Magnitude Of Reflection Coefficient Research Articles

Apertured EBG-Based Microwave Patch Antenna for Characterization of Corrosion in Steel Rebar of Civil Structures

This paper presents the design and fabrication of a prototype microwave patch antenna system for non-destructive corrosion monitoring of embedded steel rebar in civil structures. A planar rectangular patch for 2.4 GHz resonant frequency is modeled and loaded with electromagnetic bandgap (EBG) structure having an aperture slot of 2 × 10 cm2 in the center. The EBG plane provides confined radiation from its aperture and suppresses any destructive interference in the sensing area. The proposed patch sensor is fabrication using RO4450B substrate and tested on autoclaved aerated concrete and concrete block. The EBG superstrated patch is mounted on the test specimen and magnitude of reflection coefficient (S11) is measured at the near-field distance. A change in the intensity of scattered fields because of the oxide layer formed over the rebar by corrosion process is observed and measured. Analysis of weight loss of rebar due to corrosion is correlated with scattering parameter and equivalent DC voltage signaling. The measured results show that the proposed sensing technique is useful in corrosion detection of rebar inside the civil structures.

Miniaturization Techniques Using Magnetic Materials for Broadband Antenna Applications

This paper investigates the instrumentation necessary to analyze frequency-dependent complex parameters of solid magnetic materials based on T/R-based waveguide measurement technique proposed by W. B. Weir. Based on the magnitude and phase values of transmission and reflection coefficients measured using vector network analyzer, complex permittivity and permeability are calculated using the expressions defined. Equations necessary for calculating the complex parameters as a function of frequency with required step size are derived from the experimental results. To validate mathematical expressions developed for composite magnetic materials, the design of a compact 2-18 GHz cavity-backed spiral antenna using off-the-shelf magnetic materials is presented. The electrical performance of the cavity along with stack of high contrast magnetic materials of varying dimensions is modeled and simulated in ANSYS HFSS software.

Electromagnetic modeling of a reconfigurable graphene-based metasurface using MoM-GEC method for 3D digital holography

ABSTRACTViewing the trend of beam shaping, beam steering, and metasurface holograms, a reconfigurable graphene-based metasurface is proposed and modeled using the method of moments combined with the generalized equivalent circuit (MoM-GEC) approach. The scattering state of each individual unit cell of the metasurface can be controlled by applying different biased voltages into the graphene antennas. In fact, the complex surface conductivity of graphene is introduced in our mathematical formulation in order to tune the magnitude and phase of the local reflection coefficient. Benefitting from this scalability, our graphene-based metasurface will regenerate a 3D medical hologram. The proposed approach is validated and paves the way for the generation a 3D digital holograms. There are several important areas where this study makes an original contribution such as medical imaging 3D displays.

Design and optimization of a tunable W‐band subharmonic cavity based Gunn diode oscillator using computer‐aided design technique

In this work, a systematic computer-aided design technique is proposed to minimize the fabrication iteration for the design and development of W-band subharmonic Gunn diode oscillator with wideband tunable bandwidth at W-band. Gunn diode based single diode oscillator structure was divided into passive and active parts to facilitate the modeling of the component on appropriate simulation environment. Resonating structure and package of Gunn diode are modeled as passive circuit in high frequency structure simulator (HFSS). To satisfy the oscillator design equation, disc-post resonating structure is tuned in HFSS and its S-parameters are collaborated with the model of Gunn diode in advanced design system. Magnitude and phase of reflection coefficient (S11) is observed to ascertain the desired frequency of oscillation. Proper tuning of disc-post structure is done on simulation platform, which reduces the fabrication complexity and cost as well. The measurement results validate the models designed using EM and circuit simulator. The measured maximum stable RF power without any fabrication iteration is 14.2 dBm. A tunable bandwidth of 4 GHz with power output ripple of ±1 dB is measured by using a movable backshort.

