Full-wave Model Research Articles

Theoretical investigation on the optical response of the artificial magnetic–plasmonic nanoclusters

Bottom-up nanofabrication, organizing colloidal nanoparticles into ordered clusters with structural and functional complexity, provides an alternative to yield nanometric smart structures unachievable in traditional top-down approaches. Those remarkable structures assembled of colloidal nanoparticles possessing optical properties feature exotic optical responses under the illumination of incident light, showing highly dependent on the particle separation, composition, and arrangement. Here, the nanoscale electromagnetic coupling in magnetic–plasmonic nanoparticle clusters is analyzed by using a 3D full-wave computational model, and the mechanism of the resonances in a split-ring resonator out of magnetic–plasmonic nanoparticle building blocks is also illustrated. In particular, once assembling those artificial molecules (split-ring resonator) into secondary metamaterial superstructures, such as split-ring resonator dimer and Y-shaped structures, a series of exotic optical responses are presented due to the greatly enhanced near-field coupling of nanoparticles and the generation of antiparallel ring currents in the asymmetrical superstructure, exhibiting flexible sensitivity to the changes of the surrounding environment. Besides, the H-shaped arrangement exhibits high sensitivity with Q = 50.5 at λ = 2.02 μm. Our finding provides a platform for yielding complex structures with flexible tunability of the optical response through arranging those assembled artificial molecules into complex secondary structures, allowing for the development of smart sensing devices.

System-Level Simulation Suite for the Design of mmWave-Over-Fiber-Based Distributed Antenna Systems

Although challenging propagation conditions reduce its reliability, the mmWave spectrum is considered a cornerstone of (beyond-)5G networks. Recently, distributed antenna systems (DASs) surrounding the mobile users with multiple remote antenna units (RAUs) interconnected by mmWave-over-fiber technology were identified as a prime candidate to unlock high throughput and reliable coverage. This article proposes a dedicated simulation suite facilitating the deployment of such a mmWave-over-fiber-based DAS by accurately predicting system-level performance and enabling time-efficient optimization of the hardware configuration, including the RAUs, the central office (CO), and the signal processing units, toward the target application. It incorporates accurate models for the mmWave-over-fiber link and the amplifiers, including nonlinear distortion and noise, full-wave electromagnetic models for the antenna front-ends, and analytical models for the wireless channel. The simulation suite is validated by a measurement campaign, not only focusing on a single mobile user served by a fixed-beam RAU but also considering a multibeam RAU serving two users simultaneously by means of two independent mmWave-over-fiber links. The model accurately predicts up-and downlink quality over a wide range of user positions, different system parameters, and also accurately captures inter-user interference.

Full-Wave 2D Modeling of Helicons Propagation and Absorption in the Spherical Tokamak Globus-M2

Full-Wave 2D Modeling of Helicons Propagation and Absorption in the Spherical Tokamak Globus-M2

Full-wave modeling of EMIC wave packets: ducted propagation and reflected waves

Electromagnetic ion cyclotron (EMIC) waves can scatter radiation belt electrons with energies of a few hundred keV and higher. To accurately predict this scattering and the resulting precipitation of these relativistic electrons on short time scales, we need detailed knowledge of the wave field’s spatio-temporal evolution, which cannot be obtained from single spacecraft measurements. Our study presents EMIC wave models obtained from two-dimensional (2D) finite-difference time-domain (FDTD) simulations in the Earth’s dipole magnetic field. We study cases of hydrogen band and helium band wave propagation, rising-tone emissions, packets with amplitude modulations, and ducted waves. We analyze the wave propagation properties in the time domain, enabling comparison with in situ observations. We show that cold plasma density gradients can keep the wave vector quasiparallel, guide the wave energy efficiently, and have a profound effect on mode conversion and reflections. The wave normal angle of unducted waves increases rapidly with latitude, resulting in reflection on the ion hybrid frequency, which prohibits propagation to low altitudes. The modeled wave fields can serve as an input for test-particle analysis of scattering and precipitation of relativistic electrons and energetic ions.

