Measured Transmission Coefficient Research Articles

Highly-Sensitive Microwave Sensors Based on Open Complementary Split Ring Resonators (OCSRRs) for Dielectric Characterization and Solute Concentration Measurement in Liquids

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.

Reduced-scale ultrasonic modelling of Rayleigh wave transmission over seismic barriers formed by periodic arrays of vertical holes

Seismic barriers are used widely to protect buildings from traffic-induced ground vibrations, mainly from propagating Rayleigh surface waves. Experimental investigations of real size seismic barriers at frequencies typical for traffic-induced ground vibrations, i.e. at 10-100 Hz, are costly and time consuming. In the present work, an alternative and much less expensive approach is proposed - a reduced-scale experimental modelling using ultrasonic Rayleigh wave propagation over very small-scale replicas of real seismic barriers. Experimental investigations of propagation of Rayleigh wave pulses with the central frequency of 1 MHz have been carried out for seismic barriers formed by periodic arrays of vertical holes in Aluminium samples. Measurements of transmission and reflection coefficients of Rayleigh waves for different types of arrays and for different incident angles have been carried out and compared with the earlier published results obtained for real seismic barriers.

Microwave Interferometry Based On Open-ended Coaxial Technique for High Sensitivity Liquid Sensing

This paper describes a modified open-ended coaxial technique for microwave dielectric characterization in liquid media. A calibration model is developed to relate the measured transmission coefficient to the local properties of the sample under test. As a demonstration, the permittivity of different sodium chloride solutions is experimentally determined. Accuracies of 0.17% and 0.19% are obtained respectively for the real and imaginary parts of dielectric permittivity at 5.9 GHz.

Microstrip Spiral Resonator For Microwave-Based Gas Sensing

This paper presents the design of a new microstrip gas sensor based on a trapezoidal spiral resonator covered by a titanium dioxide nanoparticle layer. Reflection and transmission coefficient measurements between 1 and 8GHz revealed multiple resonances that can be used in the context of gas detection. The sensor response upon ammonia adsorption has been evaluated on the whole frequency span with a simple spectral analysis. Kinetic parameters such as response time, reversibility, and measurement stability have been determined with a monofrequential temporal analysis. The sensor showed at each resonance a good sensitivity to ammonia for concentrations between 50 and 300ppm, with a response time less than 1 min. The maximal response was reported at 7.76GHz with a reflection coefficient variation of 0.45dB after the injection of 300ppm of ammonia, which is 3.5 times higher than previously reported.

A Glucose Sensing System Based on Transmission Measurements at Millimetre Waves using Micro strip Patch Antennas

We present a sensing system operating at millimetre (mm) waves in transmission mode that can measure glucose level changes based on the complex permittivity changes across the signal path. The permittivity of a sample can change significantly as the concentration of one of its substances varies: for example, blood permittivity depends on the blood glucose levels. The proposed sensing system uses two facing microstrip patch antennas operating at 60 GHz, which are placed across interrogated samples. The measured transmission coefficient depends on the permittivity change along the signal path, which can be correlated to the change in concentration of a substance. Along with theoretical estimations, we experimentally demonstrate the sensing performance of the system using controlled laboratory samples, such as water-based glucose-loaded liquid samples. We also present results of successful glucose spike detection in humans during an in-vivo Intravenous Glucose Tolerance Test (IVGTT). The system could eventually be developed into a non-invasive glucose monitor for continuous monitoring of glucose levels for people living with diabetes, as it can detect as small as 1.33 mmol/l (0.025 wt%) glucose concentrations in the controlled water-based samples satisfactorily, which is well below the typical human glucose levels of 4 mmol/l.

Graphene-based optically transparent dipole antenna

We fabricated an optically transparent dipole antenna based on chemical vapor deposition (CVD)-grown monolayer graphene on an optically transparent quartz substrate and characterized its properties in microwave bands. The measurements of the reflection coefficients for the dipole antenna revealed that ∼90% of the microwave power transmitted to the CVD monolayer graphene of the antenna element. By measuring transmission coefficients, we demonstrated that the graphene dipole antenna radiated microwave power around the operational frequency (∼20.7 GHz). The operational frequency of the graphene dipole antenna (∼20.7 GHz) shifted to a higher frequency than that of the Au dipole antenna with the same structure (∼9.2 GHz), which suggests that monolayer graphene behaves not as a metal but as a dielectric material.