Designing a Broadband Terahertz Half-Wave Plate Using an Anisotropic Metasurface

We design a multi-resonance metasurface at terahertz frequency, which can act as a polarization manipulator in the reflective mode. The proposed polarization converter consists of periodic unit cells and each unit cell has a resonator on the top surface. While a dielectric material forms the middle layer, a gold foil constitutes the bottom layer. The proposed polarization converter converts a linearly polarized terahertz wave into an orthogonal one for a wide range of operating frequencies. It provides a maximum conversion efficiency in the frequency range of 0.64–1.47 THz where the magnitudes of the cross reflection coefficients exceed 90%. The calculated relative bandwidth of the proposed converter is 78.67%. The phase difference of the reflected wave is between − 180∘ and 180∘ depending upon the operating frequency. Further, based on the detailed numerical results, we corroborate that the proposed device is robust against the variations in the structural parameters. The proposed converter maintains the conversion efficiency for various incident angles from 0∘ to 30∘. Besides, we also demonstrate the possibility of tuning the polarization conversion ratio by integrating silicon in the metasurface. The proposed metasurface may find potential applications in communications, antenna, and radar cross-section reduction technology.

Extensions of Johnson’s Theory of Backward-Wave Oscillations in a Traveling-Wave Tube

This paper extends Johnson’s classical theory [Johnson, Proc. IRE 43, 684 (1955)] for the threshold of backward-wave oscillation (BWO) in a traveling-wave tube by considering the effects of: 1) small random variations of the circuit phase velocity along the tube axis and 2) end reflections. We find that both the BWO threshold and BWO frequency are minimally affected by random variations of the phase velocity, but we find that the oscillation threshold depends sensitively on the magnitude and phase of the composite reflection coefficients.

A Quasi-Optical Testbed for Wideband THz On-Wafer Measurements

We propose a novel quasi-optical topology to enable ultra-wideband on-wafer characterization in the THz regime. Using quasi-optical components, we implement a high-performance directional coupler and a noncontact probing scheme, to interrogate on-wafer devices and circuits. The use of quasi-optics allows superior signal isolation and wideband operation due to the frequency independent nature of focusing components. Additionally, the simple topology is amenable to integration with photonics-based THz emitters and detectors to enable a true THz testbed for vector measurement capabilities. A proof-of-concept testbed is designed and evaluated in the 300–340 GHz band using free-space and on-wafer measurements. The on-wafer characterization of passive devices shows good agreement with simulated results and repeatability of <2% for reflection coefficient magnitude and <4° for phase deviation.

An Interdigital Capacitor for Microwave Heating at 25 GHz and Wideband Dielectric Sensing of nL Volumes in Continuous Microfluidics.

This paper proposes a miniature microwave-microfluidic chip based on continuous microfluidics and a miniature interdigital capacitor (IDC). The novel chip consists of three individually accessible heaters, three platinum temperature sensors and two liquid cooling and mixing zones. The IDC is designed to achieve localized, fast and uniform heating of nanoliter volumes flowing through the microfluidic channel. The heating performance of the IDC located on the novel chip was evaluated using a fluorescent dye (Rhodamine B) diluted in demineralized water on a novel microwave-optical-fluidic (MOF) measurement setup. The MOF setup allows simultaneous microwave excitation of the IDC by means of a custom-made printed circuit board (connected to microwave equipment) placed in a top stage of a microscope, manipulation of liquid flowing through the channel located over the IDC with a pump and optical inspection of the same liquid flowing over the IDC using a fast camera, a light source and the microscope. The designed IDC brings a liquid volume of around 1.2 nL from room temperature to 100 °C in 21 ms with 1.58 W at 25 GHz. Next to the heating capability, the designed IDC can dielectrically sense the flowing liquid. Liquid sensing was evaluated on different concentration of water-isopropanol mixtures, and a reflection coefficient magnitude change of 6 dB was recorded around 8.1 GHz, while the minimum of the reflection coefficient magnitude shifted in the same frequency range for 60 MHz.