Optimal design of transmitarray antennas via low-cost surrogate modelling

Over the recent years, reflectarrays and transmitarrays have been drawing a considerable attention due to their attractive features, including a possibility of realizing high gain and pencil-like radiation patterns without the employment of complex feeding networks. Among the two, transmitarrays seem to be superior over reflectarrays in terms of achieving high radiation efficiency without the feed blockage. Notwithstanding, the design process of transmitarrays is more intricate due to the necessity of manipulating both the transmission phase and magnitude of its unit elements. For reliability, the design process has to be conducted at the level of full-wave electromagnetic models, which makes direct optimization prohibitive. The most widely used workaround is to employ surrogate modeling techniques to construct fast representations of the unit elements, yet the initial model setup cost is typically high and includes acquisition of thousands of training data points. In this paper, we propose a novel approach to cost-efficient design of transmitarrays. It is based on artificial-intelligence-enabled data-driven surrogates, which can be constructed using only a few hundreds of training data samples, while exhibiting the predictive power sufficient for reliable design. Our methodology is demonstrated by re-using the presented surrogate for the design of high-performance transmitarrays operating at various frequency ranges of 8–14 GHz, 22–28 GHz, and 28–36 GHz.

RadarSense: Accurate Recognition of Mid-air Hand Gestures with Radar Sensing and Few Training Examples

Microwave radars bring many benefits to mid-air gesture sensing due to their large field of view and independence from environmental conditions, such as ambient light and occlusion. However, radar signals are highly dimensional and usually require complex deep learning approaches. To understand this landscape, we report results from a systematic literature review of ( N =118) scientific papers on radar sensing, unveiling a large variety of radar technology of different operating frequencies and bandwidths and antenna configurations but also various gesture recognition techniques. Although highly accurate, these techniques require a large amount of training data that depend on the type of radar. Therefore, the training results cannot be easily transferred to other radars. To address this aspect, we introduce a new gesture recognition pipeline that implements advanced full-wave electromagnetic modeling and inversion to retrieve physical characteristics of gestures that are radar independent, i.e., independent of the source, antennas, and radar-hand interactions. Inversion of radar signals further reduces the size of the dataset by several orders of magnitude, while preserving the essential information. This approach is compatible with conventional gesture recognizers, such as those based on template matching, which only need a few training examples to deliver high recognition accuracy rates. To evaluate our gesture recognition pipeline, we conducted user-dependent and user-independent evaluations on a dataset of 16 gesture types collected with the Walabot, a low-cost off-the-shelf array radar. We contrast these results with those obtained for the same gesture types collected with an ultra-wideband radar made of a vector network analyzer with a single horn antenna and with a computer vision sensor, respectively. Based on our findings, we suggest some design implications to support future development in radar-based gesture recognition.

Scanning of the Near-field Focused Beam by Changing Frequency

A new antenna array system is introduced which enables the movement of a near-field focused beam by change of frequency. The shift in the beam is achieved by using multi-wavelength transmission lines between successive elements of the array. At the designed frequency the beam is focused at the focal point. Due to the differences in the transmission line lengths feeding the antenna elements, a progressive phase change occurs between these elements. These phase differences between the elements cause a shift in the beam position. An 8×8 patch antenna array system was designed at 2.4 GHz on a microstrip to implement the proposed system. Two substrates were used: one for the patch antenna array and the other one for the feed network and transmission lines. A full-wave simulation model was used to analyze this design and demonstrate the movement of the beam. For this design, the focused beam is moved 310 mm by changing the frequency from 2.2025 GHz to 2.5750 GHz.

Investigation of phase modulation and propagation-route effect from unmatched large-scale structures for Doppler reflectometry measurement through 2D full-wave modeling

To interpret the common symmetric peaks caused by the large-scale structure in the complex S(f) spectrum from the heterodyne Doppler reflectometry (DR) measurement in EAST, a 2D circular-shaped O-mode full-wave model based on the finite-difference time-domain method is built. The scattering characteristics and the influences on the DR signal from various scales are investigated. When the structure is located around the cutoff layer, a moving radial or poloidal large-scale structure k θ ≲ k θ,match (k θ,match is the theoretic wavenumber of Bragg scattering) could both generate an oscillation phase term called ‘phase modulation’, and symmetrical peaks in the complex S(f) spectrum. It was found that the image-rejection ratio A −1/A +1 (A ±1 represents the amplitudes of ±1 order modulation peaks) could be a feasible indicator for experiment comparison. In the case when the structure is near the cutoff layer with the same arrangement as the experiment for the edge DR channel, the curve of A −1/A +1 versus k θ can be divided into three regions, weak asymmetrical range with k θ /k 0 ≲ 0.15 (k 0 is the vacuum wavenumber), harmonics range with 0.15 ≲ k θ /k 0 ≲ 0.4, and Bragg scattering range of 0.4 ≲ k θ /k 0 ≲ 0.7. In the case when the structure is located away from the cutoff layer, the final complex S(f) spectrum is the simple superimposing of modulation and Bragg scattering, and the modulation peaks have an amplitude response nearly proportional to the local density fluctuation, called the ‘propagation-route effect’. Under the H-mode experiment arrangement for the core DR, a critical fluctuation amplitude ∼ 1.3–4.1 ( refers to the pedestal large-scale structure amplitude and Amp(n e,Tur.@MSA) refers to turbulence amplitude at the main scattering area) is needed for the structure in the pedestal to be observed by the core DR measurement. The simulations are well consistent with the experimental results. These effects need to be carefully considered during the DR signal analyses as the injecting beam passes through the plasma region with large-scale structures.