Wideband Measurement of the Phase Deviation and Time-Domain Response of an Open Fire

This letter presents an extension of the free-space method for the dielectric characterization of Eucalypt litter fire using the measured transmission phase shift of a propagating signal over a frequency range of 5–40 GHz. This method is especially suitable for a broadband, routine, nonintrusive, and accurate evaluation of fire dielectric properties from the measured transmission coefficient ( ${S}_{21}$ ). A 72 cm $\times $ 47 cm rectangular brick burning area prepared with Eucalypt ground litter was equipped with four K-type thermocouples at different heights to measure flame temperatures, which ranged from 283–865 K. The measured phase shift of an electromagnetic signal transmitted through the Eucalypt fire varied from 12.2 to −166.7° at 5.35 and 39.74 GHz. From this phase shift, the calculated real part of the relative permittivity of the fire was between 0.877 and 1, with the imaginary part from 0.02 to 0.09. The relative permittivity of less than unity confirms the formation of plasma in the combustion zone of fire, and shows good agreement with previous research. However, in this letter, the combustion zone region is defined with significantly higher accuracy, and characteristics are evaluated over a much broader frequency range. The extracted parameters are important for understanding of the reaction of electromagnetic waves in wildfire environments and can be used to design radar systems for forest fire detection.

A Method for the Electromagnetic Characterization of Construction Materials Based on Fabry–Pérot Resonance

The determination of the complex permittivity of low-loss construction materials at frequency bands above 6 GHz that are being proposed to allocate forthcoming mobile radio services is of critical importance for the design and deployment of future wireless systems. In this paper, a simple free-space method for the electromagnetic characterization of construction materials that does not require multiple reflection or transmission coefficient measurements for different incidence angles or complex optimization procedures is proposed and tested. The method is shown to yield permittivity and conductivity values in agreement with the literature for some common-use materials using a relatively simple measurement setup and procedure.

A modified algebraic reconstruction technique taking refraction into account with an application in terahertz tomography

Terahertz (THz) tomography is a rather novel technique for non-destructive testing that is particularly suited for the testing of plastics and ceramics. Previous publications showed a large variety of conventional algorithms adapted from computed tomography or ultrasound tomography which were directly applied to THz tomography. Conventional algorithms neglect the specific nature of THz radiation, i.e. refraction at interfaces, reflection losses and the beam profile (Gaussian beam), which results in poor reconstructions. The aim is the efficient reconstruction of the complex refractive index, since it indicates inhomogeneities in the material. A hybrid algorithm has been developed based on the algebraic reconstruction technique (ART). ART is adapted by including refraction (Snell’s law) and reflection losses (Fresnel equations). Our method uses a priori information about the interface and layer geometry of the sample. This results in the ‘Modified ART for THz tomography’, which reconstructs simultaneously the complex refractive index from transmission coefficient and travel time measurements.

Electron optics with p-n junctions in ballistic graphene.

Electrons transmitted across a ballistic semiconductor junction are expected to undergo refraction, analogous to light rays across an optical boundary. In graphene, the linear dispersion and zero-gap band structure admit highly transparent p-n junctions by simple electrostatic gating. Here, we employ transverse magnetic focusing to probe the propagation of carriers across an electrostatically defined graphene junction. We find agreement with the predicted Snell's law for electrons, including the observation of both positive and negative refraction. Resonant transmission across the p-n junction provides a direct measurement of the angle-dependent transmission coefficient. Comparing experimental data with simulations reveals the crucial role played by the effective junction width, providing guidance for future device design. Our results pave the way for realizing electron optics based on graphene p-n junctions.

A low-cost dielectric spectroscopic system using metamaterial open horn-ring resonator-inspired BSF and detection circuitry

The sensitivity in a lower microwave band dielectric spectroscopic system is relatively less compared to that of millimeter wave and terahertz system. This work reports modeling and development of an epsilon-negative metamaterial resonator-inspired microwave band-stop filter as a prototype device and its detection circuitry for the spectroscopic analysis of dielectric samples in S-band. The device structure consists of a diamond-shaped patch with a complementary open split horn-ring resonator, fabricated on a Neltech substrate of relative permittivity (e r = 3.2). The measured transmission coefficient at 2.2 GHz and simulated result at 2.24 GHz demonstrate an excellent accuracy in the device fabrication. A low-cost connector-type microwave signal detection system was assembled for the real-time transduction of device signal into an equivalent DC voltage. Further, a single channel cavity developed using polydimethylsiloxane was placed over the resonator gap for analyzing the perturbation effect of electric field intensity on the resonance and circuit output DC level for different dielectric samples under test. The performed calibrations show linearity up to 82.5 % in the device response.

Reducing Sweeping Frequencies in Microwave NDT Employing Machine Learning Feature Selection.

