Full-wave Model Research Articles

EM-simulation-driven design optimization of compact microwave structures using multi-fidelity simulation models and adjoint sensitivities

Reliable design of miniaturized microwave structures requires utilization of full-wave electromagnetic EM simulation models because other types of representations such as analytical or equivalent circuit models are of insufficient accuracy. This is primarily due to considerable cross-coupling effects in tightly arranged layouts of compact circuits. Unfortunately, high computational cost of accurate EM analysis makes the dimension adjustment process challenging, particularly for traditional methods based on parameter sweeps, but also for conventional numerical optimization techniques. In this article, low-cost simulation-driven designs of compact structures were demonstrated using gradient search with adjoint sensitivities as well as multi-fidelity EM simulation models. The optimization process was arranged sequentially, with the largest steps taken at the level of coarse-discretization models. Subsequent fine tuning was realized with the models of higher fidelity. Switching between the models was realized by means of adaptively controlled termination conditions. This allowed for considerable reduction of the design cost compared with single-level optimization. The approach was illustrated using a compact microstrip rat-race coupler with two cases considered, that is, i bandwidth enhancement, and ii minimization of the structure size. In both cases, the optimization cost corresponded to a few high-fidelity EM simulations of the coupler structure. © 2016 Wiley Periodicals, Inc. Int J RF and Microwave CAE 26:442-448, 2016.

Reduced-Order Model Accounting for High-Frequency Effects in Power Electronic Components

This paper proposes a reduced-order (RO) model of power electronic components based on the proper orthogonal decomposition. Starting from a full-wave finite-element model and several snapshots (solutions at different frequencies), the RO model is constructed. Local field values (e.g., magnetic flux density, electric current density, magnetic field, or electric field) and global quantities (e.g., characteristic complex impedance and Joule losses) can be determined for the intermediate frequencies with a very low computational cost and high accuracy. Particular attention is paid to the choice of the most suitable snapshots by means of three different greedy algorithms, the performance of which is compared. We adopt an automatic greedy algorithm that depends only on the RO model.

A Frequency Tunable High Sensitivity H-Field Probe Using Varactor Diodes and Parasitic Inductance

A frequency-tunable resonant magnetic field probe is designed for near-field scanning applications for the radio frequency interference studies. Tunable resonance is achieved by using a varactor diode providing the required capacitance and the parasitic inductance of a magnetic loop (i.e., a parallel LC circuit). An equivalent circuit model for the probe is described, analyzed, and used for designing the probe for achieving maximum sensitivity. The resonance frequency of the designed probe is tunable in the frequency range of 900–2260 MHz that covers multiple radio bands, such as the GSM900, UMTS, and GPS bands. The sensitivity of the probe at the resonance frequency is about 7–9 dB higher than that of an equivalently sized broadband magnetic field probe throughout the tunable frequency range. The measured frequency response and sensitivity over a microstrip trace using the fabricated probe shows good agreement with the simulated results of the equivalent circuit model and the full-wave simulation model.

Full-Wave Computational Model of Electromagnetic Scattering by Arbitrarily Rotated 1-D Periodic Multilayer Structure

A full-wave computational model of electromagnetic scattering of conically incident plane waves by arbitrarily rotated 1-D periodic multilayer structure is proposed. Each layer in the structure (overall sandwiched between two half-spaces) consists of a dielectric material within which an infinite chain of periodically arranged and arbitrarily orientated circular cylinders is embedded. The orientations of the cylinder arrays in different layers are arbitrary with respect to one another, offering full generality. The above is a model of fiber-reinforced composite laminates as in industry but also of multilayered photonic crystals. Combining Rayleigh’s method and mode-matching produces scattering matrices for each layer cascaded from the top to the bottom in order to relate the reflection/transmission coefficients to the incident field. Power reflection and transmission coefficients follow from Poynting’s theorem. The mathematical derivation insists on modal representations and building up proper S-matrices. Numerical simulations show that the model is accurate, known results on woodpile structures in photonics are in particular recovered. The model is also versatile in terms of illuminations, geometries, and material parameters. The results could also be used as benchmarks in view of lack of data in present-day literature on such arbitrarily orientated layerings. Calculations of pertinent dyadic Green functions is one of possible extensions in view of the imaging methods of possibly damaged multilayer structures.

