Full-wave Electromagnetic Simulation Research Articles

Reduced-Cost Microwave Design Closure by Multi-Resolution EM Simulations and Knowledge-Based Model Management

Parameter adjustment through numerical optimization has become a commonplace of contemporary microwave engineering. Although circuit theory methods are ubiquitous in the development of microwave components, the initial designs obtained with such tools have to be further tuned to improve the system performance. This is particularly pertinent to miniaturized structures, where the cross-coupling effects cannot be adequately accounted for using equivalent networks. For the sake of reliability, design closure is normally performed using full-wave electromagnetic (EM) simulation models, which entails considerable computational expenses, often impractically excessive. Available mitigation techniques include acceleration of the conventional (e.g., gradient-based) routines using adjoint sensitivities or sparse sensitivity updates, surrogate-assisted and machine learning algorithms, the latter often combined with nature-inspired procedures. Another alternative is the employment of variable-fidelity simulations (e.g., space mapping, co-kriging), which is most often limited to two levels of accuracy (coarse/fine). This work discusses an EM model management approach coupled with trust-region gradient-based routine, which exploits problem-specific knowledge for continuous (multi-level) modification of the discretization density of the microwave structure at hand in the course of the optimization run. The optimization process is launched at the lowest discretization level, thereby allowing for low-cost exploitation of the knowledge about the device under study. Subsequently, based on the convergence indicators, the model fidelity is gradually increased to ensure reliability. The simulation fidelity selection is governed by the algorithm convergence indicators. Computational speedup (i.e., reduction in the number of EM simulations required by the optimization process to converge) is achieved by maintaining low resolution in the initial stages of the optimization run, whereas design quality is secured by eventually switching to the high-fidelity model when close to concluding the process. Numerical verification is carried out using two microstrip circuits, a dual-band power divider and a dual-band branch-line coupler, with the average savings of almost sixty percent when compared to single-fidelity optimization.

Accurate Modeling of Frequency Selective Surfaces Using Fully-Connected Regression Model With Automated Architecture Determination and Parameter Selection Based on Bayesian Optimization

Surrogate modeling has become an important tool in the design of high-frequency structures. Although full-wave electromagnetic (EM) simulation tools provide an accurate account for the circuit characteristics and performance, they entail considerable computational expenditures. Replacing EM analysis by fast surrogates provides a way to accelerate the design procedures. Unfortunately, modeling of microwave passives is a challenging task due to their highly-nonlinear outputs. Frequency selective surfaces (FSSs) constitute a representative example with their multi-resonant reflection and transmission responses that need to be represented over broad frequency ranges. Deep neural networks (DNNs) seem to be the promising techniques for handling such cases. However, a serious practical issue associated with their employment is an appropriate selection of the model parameters, including its architecture. A common practice is experience-driven setup, heavily based on trial and error, which does not guarantee the optimum model determination and may lead to multiple problems such as poor generalization or high variance of the model predictive power with respect to the training data set selection. This paper proposes a novel modeling framework, referred to as a fully-connected regression model (FCRM), where the crucial role is played by Bayesian Optimization (BO), incorporated to determine the DNN-based model setup, including both its architecture and the hyperparameter values, in a fully automated manner. For validation, FCRM is applied to construct the model of a Minkowski Fractal-Based FSS. The efficacy of the methodology is demonstrated through comparisons with several benchmark techniques, including the DNN surrogates established using the traditional methods as well as conventional regression models. The numerical results indicate that FCRM exhibits considerably improved prediction power and reduced sensitivity to the training sample assignment.

Broadband and high transmission of Mie-resonance-coupled subwavelength metal aperture