Wideband Beam Steering Antenna Array of Printed Cavity-Backed Elements With Integrated EBG Structure

The letter concerns the design of a wideband printed antenna array element based on a cavity-backed patch radiator with an incorporated U-slot. An additional 2 × 3 mushroom-like electromagnetic bandgap structure is introduced in the E-plane in close proximity to the patch element on the same single-layer substrate. The proposed array unit cell design is optimized for operation in the 12% relative bandwidth in C-band and the 80° conical beam steering sector. Both simulated and experimental data demonstrate considerable wideband beam steering performance improvement compared to the basic design (without isolating structure) characteristics, provided by achieved array elements decoupling. Research results predict for the 12% relative bandwidth broadside reflection coefficient magnitude |Γ| ≤ 0.3, and maximum for the 80° conical beam steering sector |Γ| ≤ 0.37.

Ultra-wideband and high-efficiency reflective polarization converter for both linear and circular polarized waves

In this work, an ultra-wideband and high-efficiency reflective polarization converter is proposed based on an orthotropic anisotropic metasurface for both linear and circular polarized waves, which is a symmetric structure with a pair of mutually perpendicular symmetric axes. Both the simulated and measured results show that the polarization converter can realize linear polarization conversion at x- and y-polarized incidences in the frequency range from 8.77 to 24.71 GHz, which is corresponding to a 95.2% relative bandwidth; moreover, it can keep the handedness of the reflected wave the same as that of the incident wave in this band and realize reflection-type circular polarization conversion at circular polarized (CP) incidence. We have explained the root cause of these polarization conversions, and made it clear that it is just the symmetry of the metasurface structure which makes the magnitude of co-polarized reflection coefficient at CP incidence equal to that of the cross-polarized reflection coefficient at x- and y-polarized incidences, thus it is shown that the previously proposed various reflective linear polarization converters based on orthotropic anisotropic metasurfaces can all realize reflection-type circular polarization conversion.

An integral equation technique for the analysis of phased array antenna with matching step discontinuities

Applying the integral equation method of overlapping partial domains and the Schwartz alternating method to solving an electromagnetic wave diffraction problem is considered in this paper. The infinite rectangular waveguide phased array antenna scanning in H plane which waveguides have step matching discontinuities is represented. The whole field definition domain is sliced into three overlapping partial domains. The system of integral representations for unknown Ey components of the electrical field vector in each domain is set up using Greenʼs functions. Unknown functions in each domain are presented as orthogonal eigenfunction series. Using Galerkinʼs procedure, the system of integral representations is reduced to the system of linear equations for unknown expansion coefficients. For Schwartz method the system of integral representation is solved using iterative methods. The dependences of the reflection coefficient magnitude and phase on the value of scan angle are obtained. The comparison of obtained results for particular cases with known ones is performed.

“In-gap” Spectroscopy: Reflected-Wave Phase and Film Characterization

Optical methods that are used to characterize the state of a surface covered with films are based on the measurement of either the ratio between the complex reflection coefficients for mutually orthogonal light polarizations (ellipsometry) or the magnitudes of reflection coefficients themselves; afterward, the parameters of films such as their number, thicknesses, and transparencies can be determined by the fitting, while solving the corresponding inverse problem. In order to extend the set of quantities that can bemeasured experimentally, a method is proposed that allows the phase of the reflected light wave to be determined, by analyzing the spectral features for light reflected from a plane-parallel gap between the surface of analyzed specimen and the environment. In particular, the spectrum obtained, by using the “moving specimen” procedure, can be transformed into the spectral dependences of the magnitude and phase of the reflection coefficient. As a result, the inverse problem of finding the dielectric permittivity of a single-layer film is reduced to the solution of a linear matrix equation, which makes the proposed method more advantageous in comparison with the ellipsometric one, for which there is no direct relationships between the ellipsometric angles and the physical parameters of the film.

Missing Evidence of Widespread Subglacial Lakes at Recovery Glacier, Antarctica

Recovery Glacier reaches far into the East Antarctic Ice Sheet. Recent projections point out that its dynamic behavior has a considerable impact on future Antarctic ice loss (Golledge et al., 2017, https://doi.org/10.1002/2016GL072422). Subglacial lakes are thought to play a major role in the initiation of the rapid ice flow (Bell et al., 2007, https://doi.org/10.1038/nature05554). Satellite altimetry observations have even suggested several actively filling and draining subglacial lakes beneath the main trunk (B. E. Smith et al., 2009, https://doi.org/10.3189/002214309789470879). We present new data of the geometry of this glacier and investigate its basal properties employing radio‐echo sounding. Using ice sheet modeling, we were able to constrain estimates of radar absorption in the ice, but uncertainties remain large. The magnitude of the basal reflection coefficient is thus still poorly known. However, its spatial variability, in conjunction with additional indicators, can be used to infer the presence of subglacial water. We find no clear evidence of water at most of the previously proposed lake sites. Especially, locations, where altimetry detected active lakes, do not exhibit lake characteristics in radio‐echo sounding. We argue that lakes far upstream the main trunk are not triggering enhanced ice flow, which is also supported by modeled subglacial hydrology.