A Layered Vegetation Model for GPR Full-Wave Inversion

The vegetation cover strongly affects microwave remote sensing backscattered signals and makes difficulties in retrieving the surface soil water content (SWC). In this paper, we developed a vegetation cover model for full-wave inversion of ground-penetrating radar (GPR) data assuming multiple layers with effective electrical properties. Measurements were performed in laboratory conditions over spelt wheat grown in a sandy soil. A stepped-frequency continuous-wave (SFCW) GPR system was set up using a vector network analyzer and a horn antenna operating in 1–4 GHz range in far-field conditions with different incidence angles and both VV and HH polarizations. In order to control the sand surface water content, a water table was fixed at 20 cm depth and 6 time-domain reflectometry (TDR) probes were horizontally placed at 2 and 20 cm depth. Measurements were performed and recorded hourly for about 1 month and the canopy height was measured daily. The effective electromagnetic properties of the vegetation layer were modeled using a complex refractive index model (CRIM) combined with a single Debye relaxation model. A scattering loss parameter was included in the electromagnetic full-wave GPR model for the vegetation layer. The proposed method was also applied in a $\sim 7$ ha agricultural field with a vegetation canopy height of $\sim 27$ cm (spring wheat). The laboratory measurements resulted in a strong link between the vegetation scattering losses parameter and canopy height (generally, the biomass structure), which cannot be retrieved directly from the GPR measurements. Applying this vegetation model to the GPR full-wave inversion led to a significant correction on GPR-derived SWC values from the field measurements.

Homogenization of composites using full-wave point-dipole model

We propose a full-wave point-dipole-based scheme for accurate material homogenization of composites. Unlike conventional homogenization approaches which employ periodic boundary conditions for the effective permittivity extraction, the proposed technique can efficiently compute the interactions between the scatterers even in inhomogeneous mixtures as well as aperiodic arrangement of a finite number of scatterers. The method utilizes each inclusion as a basis function, and thereby sidesteps the computational burden to discretize each inclusion scatterer. In addition, the supplementary analytical calculations provide closed-form expressions to deal with the field singularities and to calculate the mutual interaction between the scatterers. We demonstrate that the proposed homogenization scheme can precisely extract the composite permittivity profile of a finite size specimen.

Particle simulations of mode conversion between slow mode and fast mode in lower hybrid range of frequencies

The propagation and mode conversion of lower hybrid waves in an inhomogeneous plasma are investigated by using the nonlinear δf algorithm in a two-dimensional particle-in-cell simulation code based on the gyrokinetic electron and fully kinetic ion (GeFi) scheme [Lin et al., Plasma Phys. Controlled Fusion 47, 657 (2005)]. The characteristics of the simulated waves, such as wavelength, frequency, phase, and group velocities, agree well with the linear theoretical analysis. It is shown that a significant reflection component emerges in the conversion process between the slow mode and the fast mode when the scale length of the density variation is comparable to the local wavelength. The dependences of the reflection coefficient on the scale length of the density variation are compared with the results based on the linear full wave model for cold plasmas. It is indicated that the mode conversion for the waves with a frequency of 2.45 GHz (ω ∼ 3ωLH, where ωLH represents the lower hybrid resonance) and within Tokamak relevant amplitudes can be well described in the linear scheme. As the frequency decreases, the modification due to the nonlinear term becomes important. For the low-frequency waves (ω ∼ 1.3ωLH), the generations of the high harmonic modes and sidebands through nonlinear mode-mode coupling provide new power channels and thus could reduce the reflection significantly.

Validation Limits of Quasi-TEM Approximation for Buried Bare and Insulated Cables

For the analysis of power system transients in overhead transmission lines and underground cables, a common assumption is related to the quasi-TEM (transverse electromagnetic) approximation. In this paper, we investigate the applicability of this approximation throughout a wide frequency band when both conduction and displacement currents in ground are considered, typically only the former is considered. A full-wave model is established to assess the behavior of the propagation constant for bare and insulated underground cables in a wide frequency band and verify the adequacy of the quasi-TEM approximation. The results demonstrate that it is possible to retain quasi-TEM approximations in the high-frequency range as well. One has only to consider different assumptions for the propagation constant. For the evaluation the adequacy of the quasi-TEM approximation, a simple, single-phase buried cable is considered. The effect of the cable coating is also taken into account and the results indicate that the quasi-TEM approximation is rather distinct for bare or insulated cables. The accuracy of the approximation is evaluated from comparison of step and impulse voltage tests considering different soil models.

