiple combinations of (i) variables (size information, Meteorological-Oceanographic (metoc), geo-location parameters) and (ii) data transformations (non-transformed, cube root, log10). Active and passive satellite-based measurements of RADARSAT, QuikSCAT, AVHRR, SeaWiFS, and MODIS were used. Results from two experiments are reported and discussed: (i) an investigation of 60 combinations of several attributes subjected to the same data transformation and (ii) a survey of 54 other data combinations of three selected variables subjected to different data transformations. In Experiment 1, the best discrimination was reached using ten cube-transformed attributes: ~85% overall accuracy using six pieces of size information, three metoc variables, and one geo-location parameter. In Experiment 2, two combinations of three variables tied as the most effective: ~81% of overall accuracy using area (log transformed), length-to-width ratio (log- or cube-transformed), and number of feature parts (non-transformed). After verifying the classification accuracy of 114 algorithms by comparing with expert interpretations, we concluded that applying different data transformations and accounting for metoc and geo-location attributes optimizes the accuracies of binary classifiers (oil spill vs. look-alike slicks) using the simple LDA technique.

Metamaterials are engineered periodic structures designed to have unique properties not encountered in naturally occurring materials. One such unusual property of metamaterials is the ability to exhibit negative refractive index over a prescribed range of frequencies. A lens made of negative refractive index metamaterials can achieve resolution beyond the diffraction limit. This paper presents the design of a metamaterial lens and its use in far-field microwave imaging for subwavelength defect detection in nondestructive evaluation (NDE). Theoretical formulation and numerical studies of the metamaterial lens design are presented followed by experimental demonstration and characterization of metamaterial behavior. Finally, a microwave homodyne receiver-based system is used in conjunction with the metamaterial lens to develop a far-field microwave NDE sensor system. A subwavelength focal spot of size 0.82λ was obtained. The system is shown to be sensitive to a defect of size 0.17λ × 0.06λ in a Teflon sample. Consecutive positions of the defect with a separation of 0.23λ was resolvable using the proposed system.

In this paper, a displacement sensor with an electrically extremely small size and high sensitivity is proposed based on an elaborately designed metamaterial element, i.e., coupled split-ring resonators (SRRs). The sensor consists of a feeding structure with a rectangular opening loop and a sensing structure with double-layer coupled SRRs. The movable double-layer structures can be used to measure the relative displacement. The size of microwave displacement sensors can be significantly reduced due to the compact feeding and sensing structures. By adjusting the position of the split gap within the resonator, the detection directions of the displacement sensing can be further expanded accordingly (along with the x- or y-axis) without increasing its physical size. Compared with previous works, the extremely compact size of 0.05λ0 × 0.05λ0 (λ0 denotes the free-space wavelength), a high sensitivity, and a high quality factor (Q-factor) can be achieved by the proposed sensor. From the perspective of the advantages above, the proposed sensor holds promise for being applied in many high-precision industrial measurement scenarios.

In this paper, a flexible microwave sensor for material dielectric characterization is presented. The sensor is composed of a two-turn complementary spiral resonator (CSR) in coplanar waveguide (CPW) technology developed on a flexible polyethylene terephthalate (PET) substrate. The sensing principle is based on the measured transmission coefficient of the designed structure, where the resonant frequency and its magnitude are in function of the dielectric properties of loaded materials under test (MUTs). A prototype of the proposed sensor is fabricated and experimentally validated by extracting the dielectric constant and loss tangent of the reference samples loaded above the region of the CSR. Specially, due to the flexibility, the fabricated sensor can be conformally attached to the palm of a dexterous robotic hand to function as robotic electronic skin for material dielectric characterization, more specifically, to obtain dielectric properties of the objects grasped by the dexterous robotic hand. Moreover, the sensing functionality of the sensor structure under different bending levels is validated by calculating the dielectric property of loaded objects with non-planar surfaces. [ABSTRACT FROM AUTHOR]