Transmission of the subwavelength metal aperture excited by the surface plasmon resonance is much higher than that from the Bethe theory. However, due to the sensitivity of resonant frequency and the loss of metal in optical band, it is difficult to achieve broadband and high transmission of the subwavelength metal aperture through surface plasmon resonance. In this article, the broadband and high transmission of the subwavelength metal aperture is realized when Mie-resonant-coupled silicon nanoparticles placed on both sides of the metal aperture are used to replace the surface plasmon resonance. The full wave simulation results show that bandwidth of the transmission coefficient more than 90% of the subwavelength aperture (<inline-formula><tex-math id="M1">\begin{document}$ {r \mathord{\left/ {\vphantom {r {\lambda = 0.1}}} \right. } {\lambda = 0.1}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20201121_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20201121_M1.png"/></alternatives></inline-formula>) reaches 65 nm by using Mie-resonance-coupled silicon nanoparticles. Compared with the transmission induced by surface plasmon resonance, the peak value is improved by 1.5 times and the 3 dB bandwidth is widened by 17 times. According to the coupled mode theory, the equivalent circuit model of transmission of the subwavelength metal aperture added with Mie-resonance-coupled silicon nanoparticles is established, and the element parameters in the circuit model are inversed under the critical coupling state. Further research shows that transmission rule of the subwavelength metal aperture added with Mie-resonance coupled silicon nanoparticles can be accurately revealed by changing the coupling coefficient in the equivalent circuit model, and the results are consistent with the full wave electromagnetic simulation results. The mathematical expression of the interaction between light and Mie-resonance-coupled subwavelength metal aperture is found, therefore it can inspire us to construct certain functional modules in optical field according to circuit design method.

Toward a More Realistic Characterization of Hand-Assembled Wire Bundles: Geometrical Modeling and EMC Prediction

In this paper, a systematic approach for EMC modelling of hand-assembled, complex cable bundles is presented. The first part of the manuscript focuses on bundles laid out above a ground plane, with layout approximately parallel to the reference ground. In order to generate smooth and physics-based trajectories, polynomial curves are used to represent the conductors in the bundle. A suitable process for the generation of bundle samples is developed, which allows for controlling the degree of randomness and avoiding conductors overlapping. Parametric representation of more general bundle structures with arbitrary orientation is then addressed, and the modeling approach extended to more complex bundle structures, such as bundles involving twisted-wire pairs. To assess the ability of Transmission Line (TL) theory to deal with complex and locally highly non-uniform bundle shapes, a fictitious bundle trajectory shaped as a trefoil knot is introduced. All the generated bundle geometries are used in combination with TL-based EMC models in order to predict the disturbances induced in the bundle terminal units due to crosstalk or coupling with external electromagnetic fields. Prediction accuracy is assessed by comparison versus full-wave electromagnetic simulation of the same bundle structures. Suitable examples are presented to show the high numerical efficiency and good accuracy of the proposed modeling approach, as well as the need for accurate and physics-based representation of the bundle geometry in order to achieve solid EMC prediction in a wide frequency range.

Overcoming Inherent Narrow Bandwidth and Low Radiation Properties of Electrically Small Antennas by Using an Active Interior-Matching Circuit

A technique is described to extend the working frequency-band and increase the radiation gain and efficiency of an electrically small antenna (ESA). The geometry of the proposed ESA is in the shape of an “H”structure. A small gap is included at the symmetry of the H-shape structure to embed an inductive load that is used to connect the two halves of the H-shaped antenna. With the lumped element inductor, the bandwidth of the H-shaped antenna is restricted by Chu-lower bound. However, it is demonstrated by analytical analysis and through 3D full-wave electromagnetic simulations that when the inductive load is replaced with negative reactance from a negative impedance converter (NIC) the antenna's bandwidth, radiation gain and efficiency performance can be significantly improved by ~40%, 3.6 dBi and 55%, respectively. This is because NIC acts as an effective interior matching circuit. The resonant frequency of the antenna structure with the inductive element was used to determine the required inductance variation in the NIC to realize the required bandwidth and radiation characteristics from the H-shaped antenna.