Ray Tracing for Simulation of Millimeter-Wave Whole Body Imaging Systems

A ray tracing algorithm for modeling millimeter waves in a whole body imaging system is presented. Ray tracing is a well-known method for approximating high-frequency wave behavior and is well suited for implementation on graphics processing units (GPUs), presenting computational speed advantages over conventional full-wave modeling techniques. This method leverages the NVIDIA OptiX engine to ensure computational efficiency. Numerical results in this work are compared with conventional two-dimensional method of moments solutions to assess accuracy and computational times are compared with a three-dimensional GPU implementation of the modified equivalent current approximation.

Comparison of analytical and numerical approaches for CT-based aberration correction in transcranial passive acoustic imaging

Computed tomography (CT)-based aberration corrections are employed in transcranial ultrasound both for therapy and imaging. In this study, analytical and numerical approaches for calculating aberration corrections based on CT data were compared, with a particular focus on their application to transcranial passive imaging. Two models were investigated: a three-dimensional full-wave numerical model (Connor and Hynynen 2004 IEEE Trans. Biomed. Eng. 51 1693–706) based on the Westervelt equation, and an analytical method (Clement and Hynynen 2002 Ultrasound Med. Biol. 28 617–24) similar to that currently employed by commercial brain therapy systems. Trans-skull time delay corrections calculated from each model were applied to data acquired by a sparse hemispherical (30 cm diameter) receiver array (128 piezoceramic discs: 2.5 mm diameter, 612 kHz center frequency) passively listening through ex vivo human skullcaps (n = 4) to emissions from a narrow-band, fixed source emitter (1 mm diameter, 516 kHz center frequency). Measurements were taken at various locations within the cranial cavity by moving the source around the field using a three-axis positioning system. Images generated through passive beamforming using CT-based skull corrections were compared with those obtained through an invasive source-based approach, as well as images formed without skull corrections, using the main lobe volume, positional shift, peak sidelobe ratio, and image signal-to-noise ratio as metrics for image quality. For each CT-based model, corrections achieved by allowing for heterogeneous skull acoustical parameters in simulation outperformed the corresponding case where homogeneous parameters were assumed. Of the CT-based methods investigated, the full-wave model provided the best imaging results at the cost of computational complexity. These results highlight the importance of accurately modeling trans-skull propagation when calculating CT-based aberration corrections. Although presented in an imaging context, our results may also be applicable to the problem of transmit focusing through the skull.

Accurate One-Sided Microwave Thickness Evaluation of Lined-Fiberglass Composites

Fiberglass is increasingly used for corrosion-resistant industrial applications. Fiberglass laminate typically consists of a structural layer and a corrosion-resistant layer (commonly referred to as the liner) on the inside of the laminate. Liners are used in all corrosion-resistant fiberglass applications to guard against corrosive chemical attack on the structural layers of the fiberglass. The ability to measure fiberglass thickness from one side of a laminate during equipment operation quickly and easily with high measurement accuracy is extremely important for life assessment and prediction. Microwave nondestructive techniques employing open-ended rectangular waveguides in conjunction with robust and full-wave electromagnetic models have tremendous potential and viability for evaluating complex layered dielectric composite structures for the purpose mentioned here. In this paper, we describe the foundation of this technique, processes involved in optimizing the approach for layered fiberglass evaluation, as well as a number of representative and diverse measurement results demonstrating the efficacy of the technique and the thickness accuracy that can be obtained. In addition, other important issues such as the effect of curvature of the structure and water with different temperatures filling it were also addressed and shown not to adversely affect measurement accuracy associated with liner thickness.

On anelliptic approximations for qP velocities in transversely isotropic and orthorhombic media