Nondestructive Testing (NDT) assessment of materials’ health condition is useful for classifying healthy from unhealthy structures or detecting flaws in metallic or dielectric structures. Performing structural health testing for coated/uncoated metallic or dielectric materials with the same testing equipment requires a testing method that can work on metallics and dielectrics such as microwave testing. Reducing complexity and expenses associated with current diagnostic practices of microwave NDT of structural health requires an effective and intelligent approach based on feature selection and classification techniques of machine learning. Current microwave NDT methods in general based on measuring variation in the S-matrix over the entire operating frequency ranges of the sensors. For instance, assessing the health of metallic structures using a microwave sensor depends on the reflection or/and transmission coefficient measurements as a function of the sweeping frequencies of the operating band. The aim of this work is reducing sweeping frequencies using machine learning feature selection techniques. By treating sweeping frequencies as features, the number of top important features can be identified, then only the most influential features (frequencies) are considered when building the microwave NDT equipment. The proposed method of reducing sweeping frequencies was validated experimentally using a waveguide sensor and a metallic plate with different cracks. Among the investigated feature selection techniques are information gain, gain ratio, relief, chi-squared. The effectiveness of the selected features were validated through performance evaluations of various classification models; namely, Nearest Neighbor, Neural Networks, Random Forest, and Support Vector Machine. Results showed good crack classification accuracy rates after employing feature selection algorithms.

Acoustical Measurement and Biot Model for Coral Reef Detection and Quantification

Coral reefs are coastal resources and very useful for marine ecosystems. Nowadays, the existence of coral reefs is seriously threatened due to the activities of blast fishing, coral mining, marine sedimentation, pollution, and global climate change. To determine the existence of coral reefs, it is necessary to study them comprehensively. One method to study a coral reef by using a propagation of sound waves is proposed. In this research, the measurement of reflection coefficient, transmission coefficient, acoustic backscattering, hardness, and roughness of coral reefs has been conducted using acoustic instruments and numerical modeling using Biot theory. The results showed that the quantification of the acoustic backscatter can classify the type of coral reef.

Peculiarities in electrical and optical properties of Cu2Zn1–x Mn x SnS4 films obtained by spray pyrolysis

Thin films of Cu2Zn1–xMnxSnS4 (0 ≤ x ≤ 1) solid solutions have been obtained for the first time by the spray pyrolysis of aqueous salt solutions (copper, zinc, manganese, and tin chlorides and thiourea) at a temperature of TS = 563 K. The films possess specific electric conductivities within σ ≈ 35–422 Ω–1 cm–1 and optical bandgap width Egop that increases with the manganese content from 1.54 eV (x = 0) to 2.25 eV (x = 1). Electrical and optical properties of the obtained films have been studied and analyzed based on a model of polycrystalline materials with grain boundaries. The energy barriers Eb between grains have been determined. The dependence of the bandgap of Cu2Zn1–xMnxSnS4 (0 ≤ x ≤ 1) solid solutions on the composition has been established using the results of measurements of the optical transmission and absorption coefficients.

RF Conductivity Measurement of Conductive Zell Fabric

This study presents a conductivity measurement technique that is applicable at radio frequencies (RF). Of particular interest is the measurement of the RF conductivity of a flexible Zell fabric, which is often used to implement wearable antennas on clothes. First, the transmission coefficient is measured using a planar microstrip ring resonator, where the ring is made of a Zell fabric. Then, the fabric’s conductivity is determined by comparing the measured transmission coefficient to a set of simulation data. Specifically, a MATLABbased root-searching algorithm is used to find the minimum of an error function composed of measured and simulation data. Several error functions have been tested, and the results showed that an error function employing only the magnitude of the transmission coefficient was the best for determining the conductivity. The effectiveness of this technique is verified by the measurement of a known copper foil before characterizing the Zell fabric. The conductivity of the Zell fabric at 2 GHz appears to be within the order of 10⁴ S/m, which is lower than the DC conductivity of 5×10 5 S/m.