Antenna Modeling Based on Two-Stage Gaussian Process Considering Sensitivity Information

For antenna modeling and optimization design, high-fidelity full-wave electromagnetic simulation software can generally be used to obtain training samples. However, this process takes a long time. To solve this problem, a two-stage Gaussian process (GP) considering electromagnetic sensitivity information is proposed. Based on the coarse grid and fine grid of electromagnetic simulation software, the first stage learns the mapping relationships of antenna performance and sensitivity of each parameter, simultaneously. In the second stage, the accurate surrogates are established based on the mapping relationships above obtained. Because the method in this study takes sensitivity information into consideration during the modeling process, it can better reflect the mapping relationship between antenna input and output, which can develop a more accurate surrogate model. The proposed two-stage GP surrogate model considering sensitivity information can significantly reduce the demand of high-fidelity training samples for modeling, and greatly save the simulation time of calling electromagnetic simulation software. Therefore, this method is more suitable for the problems of insufficient electromagnetic simulation samples. The proposed approach is evaluated by modeling and optimization of an inverted F antenna and an ultra-wideband planar monopole antenna. The results show that the method has high modeling accuracy with limited training samples given by high-fidelity electromagnetic simulation, which further verify its effectiveness and efficiency.

Resonant Frequency Modeling of Microstrip Antenna Based on Deep Kernel Learning

When modeling and optimizing electromagnetic components, it is the most time consuming for obtaining the training samples with labels from full-wave electromagnetic simulation software. The traditional machine learning (ML) model is usually effective in the training process, unfortunately, its generalization ability is limited for practical application. Inspired by the artificial neural network (ANN) and the Gaussian process (GP) kernel learning model, a deep kernel learning (DKL) model with multiple-nonlinear-mapping layers is proposed based on particle swarm optimization (PSO) algorithm. In this work, correlation characteristics of training samples are transformed by multiple layers, and then they act as inputs of the GP model. Simultaneously, we use PSO to optimize the weights, biases of each mapping layer, and GP hyperparameters, aiming to determine and optimize the DKL model structure. As a result, the proposed DKL is suitable for processing small samples, as well as has the generalization ability of a deep network, which avoids the phenomenon of gradient disappearance to some extent caused by error back propagation (BP) in the deep network and improves the modeling accuracy while effectively ensuring the modeling efficiency. In this study, the performance of the DKL model is evaluated by using the resonant frequencies of two microstrip antennas (MSAs), and the predicted results of the DKL model are compared with those via different modeling methods. The results show that prediction accuracy of the DKL is 18.906% higher than that of the GP, 38.926% higher than the ANN, and 46.660% higher than the neural network ensemble (NNE).

A comprehensive electromagnetic evaluation of an MRI anthropomorphic head phantom.

Electromagnetic simulations are an important tool for the safety assessment of RF coils. They are a useful resource for MRI RF coil designers, especially when complemented with experimental measurements and testing using physical phantoms. Regular-shaped (spherical/cylindrical) homogeneous phantoms are the MRI standard for RF testing but are somewhat inaccurate when compared with anthropomorphic anatomies, especially at high frequencies. In this work, using a recently developed anthropomorphic heterogeneous human head phantom, studies were performed to analyze the scattering parameters (S-parameters) and the electric and magnetic field distributions using (1) the B1+ field mapping method on a 7 T human MRI scanner and (2) numerical full-wave electromagnetic simulations. All studies used the following: a recently developed six-compartment refillable 3D-printed anthropomorphic head phantom (developed from MRI scans obtained in vivo), where the phantom itself is filled in its entirety with either heterogeneous loading, or homogeneous brain or water loading, in vivo imaging, and a commercial homogeneous spherical water phantom. Our results determined that the calculated S-parameters for all the anthropomorphic head phantom models were comparable to the model that is based on the volunteer (within 17% difference of the reflection coefficient value) but differed for the commercial homogeneous spherical water phantom (within 45% difference). The experimentally measured B1+ field maps of the anthropomorphic heterogeneous and homogeneous brain head phantoms were most comparable to the in vivo measured values. The numerical simulations also show that both the anthropomorphic homogeneous water and brain phantom models were less accurate in terms of electric field intensities/distributions when compared with the segmented in-vivo-based head model and the anthropomorphic heterogeneous head phantom model. The presented data highlights the differences between the physical phantoms/phantom models, and the in vivo measurements/segmented in-vivo-based head model. The results demonstrate the usefulness of 3D-printed anthropomorphic phantoms for RF coil evaluation and testing.