Anelliptic approximations for phase and group velocities of qP waves in transversely isotropic (TI) media have been widely applied in various seismic data processing and imaging tasks. We have revisited previously proposed approximations and suggested two improvements. The first improvement involves finding an empirical connection between anelliptic parameters along different fitting axes based on laboratory measurements of anisotropy of rock samples of different types. The relationship between anelliptic parameters observed was strongly linear suggesting a novel set of anisotropic parameters suitable for the study of qP-wave signatures. The second improvement involves suggesting a new functional form for the anelliptic parameter term to achieve better fitting along the horizontal axis. These modifications led to improved three-parameter and four-parameter approximations for phase and group velocities of qP-waves in TI media. In several model comparisons, the new three-parameter approximations appeared to be more accurate than previous approximations with the same number of parameters. These modifications also served as a foundation for an extension to orthorhombic media, where qP velocities involved nine independent elastic parameters. As determined by previous researchers, qP-wave propagation in orthorhombic media could be adequately approximated using just six combinations of those nine parameters. We have developed novel six-parameter approximations for phase and group velocities for qP-waves in orthorhombic media. The orthorhombic phase-velocity approximation provides a more accurate alternative to previously known approximations and can find applications in full-wave modeling, imaging, and inversion. The group-velocity approximation is also highly accurate and can find applications in ray tracing and velocity analysis.

Computed tomography-based aberration correction for trans-skull acoustic focusing: Comparison of simulations and measurements with focused ultrasound brain systems

Clinical focused ultrasound brain systems currently employ computed tomography (CT)-based phase and amplitude corrections to mitigate skull-induced distortions and restore an acoustic focus at the intended target. In this study, acoustic measurements were conducted using two transcranial magnetic resonance-guided focused ultrasound systems (ExAblate 4000, InSightec, Haifa, Israel) operating at 230 and 650 kHz. The acoustic fields generated by the devices within intact, water-filled ex-vivo human skulls near the geometric focus were mapped using a 0.5 mm diameter needle hydrophone. In addition, the signals transmitted from each individual array element were captured at various locations within the skull cavity. These measurements were repeated without the presence of the skull in order to determine the element-specific aberrations induced by the cranial bone for each target position. The experimental measurements were simulated using three previously developed transcranial ultrasound propagation models: an analytical method similar to that currently employed by clinical brain systems, a multi-layered ray-acoustic approach, and a three-dimensional full-wave propagation model based on the Westervelt equation. We will present a comparison of the models based on their computational complexity as well as their ability to both predict the phase and amplitude aberrations induced by the skull and reproduce the measured in-situ pressure field distributions.

Full-wave modeling of THz RTD-gated GaN HEMTs

Modeling transistors at terahertz frequencies is challenging, because electromagnetic and quantum effects that are negligible in lower frequencies become limiting factors in device performance. Though previous work has focused on modeling the channel of a high-electron mobility transistor (HEMT) using hydrodynamic equations, a more complete toolset is needed to describe submillimeter-wave device gain performance. This paper introduces a simulator that couples full-wave Maxwell’s equations with Schrodinger-based charge transport equations, and is used to evaluate the gain performance of a GaN HEMT at THz. This novel simulator is also used to evaluate the effect on gain when a resonant tunneling diode (RTD) is integrated with a HEMT. Upon validation with published work, we state the feasibility of RTD-gated GaN HEMT structures that have resonances up to 2.3THz and gain up to 6dB.

Efficient Design of Waveguide Manifold Multiplexers Based on Low-Order EM Distributed Models

In this paper, a new systematic technique to design manifold-coupled multiplexers in waveguide technology is proposed. The new technique uses generalized low-order electromagnetic (EM) distributed models, which constitute a half-way point between the fast, but imprecise, analytical models, and the more accurate, but costly, full-wave EM models. The method can be applied to contiguous and noncontiguous channel multiplexers, in both $ { E}$ -plane or $ { H}$ -plane configurations. This paper covers the complete design procedure for manifold multiplexers, starting from the required specifications and finishing with the physical dimensions. After explaining the general design technique for multiplexers with channel filters based on transmission-line resonators, the method is specialized for the design of classic multiplexers with circular-waveguide dual-mode filters. As a further application example, the same methodology is also used for the design of a nonconventional tuningless wideband multiplexer with a new type of rectangular waveguide filters. This paper is concluded with a comparison between simulation and measurements for the wideband multiplexer. Excellent agreement between measurements and simulations are demonstrated, thereby validating the complete design procedure.

Method of Effective Roughness Dielectric in a PCB: Measurement and Full-Wave Simulation Verification

Surface roughness topography of printed circuit boards (PCBs) needs to be included in signal integrity simulations in order to accurately predict the insertion loss of the structure and its delay time. An effective roughness dielectric (ERD) model can be used to substitute an inhomogeneous interface between copper foil and laminate dielectric in a PCB. Herein, this approach is tested for verification using 3-D full-wave numerical simulations. These ERD layers with the appropriate complex permittivity are included in the modeling of stripline examples. The parameters of an ambient laminate dielectric refined from conductor roughness in the stripline are determined using differential extrapolation roughness measurement technique. The agreement of the results of 3-D full-wave modeling simulations and measurements on multiple test structures justifies the proposed approach. Based on the extracted ERD parameters “design curves” can be built and used in numerical simulations of PCB high-speed designs.