Compact Size, Equal-Length and Unequal-Width Substrate Integrated Waveguide Four Channel Phase Shifter

Objectives: In the past decade, Substrate Integrated Waveguide (SIW) phase shifters have attracted a lot of attentions, they are key components extensively deployed in phased array antennas, electronic scanning radars and other applications. In this paper a novel SIW four channel phase shifter is proposed. Methodology: It consists of five ports from which four ports act as output channels and the remaining is applied as a feed port. Also this substrate is integrated with a waveguide phase shifter designed to operate at 10 GHz. Phase channels are made by SIW with equal length and unequal width. The width of the phase channels differ from each other and can be controlled. The equations and the process of the whole design are preceeded by mathematical analysis. The propagation constant of the output signals and output phases in output channels have been adjusted by changing only the width of the output arms. Findings: As a result, a novel phase shifter is obtained. The experimental results of the prototype at 10GHz represents 22.5, 45, 67.5 degrees phase difference between one of the outputs in comparison to other outputs respectively. The measured bandwidth is 1.3 GHz bandwidth which has conformity with the simulation results. With the change of the position, the number of added vias and the width of output channels, a different phase shift can be obtained. The measurement of return loss and transmission coefficients are in good agreement with the simulation results in the proposed band. Application/Improvements: Therefore, such a SIW phase shifter is a good candidate for development of a compact size and integrated microwave systems.

Identification of the mechanical moduli of closed-cell porous foams using transmitted acoustic waves in air and the transfer matrix method

Closed-cell cellular polymer foams are the most efficient insulating materials commercially available. Their lightweight and mechanical resistance also make them ideal materials for packaging protection in the transport industry. A new method using guided acoustic waves in air transmitted through samples of cellular foam panels, has been developed to recover their P-wave moduli and Poisson’s ratios. These parameters were recovered from measured transmission coefficients of the foams through fitting to an elastodynamic transfer matrix method (TMM). The TMM integrates both the P and shear waves propagating in the layer. It was shown by using a finite element fluid–solid (elastic) interaction and an analytic P-waves-only model, that the two types of waves should be modeled to give a more precise representation of the data. The retrieved values were validated using vibration spectroscopy and from the measured velocity of a transient mechanical stress wave propagating in thin, long rod specimens cut from the panels. The problems inherent to these simple 1D characterization methods were pointed out and solved.

MICROWAVE CHARACTERIZATION OF BIO-COMPOSITES MATERIALS BASED FINITE ELEMENT AND NICHOLSON–ROSS–WEIR METHODS

In this work, Bio-composite of oil palm empty fruit bunch fibre (OPEFB)-filler andpolycaprolactone (PCL)-polymer has been prepared and characterized. The functional groups and morphology of theprepared samples were characterized by Fourier transform infrared spectroscopy (FT-IR). By using the NicholsonRoss-Weir(NRW) mode, both of real and imaginary relative permittivity values of the samples were obtainedsimultaneously from the reflection and transmission coefficient measurements of the materials. Whereas, theattenuation with the field distribution at the waveguide filled with a sample were considered by using the FiniteElement Method (FEM). The magnitude of the reflection and transmission (R/T) coefficients of the composite withdifferent filler percentages were measured using rectangular waveguide in conjunction with a microwave vectornetwork analyzer (VNA) in X-band range of frequency. The computations of the S-parameters were achieved byusing the FEM technique along with NRW mode. Then, the obtained results were compared with the measured R/Tcoefficients. Relative error results nominated the FEM mode due to its highly accurate results than the other method. Keywords: Microwave measurements, OPEFB, PCL, Bio-composite

Tunable multiport system for measurement of two-port scattering parameters

A novel multiport system, allowing for measurements of scattering parameters in over-two-octave frequency range is proposed. It is composed of two directional couplers and a standard 4 × 4 Butler matrix, and does not require any isolators. The presented system features a uniform power distribution providing the high precision of measurements, which can be further enhanced by a simple adjustment of the system's parameters. A comprehensive analysis of the proposed system configurations, a fully analytical calibration for transmission coefficient measurement, and the estimation of maximum measurement error are given. The proposed measuring system has been experimentally verified in a wide frequency range 1–5 GHz, by measurements of S-parameters of exemplary components. The measurement results are very close to the values obtained with the use of a commercial vector network analyser within the 50 dB range of measured values’ magnitude.

Wideband circular polarizer based on dielectric gratings with periodic parallel strips.

A wideband linear-to-circular polarizer is proposed, which consists of an array of dielectric slabs with double-sided parallel metallic strips. The polarization conversion is achieved by decomposing the linearly polarized incident plane wave into two orthogonal components of equal amplitude, which are subjected to an unequal phase shift such that the resultant phase difference between two components is 90° after an appropriate propagation path. The metallic strips are introduced to enhance the axial ratio bandwidth. Microwave experiment is performed to successfully realize these ideas and measured results are in good agreement with numerical simulations. The frequency range over which the measured transmission coefficient is higher than 0.95 is from 7 to 13.7 GHz, and the 3 dB axial ratio bandwidth under normal incidence ranges from 7 to 13 GHz, corresponding to a 60% fractional bandwidth. In addition, the proposed polarizer shows a good stability with respect to the oblique incidence.