Design and performance enhancement of terahertz photoconductive antenna based on nano-crossline contacts

Photoconductive antennas (PCAs) of high conversion efficiency is an essential device for generating or detecting terahertz (THz) waves. This paper presents an enhanced THz-PCA by incorporating buried nano-crosslines (BN-CL) contacts to offer high optical-to-terahertz power conversion efficiency. The BN-CL contact makes that most photocarriers are created at nanoscale distances from the PCA electrodes and, hence, carries drift in a sub-picosecond duration to generate THz radiation. The full-wave multiphysics model is used for the hybrid simulation of THz PCAs and investigated the impact of geometrical variables on a proposed PCA's transient photocurrent. The designed THz antenna has a bandwidth of more than 4 THz and high efficiency of 94%. Both designs of BN-CL and N-CL are implemented with and without double dielectric layers of anti-reflection coating (ARC) and distributed Bragg reflector (DBR) layers. The obtained results of the transient photocurrent are 891 μA and 1089 μA for 10 elements of implemented BN-CL and N-CL contacts, respectively. The photocurrent of BN-CL contacts is enhanced to 1464 μA when 50 element contacts are used. Also, the conversion efficiency of this PCA reaches a maximum value of 1.032% at a pump power of 1.5 mW.

Human Metal ESD: Design of a Full-Wave Model and an Improved Compact Generator

A full-size, full-wave human body model has been designed. Compared with a real human-metal electrostatic discharge (ESD), similar fields and currents are obtained from the numerical model. A resistive surface sheet is used in microwave studio to control the currents on the air-filled body. Its performance is verified by comparing it to the currents and fields from the real human ESD. A subcircuit models the time-dependent resistance of the spark using the well-known Rompe–Weizel spark resistance model. Real human metal ESD does not reveal the double hump shape as shown in the IEC standard. To create a generator that reproduces the real human metal ESD current waveform, a compact ESD generator was designed, which is based on matching the impedance of real human metal mode ESD with the impedance of the compact ESD generator. Furthermore, a circuit-based simulator is presented. It is based on a PCB implementation of the equivalent circuit of the human metal ESD.

Can Perfectly Electric Conducting Obstacles Modify the Permeability of Metamaterial? An Investigation Using Lorentz, Floquet, and Full-Wave Models

The Lorentz model shows that inclusion of perfectly electric conducting (PEC) obstacles in a host medium introduces both electric and magnetic polarizabilities that modify the permittivity and permeability tensors of the composite medium. On the other hand, the Floquet eigenmodal analysis shows that the PEC obstacles, while modify the permittivity tensor, have no effect on the permeability tensor. Interestingly, numerical results obtained from full wave analyses of metamaterial-slabs support the Floquet model. The source for the inaccuracy in Lorentz model is critically examined. Characteristic features of metamaterials made with non-resonant and resonant types of obstacles are investigated and the numerical results obtained from four independent models are compared and discussed.

Swarm Intelligence-Homotopy Hybrid Optimization-Based ANN Model for Tunable Bandpass Filter

High-performance tunable radio frequency (RF)/ microwave and millimeter-wave filter design is a challenging task due to the lack of a basic theory. The filtering characteristics are highly sensitive to the variation of tuning elements that are commonly modeled and achieved by optimization algorithms. However, those optimizations only provide satisfactory results with a good set of initial parameters. Such range-limited optimization algorithms generally have issues of falling into local optima, slow convergence, and cumbersome implementation. To mitigate this problem, for the first time, a topology-based local optimizer is integrated with metaheuristic global optimization algorithms in this work. We have hybridized the homotopy method with an improved whale optimization algorithm (WOA) and a gray wolf optimization (GWO) algorithm. In this work, an artificial neural network (ANN) is formulated and studied, which has twofold applications. First, ANN is used as a surrogate model to represent the time-consuming electromagnetic (EM) model in expediting the hybrid optimization process of tunable filters. Second, an ANN model is developed on data generated by the proposed optimization algorithm for predicting tunable circuit parameters at different tuning stages. The proposed ANN model-based algorithm is then applied to a fifth-order lumped-element tunable circuit and two fourth-order full-wave EM simulation models of two tunable bandpass filters (tBPFs). The calculated results out of the ANN model demonstrate a good agreement with simulation and measurement counterparts.