Fluid balance is important for a healthy human being. In this paper, a method to measure hydration status was developed and tested towards non-invasive measurement from human skin. Measurement of hydration status was performed by a microwave sensor utilizing a complementary split ring resonator (CSRR). The sensor was modeled, manufactured and then characterized by measuring tailor made skin phantoms based on the realistic electrical properties of skin with different degrees of hydration status. Qualitative longer term (>24 h) evaluation of the sensor was also performed by measuring polyester tissue that was drying over the time. Hydration status, represented by dehydrated, normal and hydrated skin phantoms, based on polyurethane with carbon and ceramic additives, was measured successfully by monitoring the changes in resonance frequency around 5.52 GHz. All results were compared to the dielectric reference measurements done by a commercial laboratory instrument. [ABSTRACT FROM AUTHOR]

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]

A simple analytical method useful to optimize the sensitivity in differential sensors based on a pair of meandered microstrip lines is presented in this paper. Sensing is based on the phase difference of the transmission coefficients of both lines, when such lines are asymmetrically loaded. The analysis provides the combination of operating frequency and line length (the main design parameters) that are necessary to obtain the maximum possible differential phase (±180°) for a given level of the differential dielectric constant (input dynamic range). The proposed sensor is useful to detect tiny defects of a sample under test (SUT) as compared to a reference (REF) sample. It can also be applied to the measurement of the complex dielectric constant of the SUT, where the real part is inferred from the differential phase, whereas the imaginary part, or the loss tangent, is derived from the modulus of the transmission coefficient of the line loaded with the SUT. It is experimentally demonstrated that the proposed device is able to detect the presence of few and small (purposely generated) defects in a commercial microwave substrate, as well as subtle variations in their density, pointing out the high achieved sensor sensitivity. Sensor validation is also carried out by determining the dielectric constant and loss tangent of commercial microwave substrates. [ABSTRACT FROM AUTHOR]

The phenomenon of microwave energy tunneling (ET) has been reported in guided microwave structures loaded with either epsilon-near-zero (ENZ) materials or resonant metallic wires [1]-[14]. A bit of confusion sometimes can arise over the term tunneling, which is often used by physicists, so it seems appropriate here to point out the difference between ET and the well-known particle tunneling encountered in quantum mechanics (Figure 1). [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]

The integration of nanomaterials with the micro-wave technology is a significant step toward the development of low-cost and low-power RF gas sensors. But, the issues associated with materials like insignificant surface area and poor conductivity, are barriers for the rapid detection of wide range of gas concentrations. In this paper, we demonstrate the proof-of concept using an RF complementary split ring resonator (CSRR) integrated ZnO/N-doped graphene (NGN) nano-hybrid to study the gas sensing properties. This RF device senses the presence of ammonia based on the change in the electrical conductivity of the nanohybrid material upon gas exposure. For this, we have designed and fabricated various new CSRR structures, namely, the meander (M-CSRR) and the CSRR, with the concentric-etched square pattern (SP-CSRR) other than the conventional CSRR (C-CSRR). The fabricated CSRRs are coated with the solvothermal methodically prepared ZnO/NGN nanomaterials and a comparison among them is also made. The comparison revealed that the SP-CSRR coated ZnO/NGN sensor had the maximum shift in the S21 resonance frequency ($\Delta \text {f}_{\mathbf {S21}} = 13$ MHz at 100 ppm) among all at room temperature. Furthermore, the SP-CSRR-ZnO/NGN sensor exhibited a large-S21 frequency shift of 133 MHz for 500 ppm of ammonia and has shown excellent reproducibility with 0.05% of variation. We have also projected the plausible sensing mechanism for this RF sensing device. [ABSTRACT FROM AUTHOR]

A differential microwave sensor, based on a pair of uncoupled microstrip lines each one loaded with a split ring resonator (SRR), is applied to the measurement of electrolyte concentration in deionized (DI) water. For that purpose, fluidic channels are added on top of the SRR gaps, the most sensitive parts of the structure. The operating principle is based on the measurement of the cross-mode insertion loss, highly sensitive to small asymmetries caused by differences between the reference liquid and the liquid under test (LUT). In this paper, the reference liquid is pure DI water (the solvent), whereas the solution, DI water with electrolyte content, is injected to the LUT channel. The proposed sensor is able to detect electrolyte concentrations as small as 0.25 g/L, with maximum sensitivity of 0.033 (g/L)–1. The sensor is validated by measuring the concentration of three types of electrolytes, i.e., NaCl, KCl, and CaCl2. Finally, the sensor is applied to monitor variations of total electrolyte concentration in urine samples. [ABSTRACT FROM AUTHOR]