Dielectric property characterisation of thin films based on iterative comparisons of full-wave simulations and measurements

A new research on characterising the dielectric constant and loss tangent of thin-film material with a convenient technique based on simulation is proposed. The authors utilise a set of full-wave electromagnetic simulation data to retrieve the dielectric properties from the measured data. More specifically, a set of simulation data is collected from a commercial full-wave simulator (Ansys HFSS) by solving the electromagnetic model the same as the measurement set-up which employs a grounded coplanar waveguide with a thin film placed on the top surface. The measured transmission coefficients (S21) are iteratively compared with its simulation data to estimate the dielectric constant and loss tangent using an efficient comparison algorithm. For adaptive data acquisitions and comparisons, they have developed a MATLAB code using hfss-matlab-toolbox that makes the whole process quick and automatic. They have successfully retrieved the dielectric properties of four different sprayed thin films at 8, 10, and 12 GHz, and observed that the dielectric constant and loss tangent strongly depends on their compositions, thickness, and frequency.

Antenna-on-Package Design: Achieving Near-Isotropic Radiation Pattern and Wide CP Coverage Simultaneously

Internet of Things (IoT) applications require orientation insensitive wireless devices to maintain stable and reliable communication as well as the localization capability using the global positioning system (GPS). For those reasons, antennas providing near-isotropic radiation pattern with wide circularly polarized (CP) coverage are high in demand for IoT applications. In this work, we derive conditions for 3-D antenna on package (AoP), through a combination of three orthogonal dipoles model and particle swarm optimization (PSO) technique to achieve near-isotropic radiation pattern and wide CP coverage simultaneously. Design guidelines have been provided, and a practical AoP has been designed on a 3-D cube package by following this guideline. A prototype has been fabricated and the measured results demonstrate an isotropy of ~93% and CP coverage of ~18%, which matches well with the predicted results from the theoretical analysis as well as the full-wave electromagnetic simulations.

Analytical Modeling and Experimental Studies on Tapered Post Re-Entrant Cavity Resonator

Compact re-entrant cavities, widely used as microwave resonators, can be employed as high-performance sensors. A new approach is proposed here to calculate the resonance frequency, its sensitivity, and quality factor (Q-actor) of these cavities, having diverse taper shapes of the re-entrant post. The proposed closed-form expressions are based on the chain matrix approach for tapered coaxial transmission lines, and the calculated results for the resonance frequency and quality factor have been verified with full-wave electromagnetic simulations and experiments. The formulation is mathematically simple and accurate (<; 4%). Since a cavity with taper toward the gap-end is found to be more sensitive to gap variations, a robust, tunable re-entrant cavity resonator with a tapered post and an adjustable gap was fabricated. The performance of this cavity as a displacement sensor was evaluated by numerically calculating the deflection of a thin diaphragm from measured resonance frequencies using the expressions derived here. This design of the sensor is found to be immune to manufacturing tolerance in the re-entrant gap.

In-situ monitoring for liquid metal jetting using a millimeter-wave impedance diagnostic

This article presents a millimeter-wave diagnostic for the in-situ monitoring of liquid metal jetting additive manufacturing systems. The diagnostic leverages a T-junction waveguide device to monitor impedance changes due to jetted metal droplets in real time. An analytical formulation for the time-domain T-junction operation is presented and supported with a quasi-static full-wave electromagnetic simulation model. The approach is evaluated experimentally with metallic spheres of known diameters ranging from 0.79 to 3.18 mm. It is then demonstrated in a custom drop-on-demand liquid metal jetting system where effective droplet diameters ranging from 0.8 to 1.6 mm are detected. Experimental results demonstrate that this approach can provide information about droplet size, timing, and motion by monitoring a single parameter, the reflection coefficient amplitude at the input port. These results show the promise of the impedance diagnostic as a reliable in-situ characterization method for metal droplets in an advanced manufacturing system.