Recursive matrix schemes for composite laminates under plane-wave and Gaussian beam illumination

A full-wave computational model of electromagnetic scattering by composite laminates made of dielectric panels reinforced by periodically arranged circular cylindrical fibers parallel to one another is proposed. Incident plane waves, with plane of incidence possibly differing from the plane orthogonal to the fibers’ axes, and Gaussian beams are assumed. Approaches based on transfer matrices and scattering matrices are developed in parallel, the latter being shown more stable in the present study as expected. In practice, Rayleigh’s and mode-matching methods yield scattering matrices that are cascaded between layers to provide reflection and transmission coefficients, producing power reflection and transmission coefficients from Poynting’s theorem. Typical numerical results, with emphasis on graphite-fiber and glass-fiber laminates, are proposed, completed by results about photonic crystals.

Numerical Issues in Line Models Based on a Thin Wire Above a Lossy Ground

This paper investigates the causes of numerical instabilities that might occur in transmission line models if the so-called image approximations are used for the evaluation of the per-unit-length parameters when the ground is assumed lossy and a wide frequency range is considered. To assess whether these instabilities are inherent to the formulation of the line parameters, results were compared with two well-established models: a full-wave model and a quasi-transverse electromagnetic approximation. Both approaches provided stable models throughout the whole frequency range. Two different methodologies are used for the determination of the modal propagation constant in a full-wave model leading to similar results. Time-domain responses based on the numerical Laplace transform and the method of characteristics were used to verify the accuracy and stability of the tested models. The input voltage considered for the time-domain responses included a small perturbation to excite the passivity violations found in the modeling using image approximations. Some aspects related to a full-wave model based on the method of characteristics using rational fitting are also presented.

Lower thermospheric response to atmospheric gravity waves induced by the 2011 Tohoku tsunami

AbstractPrevious GPS observations have revealed that while ionospheric TIDs were seen propagating in all directions away from the 2011 Tohoku earthquake epicenter, the total electron content (TEC) fluctuations associated with the subsequent tsunami were largest for waves propagating toward the northwest of the epicenter. Ionospheric motions observed approximately 10 min after the earthquake were attributed to fast acoustic waves directly produced by the earthquake. Waves that first appeared about 40 min after the tsunami onset in TEC measurements were attributed to atmospheric gravity waves. In this paper, we conjecture that the remarkably different responses observed for the eastward and westward propagating waves noted in previous observations can be explained by the different ocean depths associated with the two directions of travel and by the effects of the mean winds. The former has consequences for the generated gravity waves (wave spectrum), while their combination has consequences for the ability of the waves to propagate to higher altitudes. Because the ocean depth to the east of the epicenter is greater than that to the west, the eastward propagating tsunami travels faster than the westward propagating tsunami; and hence, the eastward propagating gravity waves that are generated will be faster than the westward waves. We demonstrate that the faster eastward waves encounter regions of evanescence that inhibits their upward propagation, with the result that the westward propagating waves reach the lower thermosphere sooner and with much larger amplitudes than those of the eastward propagating waves. However, at much higher altitudes the slower westward propagating waves are severely damped by viscosity, with the result that only the eastward propagating waves survive to F region altitudes. These results are clearly seen in our full‐wave model simulations and also in the evolution of the wave momentum flux calculated using our 2‐D, time‐dependent model.

The stratospheric arrival pair in infrasound propagation

The ideal case of a deep and well-formed stratospheric duct for long range infrasound propagation in the absence of tropospheric ducting is considered. A canonical form, that of a pair of arrivals, for ground returns of impulsive signals in a stratospheric duct is determined. The canonical form is derived from the geometrical acoustics approximation, and is validated and extended through full wave modeling. The full caustic structure of the field of ray paths is found and used to determine phase relations between the contributions to the wavetrain from different propagation paths. Finally, comparison with data collected from the 2005 fuel gas depot explosion in Buncefield, England is made. The correspondence between the theoretical results and the observations is shown to be quite good.