Optical and magnetic linear/rotary encoders are well-known systems traditionally used in industry for the accurate measurement of linear/angular displacements and velocities. Recently, a different approach for the implementation of linear/rotary encoders has been proposed. Such an approach uses electromagnetic signals, and the working principle of these electromagnetic encoders is very similar to the one of optical encoders, i.e., pulse counting. Specifically, a transmission line based structure fed by a harmonic signal tuned to a certain frequency, the stator, is perturbed by encoder motion. Such encoder consists in a linear or circular chain (or chains) of inclusions (metallic, dielectric, or apertures) on a dielectric substrate, rigid or flexible, and made of different materials, including plastics, organic materials, rubber, etc. The harmonic signal is amplitude modulated by the encoder chain, and the envelope function contains the information relative to the position and velocity. The paper mainly focuses on linear encoders based on metallic and dielectric inclusions. Moreover, it is shown that synchronous electromagnetic encoders, able to provide the quasi-absolute position (plus the velocity and direction of motion in some cases), can be implemented. Several prototype examples are reviewed in the paper, including encoders implemented by means of additive process, such as 3D printed and screen-printed encoders.

Planar phase-variation microwave sensors have attracted increasing interest in recent years since they combine the advantages of planar technology (including low cost, low profile, and sensor integration with the associated circuitry for post-processing and communication purposes, among others) and the possibility of operation at a single frequency (thereby reducing the costs of the associated electronics). This paper reviews and compares three different strategies for sensitivity improvement in such phase-variation sensors (devoted to material characterization). The considered approaches include line elongation (through meandering), dispersion engineering (by considering slow-wave artificial transmission lines), and reflective-mode sensors based on step-impedance open-ended lines. It is shown that unprecedented sensitivities compatible with small sensing regions are achievable with the latter approach.

In this paper, reflective-mode phase-variation sensors based on open-ended stepped-impedance transmission lines with optimized sensitivity for their use as defect detectors and dielectric constant sensors are reported. The sensitive part of the sensors consists of either a 90° high-impedance or a 180° low-impedance open-ended sensing line. To optimize the sensitivity, such a sensing line is cascaded to a 90° transmission line section with either low or high characteristic impedance, resulting in a stepped-impedance transmission line configuration. For validation purposes, two different sensors are designed and fabricated. One of the sensors is implemented by means of a 90° high impedance (85 Ω) open-ended sensing line cascaded to a 90° low impedance (15 Ω) transmission line section. The other sensor consists of a 180° 15-Ω open-ended sensing line cascaded to a 90° 85-Ω line. Sensitivity optimization for the measurement of dielectric constants in the vicinity of that corresponding to the Rogers RO4003C substrate (i.e., with dielectric constant 3.55) is carried out. The functionality as a defect detector is demonstrated by measuring the phase-variation in samples consisting of the uncoated Rogers RO4003C substrate (the reference sample) with arrays of holes of different densities.

In this paper, 3D-printed electromagnetic (or microwave) encoders with synchronous reading based on permittivity contrast, and devoted to the measurement of displacements and velocities, are reported for the first time. The considered encoders are based on two chains of linearly shaped apertures made on a 3D-printed high-permittivity dielectric material. One such aperture chain contains the identification (ID) code, whereas the other chain provides the clock signal. Synchronous reading is necessary in order to determine the absolute position if the velocity between the encoder and the sensitive part of the reader is not constant. Such absolute position can be determined as long as the whole encoder is encoded with the so-called de Bruijn sequence. For encoder reading, a splitter/combiner structure with each branch loaded with a series gap and a slot resonator (each one tuned to a different frequency) is considered. Such a structure is able to detect the presence of the apertures when the encoder is displaced, at short distance, over the slots. Thus, by injecting two harmonic signals, conveniently tuned, at the input port of the splitter/combiner structure, two amplitude modulated (AM) signals are generated by tag motion at the output port of the sensitive part of the reader. One of the AM envelope functions provides the absolute position, whereas the other one provides the clock signal and the velocity of the encoder. These synchronous 3D-printed all-dielectric encoders based on permittivity contrast are a good alternative to microwave encoders based on metallic inclusions in those applications where low cost as well as major robustness against mechanical wearing and aging effects are the main concerns.