Partially Filled Half-Mode Substrate Integrated Waveguide Leaky-Wave Antenna for 24 GHz Automotive Radar

A novel highly efficient leaky-wave antenna (LWA), based on a periodic set of holes created in a half-mode substrate integrated waveguide (SIW), is conceived for automotive radar. The computer-aided design of this partially filled SIW (PFSIW) LWA is carried out using a full-wave electromagnetic simulator. As a proof-of-concept, an LWA prototype having a plate size of 15 × 126 mm $^2$ is fabricated through a standard low-cost printed circuit board (PCB) manufacturing process. Our manufactured prototype yields more than 25% impedance bandwidth, targeting the 24 GHz automotive short-range-radar band. Moreover, the measured peak gain and total antenna efficiency reach up to 15.5 dBi and 85% at 24 GHz. Furthermore, the main radiating direction steers continuously from 38 $^\circ$ to 58 $^\circ$ with an average half-power beamwidth of 12 $^\circ$ when the operating frequency changes from 22 to 26 GHz. In comparison, a 160 mm long dielectric-filled SIW (DFSIW) LWA yields a measured scanning range of 49 $^\circ$ and a total antenna efficiency of 43.8%, whereas an 80.3 mm long air-filled SIW (AFSIW) LWA exhibits a measured scanning range of 9 $^\circ$ and a total antenna efficiency of 80%. Therefore, PFSIW technology provides an optimal tradeoff between DFSIW and AFSIW for the envisaged short-range automotive radar application.

A Wideband High-Power Ridge Gap Waveguide Power Divider for High-Power Division Sub-Systems Applications

ABSTRACT In this paper, a wideband high-power ridge gap waveguide (RGW) T junction power divider within the frequency band from 10 to 13.5 GHz is introduced. In this configuration, the air has the ridge gap to have maximum power handling capability. An excitation transition, from coaxial cable to the ridge gap waveguide, is introduced. The ridge gap waveguide power divider was fabricated using aluminum sheets to gain the advantage of low cost. The power divider has a wider bandwidth (30%) than the recent most successful RGW power divider along with the high handling power capability (more than 20 K Watts). The power divider design was validated using circuit and 3D full wave electromagnetic simulations and confirmed using experimental measurements with all agreements between results. It has been proved that the power divider attenuation has good matching (15 dB return loss) and equal 3 dB power division with less than 0.05 dB insertion loss. The power divider can be used as a feeder for many high-power application components.

Low concentration noroxin detection using terahertz spectroscopy combined with metamaterial.

The unreasonable use of antibiotics in poultry breeding has led to the frequent occurrence of antibiotic residues in poultry products, affecting food safety and posing a threat to human health. Accurate and rapid detection of antibiotic drug content is therefore of great significance. In this study, a new metamaterial method based on terahertz (THz) spectroscopy for the detection of the quinolone antibacterial drug noroxin in low concentrations was developed and tested. First, a new metamaterial structure was designed using a computational electromagnetics simulation tool with experimental verification. Three-dimensional full-wave electromagnetic field simulations and measured transmission spectra for the metamaterials were observed. Second, the transmission spectra of photoresists with different thicknesses (0-40µm) on the metamaterials surface were simulated, and the transmission spectra of noroxin thin films of different concentrations on the metamaterial surface were measured. Finally, using a purchased standard sample of noroxin in ethanol (100µg/mL) as the mother liquor, test samples of different concentrations were prepared, and experiments and multivariate data analysis were carried out. The noroxin detection limit for the method presented in this paper was determined as 0.01µg/mL. Thus, the new metamaterial method based on terahertz spectroscopy designed in this study was shown to effectively detect low concentrations of noroxin, providing a theoretical and experimental basis for the rapid detection of quinolone antibiotic residues in food matrices.

Optimization of Microwave Wireless Power Transmission With Specific Absorption Rate Constraint for Human Safety

In this communication, we propose a new convex optimization algorithm for exciting the transmitting antennas of a microwave wireless power transmission (MPT) system that transfers maximum power under a specific absorption rate (SAR) constraint for human safety. The transformation of the initial NP-hard problem to a convex optimization problem is described in detail. We applied the proposed algorithm to several MPT scenarios with multiple transmitting antennas around and one receiver placed near a box-shaped phantom model. The channel response and the electric field response for the receiver and phantom required for the optimization process are obtained using full-wave electromagnetic simulations. The received power and the power transfer efficiency (PTE) of the proposed optimization (OPT) technique are compared with those of the time-reversal (TR) technique at 0.9 GHz. The results show that the OPT technique can transfer more power than the TR technique with a lower PTE within the SAR limit and that the proposed technique can be applied to various MPT scenarios.