A method for density fluctuation measurements using pulse reflectometry

An analytical 1D model based on the WKB approximation is used to study the properties of a short pulse reflectometry diagnostic in a magnetic fusion device. Expressions linking the pulse delay with the parameters of the turbulence near the cut-off layer are derived for both ordinary and extraordinary polarizations of the probing beam. These results are used to develop a method for measuring the turbulence amplitude and the radial correlation length. The analytical conclusions and the proposed method are validated using full-wave numerical modelling. The latter is also used to study the limitations of the method and potential experimental effects not included in the reduced model.

On nature-inspired design optimization of antenna structures using variable-resolution EM models

Numerical optimization has been ubiquitous in antenna design for over a decade or so. It is indispensable in handling of multiple geometry/material parameters, performance goals, and constraints. It is also challenging as it incurs significant CPU expenses, especially when the underlying computational model involves full-wave electromagnetic (EM) analysis. In most practical cases, the latter is imperative to ensure evaluation reliability. The numerical challenges are even more pronounced when global search is required, which is most often carried out using nature-inspired algorithms. Population-based procedures are known for their ability to escape from local optima, yet their computational efficiency is poor, which makes them impractical when applied directly to EM models. A common workaround is the utilization of surrogate modeling techniques, typically in the form of iterative prediction-correction schemes, where the accumulated EM simulation data is used to identify the promising regions of the parameter space and to refine the surrogate model predictive power at the same time. Notwithstanding, implementation of surrogate-assisted procedures is often intricate, whereas their efficacy may be hampered by the dimensionality issues and considerable nonlinearity of antenna characteristics. This work investigates the benefits of incorporating variable-resolution EM simulation models into nature-inspired algorithms for optimization of antenna structures, where the model resolution pertains to the level of discretization density of an antenna structure in the full-wave simulation model. The considered framework utilizes EM simulation models which share the same physical background and are selected from a continuous spectrum of allowable resolutions. The early stages of the search process are carried out with the use of the lowest fidelity model, which is subsequently automatically increased to finally reach the high-fidelity antenna representation (i.e., considered as sufficiently accurate for design purposes). Numerical validation is executed using several antenna structures of distinct types of characteristics, and a particle swarm optimizer as the optimization engine. The results demonstrate that appropriate resolution adjustment profiles permit considerable computational savings (reaching up to eighty percent in comparison to high-fidelity-based optimization) without noticeable degradation of the search process reliability. The most appealing features of the presented approach—apart from its computational efficiency—are straightforward implementation and versatility.

An Efficient Full Wave Model for Investigating Layered Media Using Bi-Static Ground Penetrating Radar

Ground penetrating radar (GPR) is a popular non-destructive sensor for subsurface media investigation. Modelling of the GPR system has a significant role in the successful estimation of sub-surface media properties, imaging, and detection of buried object. A model should provide good accuracy and computational efficiency while taking care of numerous GPR scenarios. However, achieving accuracy and speed of computation is a difficult task. In this respect, analytical modellings are promising techniques to represent a specific GPR scenario. In this work, a simplified full wave model (FWM) for bi-static GPR has been developed to represent radar transceivers with layered media. The model has been derived from an existing full wave model (FWM) solution. The accuracy of the proposed model has been verified by comparing it with existing FWMs, and by laboratory experiments based on stepped frequency continuous wave (SFCW) GPR. The corresponding results show that the proposed bi-static model is as accurate as FWM and its computational efficiency is enormous in estimating the sub-surface media properties.

Insert Misalignment Studies of a Coaxial-Cavity Gyrotron—Full-Wave Approach

Insert misalignment studies are carried out for a coaxial-cavity gyrotron with triangular corrugations on the insert wall using the full-wave approach, space harmonics method (SHM). By applying electromagnetic boundary conditions, dispersion relation is derived for a coaxial-cavity with misaligned insert. Misalignment of the insert caused by axial displacement as well as tilting of the insert axis from the outer resonator axis are taken into consideration in the study. As the electromagnetic fields in the interaction space vary with the insert misalignment, due to this the mathematical formulations of the beam-coupling coefficient, wall losses, RF interaction efficiency, and output power are modified. For a 2-MW, 220-GHz gyrotron, the insert misalignment studies are carried out using the proposed full-wave model with the help of our in-house code Gyrotron Design Suite. For validation, the results obtained using the SHM approach are compared with that of the surface impedance model (SIM) approach. In addition, comparison studies between SIM and SHM approaches are performed for the practically developed 170-GHz, 2-MW coaxial-cavity gyrotron with both triangular and rectangular corrugations in the insert.