This paper highlights interest in the implementation of microwave sensors based on resonant elements, the subject of a special issue in the journal. A classification of these sensors on the basis of the operating principle is presented, and the advantages and limitations of the different sensor types are pointed out. Finally, the paper summarizes the different contributions to the special issue.

We report a novel approach to ultrahigh quality factor Q) in the split-ring resonator with an active feedback loop, which utilizes a phase shifter to tune equivalent negative resistance for accurate loss compensation. Based on the proposed technique, a microwave detector is designed for on-column liquid chromatography. With lossy solvents loaded, the measured Q is as high as 4.0 × 106 at ~ 3.89 GHz. We demonstrate the highly sensitive and contactless measurement of glucose–water solution in a capillary tube under stop–flow and plug–unplug modes, respectively. With the stop–flow mode, different concentrations between 40 and 150 mg/dL are measured, which covers the normal blood-glucose levels. The minimum detectable quantity of glucose is ~18.8 ng with an effective sample volume of ~47 nL. The plug–unplug mode provides fast and flexible sampling and analysis. Further investigations are required for quantification of material-under-test permittivity and promotion of sensitivity. [ABSTRACT FROM AUTHOR]

This paper reviews the recent progress in the analysis and applications of the symmetry-related electromagnetic properties of transmission lines loaded with symmetric configurations of resonant elements. It will be shown that the transmission characteristics of these reactively loaded lines can be controlled by the relative orientation between the line and the resonant elements. Two main types of loaded lines are considered: (i) resonance-based structures; and (ii) frequency-splitting structures. In resonance-based transmission lines, a line is loaded with a single resonant (and symmetric) element. For a perfectly symmetric structure, the line is transparent if the line and resonator exhibit symmetry planes of different electromagnetic nature (electric or magnetic wall), whereas the line exhibits a notch (resonance) in the transmission coefficient if the symmetry planes behave as either electric or magnetic walls (symmetric configuration), or if symmetry is broken. In frequency-splitting lines, paired resonators are typically loaded to the transmission line; the structure exhibits a single notch for the symmetric configuration, whereas generally two split notches appear when symmetry is disrupted. Applications of these structures include microwave sensors (e.g., contactless sensors of spatial variables), selective mode suppressors (of application in common-mode suppressed differential lines, for instance) and spectral signature barcodes, among others.

Microwave sensors have attracted a considerable attention since they can provide non-invasive, label-free, and real-time detection; however, they often suffer from low sensitivity and resolution. In this paper, we present a novel highly sensitive microwave sensor based on half-mode substrate integrated waveguide (HM SIW) that supports surface plasmon polariton (SPP)-like propagation. Two comprising guiding parts of HM SIW, one of which contains the sensed analyte, exhibit the real parts of the effective permittivities of the opposite sign in a specific frequency range, which allows for SPP to propagate at their interface. Ultimately, such behavior causes a sharp transmission zero to occur in HM SIW response, whose spectral position is highly sensitive to the real part of the dielectric constant of the sensed analyte. To demonstrate the sensing potential, HM SIW sensor has been fabricated in low-cost PCB technology and its response is measured for different toluene/methanol mixtures whose real part of the dielectric constants range from 2.4 to 3.8. Measurement results together with a detailed theoretical and numerical analysis, confirm that the sensor is an excellent candidate for very sensitive dielectric constant measurements with low relative error, which can be applied in the engine and edible oil quality analysis. [ABSTRACT FROM AUTHOR]