Finite Element Analysis of Simple Rectangular Microstrip Sensor for Determination Moisture Content of Hevea Rubber Latex

The Finite Element Method, FEM has been successfully used to model a simple rectangular microstrip sensor to determine the moisture content of Hevea rubber latex. The FEM simulation of sensor and samples was implemented by using COMSOL Multiphysics software. The simulation includes the calculation of magnitude and phase of reflection coefficient and was compared to analytical method. The results show a good agreement in finding the magnitude and phase of reflection coefficient when compared with analytical results. Field distributions of both the unloaded sensor as well as the sensor loaded with different percentages of moisture content were visualized using FEM in conjunction with COMSOL software. The higher the amount of moisture content in the sample the more the electric loops were observed.

A compact dual, triple band resonators for negative permittivity metamaterial

We report, two different dual and triple-band designs of single negative meta-atoms based on combination of individual Z-shaped and S-shaped inclusions. The multi-band responses for proposed resonators are obtained based on combination of individual resonant structures which exhibit electric resonances at different frequencies in microwave region. Numerical simulations and parameter extractions using retrieval method for normally incident wave demonstrate negative permittivity response over frequencies 3.27–4.5 GHz and 7.37–9 GHz for metamaterial made of dual-resonant meta-atom (DRMA). Further, metamaterial made of modified version i.e., triple-resonant meta-atom (TRMA) has negative values for permittivity over frequencies 2.79–3.5 GHz, 6.4–7.5 GHz and 8.93–11.0 GHz. The magnitude of reflection and transmission coefficient obtained from numerical simulations are further validated with microwave free space measurements. The equivalent circuit model developed for TRMA is validated with numerically simulated results. The effective medium ratio (λ0/p) for proposed resonators is compared with various other metamaterial resonators to indicate compact nature. The proposed DRMA/TRMA resonators can find applications in performance improvement of multiband devices such as antennas, antenna arrays, filters and in realization of microwave sensors, detectors at microwave frequency regime.

Hybrid Genetic Programming for the Development of Metamaterials Designs With Improved Characteristics

The expansion of a hybrid genetic program's (HGP) functionality to allow for the development of broadband two-dimensional metamaterials designs with advanced characteristics is presented. Artificial magnetic conductor (AMC) ground planes and a tunable terahertz absorber generated by the HGP are compared with designs available in the literature. The HGP is shown to produce human-competitive results. The AMC ground planes synthesized by the HGP were found to produce improved results including wider bandwidths and larger reflection coefficient magnitudes than that of the human designs. For one of the designed AMCs, the HGP's bandwidth is 73.3% larger and the minimum reflection magnitude is 1.0% larger than the reference AMC. Similarly, the absorber synthesized by the HGP has larger bandwidths than that of a recently published absorber optimized by random hill climbing. Three bias voltages were tested with the tunable absorber. The bandwidths of the HGP absorber are 23.1%, 37.6%, and 400% larger than the reference absorber, for biases of 4, 2, and 0.5 V/nm, respectively. Four example designs are discussed together with comparative results to illustrate the advantages of the developed HGP-enabled design method.

Measurement and Simulation of Combustion Noise and Dynamics of a Confined Swirl Flame

The combustion noise produced by a confined, turbulent, premixed swirl burner is predicted with large-eddy simulation of compressible, reacting flow. Characteristics-based state-space boundary conditions are coupled to the large-eddy simulation to impose precisely and independently from each other magnitude and phase of the acoustic reflection coefficients at the boundaries of the computational domain. The coupling approach proves to be accurate and flexible in regard to the estimation of sound pressure spectra in a confined swirl combustor for different operating conditions. The predicted sound pressure levels and its spectral distributions are compared to measurements. Excellent qualitative and quantitative agreement is achieved not only for a stable configuration but also for configurations that exhibit a thermoacoustic instability. This indicates that the flow and flame dynamics are reasonably well reproduced by the simulations.