Design and analysis of Matts-shaped perfect metamaterial absorber using equivalent circuit model

A mid-infrared perfect metamaterial absorber (PMA) consisting of well-engineered unit cells, composed of metallic backed layer (Cu), dielectric layer (Al2O3), matts-shaped meta cell (Cu), and an anti-oxidation layer from bottom to top, is proposed in this paper. The electromagnetic response of the designed PMA is analyzed using 3D full-wave computational model. Our analysis demonstrates that the maximum absorption is enhanced to 99.9% at the resonance wavelength of 7.35 µm, which is caused by magnetic polaritons (MPs) resonance between the incident beam and magnetic polaritons formed by three fluxing currents. Specially, a three-circulation equivalent circuit model is proposed to predict the resonant wavelength of the designed PMA, which comprehensively considers the influence of coupling capacitance not only between neighboring unit cells, but also between the metal lines inside the top metal pattern for the first time. The influence of geometrical parameters on resonant wavelength is predicted by the equivalent circuit model proposed in this paper, and the prediction error is less than 5% as compared with that of computational analysis. With well-engineered plasmonic meta-atoms, the designed PMA presents the characteristics of angle and polarization- independent with a smaller relative size (0.245λ) and a thinner relative thickness (0.053λ). Besides, an obvious red-shift of the resonance frequency of PMA is observed as the permittivity of dielectric layers is increased. The approach of establishing equivalent circuit model will provide methods for the analysis of PMA based on MPs with complex electromagnetic behavior.

Semi-analytical and Full Wave Solutionfor the FMCW-signal Path of a Radar for the Metal Rod in a Concrete Medium Detection

This article describes the systematic development of an analytical and full wave model of a radio channel, which may be included in the operation algorithm of detecting metal structures in a concrete medium for the radar device. The analytical model is based on the geometric formulation and solution of an inverse problem to find the required refraction angle to accurately target incidence. In the full wave model, which was developed in CAD Altair Feko, it is possible to take into account both the real antenna or antenna array, which is used in the device, and the real signal reflection object, which is located in the concrete medium. It is important to note that the analytical model takes into account the dispersion effect of electromagnetic waves propagating in the medium. This fact turns out to be especially relevant for the forming of the radar complex based on chirp signals (or FMCW) in a certain frequency range. In this paper, an algorithm is developed for the frequency range of 0.8–5 GHz, however, if necessary, the band boundaries may be changed. The algorithm is established created for its use in radar devices for detecting metal structures in a concrete medium. A measuring module of restoring the actual material parameters may be included in the complex of the radar device for a posteriori accounting of the electrophysical parameters of the concrete.

Development of an Equivalent Circuit Model for the Design of Array of Electrically Small Antennas

In this article, we present a fully parameterized circuit model of an array antenna that relates antenna performance and field coupling to the physical parameters of the elements as represented through a circuit model. We demonstrate that the simplified model can be used as a surrogate for full-wave modeling during the initial optimization steps in array design. The model accounts for mutual coupling between array elements both through the feeding network and through free space. We present a method to calculate the mutual coupling between elements in the array antenna that is parameterized in terms of the size and location of the individual elements, and these circuit models provide excellent agreement with the scattering parameters calculated by the full-wave numerical model. The model is applied to the analysis of a specific built array configuration, and we demonstrate that the simplified circuit model can have strong predictive effects for designing arrays of electrically small antennas (ESAs) and predicting their ultimate performance by comparing with numerical and limited experimental measurements. The study is motivated by high power electromagnetic (HPEM) applications, where arrays of densely space elements can help reduce the antenna size necessary to radiate extremely large power signals.

SIMULATION THE VIBRATIONS OF RAILWAY STRUCTURES BY THE GRID-CHARACTERISTIC METHOD

The aim of the study is to apply the grid-characteristic method on structured grids in various problems related to railway traffic safety; obtaining the results of full wave modeling using this method and analyzing them for a better understanding of the physical processes occurring in conditions of heavy and high-speed traffic on various sections of the track. The problem is to consider the traffic on a ballast and non-ballast bridge desk. In the course of computer simulation, various wave fields and dynamic distributions of the pressure and components of the Cauchy stress tensor were obtained during the traffic. An estimate of the calculation time during which the propagation of wave processes occurs in various bridge structures was made. The results obtained give an idea of wave phenomena during the traffic in the area of bridges and along the railway track as a whole.