In this paper, a planar microstrip sensor based on a band-stop filter for determining relative permittivity of solid materials is presented. The proposed sensor has been composed of the structure of meandered microstrip line with a T-shaped resonator and an interdigital structure that is connected ground by via. Interaction between the T-shaped resonator and the interdigital structure is caused to increase electric field intensity and the high field increase the sensor sensitivity and resolution. The microwave sensor by using single layer technology is fabricated on the substrate RO4003. The samples of FR4 and RO4350 in single and double layer forms as multilayer are placed on the sensor, and the sensor is demonstrating different resonance frequencies. The relationship between changing the resonance frequency and variation of the relative permittivity is linear, and it determines unknown materials’ dielectric characterizations. The permittivity of samples changes from 3 to 11; therefore, the resonance frequency varies from 3.7 to 5.65 GHz linearly. The proposed sensor shows improvement relative to other similar works in the fields of sensitivity and quality factor. [ABSTRACT FROM AUTHOR]

This paper suggests the usage of an optimized structure of the embedded wafer-level ball grid array technology for millimeter-wave antennas-in-package. Multiple antenna arrays in a second redistribution layer have been designed, evaluated, and integrated with different transceivers to form very compact sensor packages for 61- and 122-GHz radar applications. The resulting packages contain a complete radar front end while featuring dimensions of $8\,\,\text {mm}\times 8~\text{mm}$. The packages are easy to assemble and replaceable. Theoretical considerations, numerical results, and measurements have been carried out for the single components. For the verification of the sensor packages, a system demonstrator was developed, and measurements have been performed in continuous-wave mode. At both frequencies, promising system performance regarding range and distance errors could be confirmed without the usage of complex processing and calibration algorithms. [ABSTRACT FROM AUTHOR]

A fast living-body localization algorithm using time-differential channel suitable for multipath environments is introduced. In this method, a time-differential channel is calculated from the difference among the observed channels that correspond to known biological activities such as respiration and heartbeat. The living-body locations are estimated by applying 2-D multiple signal classification to the correlation matrix calculated from the time-differential channels. Experiments were carried out in an indoor environment, and the results show that the proposed localization algorithm could well estimate multiple device-free living-bodies simultaneously. The experimental results demonstrate that the locations of up to three persons could be estimated faster, within a few seconds, and more accurately than the conventional method. One, two or three targets yield 90% values of localization errors of 0.23, 0.41, and 1.29 m, respectively. [ABSTRACT FROM AUTHOR]

A radar sensor suitable to record carotid wall movements has been developed and tested. The sensor is based on a double-sideband continuous-wave coherent transceiver, operating at 2.45 GHz, and on a bow-tie planar antenna optimized to operate in contact with the neck. Measured transceiver responses on a sliding short load are in good agreement with an analytical model and simulation results. An artificial model of the neck with an embedded pulsatile carotid-artery mockup has been built to test the radar. When the radar antenna is placed in contact with the neck model, an output voltage signal following the carotid diameter variations is measured. The signal amplitude follows the pressure wave imposed on the carotid artery mockup. [ABSTRACT FROM AUTHOR]

In this paper, a strategy to detect the rotation direction in angular displacement and velocity sensors based on rotors loaded with circular chains of split ring resonators (SRRs) is presented. The rotor (made of a dielectric material) is loaded with two concentric SRR chains for measuring the angular velocity (angular velocity chains) and with one additional (also concentric) non-periodic resonator chain for detecting the direction of motion (rotation direction chain). The stator is a CPW transmission line placed below the rotor chains, in close proximity to them, with the line axis oriented in the radial direction of the rotor. A novel relevant contribution of this paper concerns the detailed study of the rotation direction sensor, from which we have concluded that the design must be completely different than the one of the angular velocity sensor (single SRR on the rotor). By this means, the transmission coefficient of the CPW is modulated by rotor motion (through electromagnetic coupling), as the SRRs of the different chains cross the CPW axis. By injecting a double-tuned signal to the line (with carrier frequencies tuned to the SRR resonance frequencies of both chains), two superposed amplitude modulated (AM) signals at the output port of the CPW arise. From the envelope of both AM signals, discriminated by means of a designed diplexer, the angular velocity and the rotation direction can be determined. [ABSTRACT FROM AUTHOR]