Inkjet-Printed Wideband Circularly Polarized Microstrip Patch Array Antenna on a PET Film Flexible Substrate Material

In this letter, a wideband right-hand circularly polarized high-gain 4 × 4 microstrip patch array antenna on a polyethylene terephthalate (PET) film flexible substrate material is investigated. A comparison between the planar and curved surface study is also presented to show impact of curving the array on antenna performance parameters since it is employing a flexible substrate material. Due to some unknown properties of the PET material, some disagreement is noticed between the simulated and measured results in reflection coefficient magnitude, axial ratio, and gain in addition to radiation patterns. The radiation patterns are consistent with some rise in cross-polarization levels.

Uncertainty quantification and sensitivity analysis of thermoacoustic stability with non-intrusive polynomial chaos expansion

In this paper, non-intrusive polynomial chaos expansion (NIPCE) is used for forward uncertainty quantification and sensitivity analysis of thermoacoustic stability of two premixed flame configurations. The first configuration is a turbulent swirl combustor, modeled by the Helmholtz equation with an n−τ flame model. Uncertain input parameters are the gain and the time delay of the flame, as well as the magnitude and the phase of the outlet reflection coefficient. NIPCE is successfully validated against Monte Carlo simulation. It is observed that the first order expansion suffices to yield accurate results. The second configuration under investigation is a low order network model of a laminar slit burner, with the flame transfer function identified from weakly compressible CFD simulations of laminar reacting flow. Firstly the uncertainty and sensitivity of the growth rate due to three uncertain input parameters of the CFD model – i.e., flow velocity, burner plate temperature and equivalence ratio – are analyzed. A Monte Carlo simulation is no longer possible due to the computational cost of the CFD simulations. Secondly, two additional uncertain parameters are taken into account, i.e., the respective magnitudes of inlet and outlet reflection coefficients. This extension of the analysis does not entail a considerable increase in computational cost, since the additional parameters are included only in the low order network model. In both cases, the second order expansion is sufficient to model the uncertainties in growth rate.

Feasibility of using near-field microwave reflectometry for monitoring autogenous crack healing in cementitious materials

This study demonstrates the feasibility of using the near-field microwave reflectometry technique to nondestructively monitor the evolution of autogenous crack healing of mortar containing high volume of supplementary cementitious materials. Mortar samples were subjected to controlled compressive loading to generate cracks, and subsequently exposed to wetting/drying cycles to initiate the autogenous crack healing process. Test results indicate that cracked mortar samples exhibit higher point-to-point microwave reflection coefficient variations caused by cracking and moisture ingress (i.e., larger coefficient of variation (COV) values of magnitude of reflection coefficient, |Γ|, obtained from microwave reflectometry). When subjected to wetting/drying cycles, samples with higher crack healing capability are found to undergo less variation in microwave reflection coefficient. Based on the results for cracked samples, the COV trends obtained for microwave reflection properties can be divided into three parts as a function of wetting/drying cycles: part (I) corresponding to a significant point-to-point microwave reflection variations resulting from crack formation and moisture ingress after the first wetting/drying cycle; part (II) indicating the onset of the crack healing process identified by the reduction in the COV values; and part (III) representing slow-down of crack healing process for the regions exposed to microwave radiation as indicated by the relatively constant COV values during additional wetting/drying cycles. Such variations in microwave reflection properties can be linked to changes in moisture transport properties and subsequent crack healing process. To corroborate the microwave reflectometry results, concurrent ultrasonic measurements were conducted on the mortar samples during the self-healing process, and a good correlation was observed between the outcomes of these two test methods. The results of material characterization assessments including capillary water absorption, crack healing quantification, as well as X-ray diffraction and scanning electron microscopy of crack healing products were also used to quantify the crack healing evolution for the investigated mortar samples.