Effects of Filling Material Roughness on Scattering from a Rectangular Groove

This study investigated the effects of filling material roughness on the H-polarized scattering signatures of a two-dimensional (2D) rectangular groove embedded on an infinite ground plane. Under the assumption of a weakly rough surface on the groove, simplifying suppositions can be used to estimate Green’s functions inside and outside the groove. Enforcing continuity in the tangential magnetic field on the rough surface enables the construction and resolution of a Fredholm’s integral equation with logarithmic singularity. The results were verified via two full-wave electromagnetic field simulations carried out using the finite element method and the method of moments. The findings showed that even a weakly rough surface, such as a polished exterior with weak residual roughness, can considerably affect and change 2D scattering patterns.

Modeling the Self-Capacitance of an Irregular Solenoid

Irregular solenoids, i.e., sparse solenoids, varied-pitch solenoids, and noncylindrical solenoids, has started to appear recently for specific applications, and an accurate and fast modeling of their self-capacitance is necessary to accelerate the design and optimization process of such coils. Conventional capacitance modeling methods only work for coils, which are standard tightly wound cylindrical solenoids, but fail for irregular solenoids. Full-wave electromagnetic (EM) simulators provide good accuracy, but the computation time is usually long. In this article, a fast and accurate model to calculate the self-capacitance of an irregular solenoid is proposed. A modified turn-to-turn capacitance model, which considers the capacitance in the insulation layer of a wire as well as the air capacitance between the nearest neighbouring loops and the second nearest neighbouring loops is adopted. It works for solenoids of different types (e.g., sparse, varied-pitched, and noncylindrical ones). The accuracy of the proposed model is validated by comparing the calculation results to both those obtained by using commercial full-wave simulators and experimental results. For sparse solenoids, the proposed model is always accurate for coils with either small or large pitches. For varied-pitch solenoid and noncylindrical solenoid, the error rate of the proposed model is around 10%. Computational time wise, the computation time of the proposed model is reduced by over $10^4$ times compared to that of a commercial EM simulator whereas the accuracy is comparable.

High power characterisation of the ECRH transmission lines and power deposition calculations in the TJ-II stellarator

Abstract In the TJ-II stellarator, the last mirror of both ECRH transmission lines is an in-vessel steerable phase correcting mirror that distorts the power distribution of the reflected beam whenever the mirror orientation angles deviate from the design position. Beam distortions cannot be measured due to space restrictions when accessing the inner part of the device, particularly if we have in mind the wide range of orientation angles needed to carry out ECRH and ECCD experiments, for which such experimental characterisation should be performed. Instead, a set of full wave electromagnetic simulations allows us to estimate the power distribution of the beam coupled to the plasma and how this distribution changes when the in-vessel mirror is steered. High power characterisation of the transmission lines is first accomplished and documented in order to determine the beam power distribution before entering the vessel. For the perpendicular injection case, the additional contribution to the beam distortion produced by its passage through the structure supporting the window is calculated. Leaving aside the power losses due to beam trimming (7% and 10% for the first and second line respectively), the added distortion produced by the window housing is of minor importance compared to the effect of the internal mirror. The impact of beam distortion on the width of the power deposition profiles for several launching directions is addressed using the ray/beam tracing code TRUBA. Results show that the beam waist in the plasma appears smaller and ahead of the position predicted by quasi optical theory, leading to beam widening in the magnetic axis where the resonant layer is located. Despite the evident distortion, power deposition profiles calculated with beam tracing, refined with a Gabor expansion of the wave field distribution in case of strongly oblique incidence, do not differ much from the predictions of the standard ray tracing calculation.

Dyakonov-like waveguide modes in an interfacial strip waveguide

We study Dyakonov surface waveguide modes in a waveguide represented by an interface of two anisotropic media confined between two air half-spaces. We analyze such modes in terms of perturbation theory in the approximation of weak anisotropy. We show that in contrast to conventional Dyakonov surface waves that decay monotonically with distance from the interface, Dyakonov waveguide modes can have local maxima of the field intensity away from the interface. We confirm our analytical results by comparing them with full-wave electromagnetic simulations. We believe that this work can bring new ideas in the research of Dyakonov surface waves.