Existing layouts of sensors on freeways usually have room for improvement. The optimal allocation of multi-type sensors could increase the accuracy and coverage of data acquisition for traffic control and management. This paper aims to explore the appropriate method to solve the multi-type sensor allocation problem on freeways. Two types of sensors, the micro-loop sensor and the microwave sensor, are considered in this study. Three sensor allocation methods, the Integer Programming-Genetic Algorithm (IP-GA) method, the cluster method, and the two-stage method, are proposed to identify the optimal sensor locations. To investigate the performances of our proposed methods, a case study of sensor allocations on Ning-Hang freeway in Jiangsu province, China was conducted. The analysis results demonstrated that the IP-GA method has the best performance among the three proposed algorithms in view of the Mean Absolute Relative Errors (MAREs) of link travel time. Besides, this study also found that optimizing the composition of sensors has a far greater impact on data accuracy than just increasing the number of sensors. Within a specific range, data accuracy increases along with the number of sensors. Under the cases of the total number being constant, the greater the proportion of high accuracy sensors, the more accurate of the estimated traffic information. [ABSTRACT FROM PUBLISHER]

Aspergillus Niger is an airborne fungal pathogen which commonly affects the vegetation. More importantly, the toxins carried by its micrometer-sized spores can be easily inhaled through surrounding atmosphere and causes respiratory diseases in human. Thus, their detection in the surrounding media is important. In this paper, a microwave sensor based on epsilon negative structure is realized. The sensor has two complementary ring resonators in the horn-shape and U-shape. A label-free detection approach is presented in which a functionalized glass slide attached with fungi specific enzyme is mounted on the sensor. The air fringing fields of the complementary rings interact with the fungi immobilized glass slide to produce a change in the resonant frequency. A prototype sensor system is also proposed for low-cost measurement of sensor frequency response into equivalent DC signaling. The proposed sensor is suitable for real-time monitoring of fungal contamination of air in the industrial processes. [ABSTRACT FROM AUTHOR]

A chipless radio-frequency identification (chipless-RFID) and sensing system, where tags are read by proximity (near-field) through a switch, is presented. The tags consist of a set of identical resonant elements (split-ring resonators or SRRs), printed or etched at predefined and equidistant positions, forming a linear chain, each SRR providing a bit of information. The logic state (‘1’ or ‘0’) associated with each resonator depends on whether it is present or not in the predefined position. The reader is an array of power splitters used to feed a set of SRR-loaded transmission lines (in equal number to the number of resonant elements, or bits, of the tag). The feeding (interrogation) signal is a harmonic (single-tone) signal tuned to a frequency in the vicinity of the fundamental resonance of the SRRs. The set of SRR-loaded lines must be designed so that the corresponding SRRs are in perfect alignment with the SRRs of the tag, provided the tag is positioned on top of the reader. Thus, in a reading operation, as long as the tag is very close to the reader, the SRRs of the tag modify (decrease) the transmission coefficient of the corresponding reader line (through electromagnetic coupling between both SRRs), and the amplitude of the output signal is severely reduced. Therefore, the identification (ID) code of the tag is contained in the amplitudes of the output signals of the SRR-loaded lines, which can be inferred sequentially by means of a switching system. Unlike previous chipless-RFID systems based on near-field and sequential bit reading, the tags in the proposed system can be merely positioned on top of the reader, conveniently aligned, without the need to mechanically place them across the reader. Since tag reading is only possible if the tag is very close to the reader, this system can be also used as a proximity sensor with applications such as target identification. The proposed chipless-RFID and sensing approach is validated by reading a designed 4-bit tag. For identification purposes, this system is of special interest in applications where a low number of bits suffice, and tag reading by proximity is acceptable (or even convenient). Applications mostly related to secure paper, particularly involving a limited number of items (e.g., exams, ballots, etc.), in order to provide authenticity and avoid counterfeiting, are envisaged. As a proximity sensor, the system may be of use in detecting and distinguishing different targets in applications such as smart packaging. [ABSTRACT FROM AUTHOR]

In this paper, an analytical method to estimate the complex dielectric constant of liquids is presented. The method is based on the measurement of the transmission coefficient in an embedded microstrip line loaded with a complementary split ring resonator (CSRR), which is etched in the ground plane. From this response, the dielectric constant and loss tangent of the liquid under test (LUT) can be extracted, provided that the CSRR is surrounded by such LUT, and the liquid level extends beyond the region where the electromagnetic fields generated by the CSRR are present. For that purpose, a liquid container acting as a pool is added to the structure. The main advantage of this method, which is validated from the measurement of the complex dielectric constant of olive and castor oil, is that reference samples for calibration are not required. [ABSTRACT FROM AUTHOR]

Elevated intracranial fluid volume can drive intracranial pressure increases, which can potentially result in numerous neurological complications or death. This study’s focus was to develop a passive skin patch sensor for the head that would non-invasively measure cranial fluid volume shifts. The sensor consists of a single baseline component configured into a rectangular planar spiral with a self-resonant frequency response when impinged upon by external radio frequency sweeps. Fluid volume changes (10 mL increments) were detected through cranial bone using the sensor on a dry human skull model. Preliminary human tests utilized two sensors to determine feasibility of detecting fluid volume shifts in the complex environment of the human body. The correlation between fluid volume changes and shifts in the first resonance frequency using the dry human skull was classified as a second order polynomial with R2 = 0.97. During preliminary and secondary human tests, a ≈24 MHz and an average of ≈45.07 MHz shifts in the principal resonant frequency were measured respectively, corresponding to the induced cephalad bio-fluid shifts. This electromagnetic resonant sensor may provide a non-invasive method to monitor shifts in fluid volume and assist with medical scenarios including stroke, cerebral hemorrhage, concussion, or monitoring intracranial pressure. [ABSTRACT FROM AUTHOR]

This paper presents a microwave angular displacement sensor that consists of a curved two-metal-layer coplanar waveguide (TML CPW) loaded by an electromagnetic bandgap structure (two semi-cylindrical periodic air-gap structures of dielectrics, allocated at the top and bottom of the curved TML CPW). Rotating one of the dielectrics against the other one, produces a variation in the depth of the stopband at the central frequency. The magnitude change is proportional to the angle of the rotation. The curved TML CPW line with the length of 140 mm is fabricated on a 254~\mu \textm thick RO3010 Rogers substrate ( \varepsilon r = 10.2 and \tan \delta = 0.0022 ), which is arched in a semi-cylindrical shape with a radius of 50 mm. The periodic air-gap structures, in order to create the electromagnetic bandgap, are manufactured by laser cutting of an optically transparent polycarbonate ( \varepsilon _{r} = 2.8 and \tan \delta =0.01$ ) in semi-cylindrical shapes. The analysis, finite element method simulation, realization, and measurements have been performed for the sensor with 15 unit cells. An average sensitivity and a dynamic range of about 3 dB/deg and 5° degrees, respectively, have been obtained at 10.2 GHz. The fixed operating frequency and the wide bandwidth of the sensor (>1 GHz) will simplify the design and fabrication of the data readout circuit. The impact of the structure curvature on the sensor response is also investigated as an important practical issue. [ABSTRACT FROM PUBLISHER]

A differential microwave permittivity sensor and comparator is designed using a microstrip transmission line loaded with a magnetic-LC resonator. The microstrip transmission line is aligned with the electric wall of the resonator. The sensor shows a single transmission zero, when it is unloaded or loaded symmetrically on both halves. A second notch appears in the transmission response by asymmetrical dielectric loading on the two halves of the device. The frequency splitting is used to characterize the dielectric properties of the samples under test. The sensitivity of the sensor is enhanced by removing the mutual coupling between the two halves of the magnetic-LC resonator using a metallic wall. The sensors’ operation principle is explained through a circuit model analysis. A prototype of the designed sensor is fabricated and measurements are used for validation of the sensing concept. The sensor can be used for determination of the dielectric properties in solid materials or detecting defects and impurities in solid materials through a comparative measurement with a reference sample.