Nada J. Sekeljic

Optimal Modeling Parameters for Higher Order MoM-SIE and FEM-MoM Electromagnetic Simulations

General guidelines and quantitative recipes for adoptions of optimal higher order parameters for computational electromagnetics (CEM) modeling using the method of moments and the finite element method are established and validated, based on an exhaustive series of numerical experiments and comprehensive case studies on higher order hierarchical CEM models of metallic and dielectric scatterers. The modeling parameters considered are: electrical dimensions of elements (h -refinement), polynomial orders of basis functions (p-refinement), orders of Gauss-Legendre integration formulas (integration accuracy), and geometrical (curvature) orders of elements in the model. The goal of the study, which is the first such study of higher order parameters in CEM, is to reduce the dilemmas and uncertainties associated with the great modeling flexibility of higher order elements, basis and testing functions, and integration procedures (this flexibility is the principal advantage but also the greatest shortcoming of the higher order CEM), and to ease and facilitate the decisions to be made on how to actually use them, by both CEM developers and practitioners.

Efficient and Accurate Computational Electromagnetics Approach to Precipitation Particle Scattering Analysis Based on Higher-Order Method of Moments Integral Equation Modeling

AbstractA new full-wave computational electromagnetics (CEM) approach to precipitation particle scattering analysis based primarily on a higher-order method of moments (MoM) for solving surface integral equations (SIEs) is proposed, as an alternative and addition to the conventionally used tools in this area. This is a well-established CEM approach that has not been applied, evaluated, discussed, or compared with other approaches in the scattering analysis of precipitation particles so far. Several characteristic examples of scattering from precipitation particles of various shapes demonstrate the capabilities and potential of the presented numerical methodology, and discuss its advantages over both discrete dipole approximation (DDA) and -matrix methods in cases considered. In particular, it is shown that the higher-order MoM-SIE approach is much faster, more accurate, and more robust than the DDA method, and much more general and versatile than the -matrix method. In addition, the paper illustrates prob...

Investigating raindrop shapes, oscillation modes, and implications for radio wave propagation

Studies of raindrop shapes, oscillation modes, and implications for radio wave propagation are presented. Drop shape measurements in natural rain using 2-D video disdrometers (2DVDs) are discussed. As a representative exception to vast majority of the cases where the “most probable” shapes conform to the axisymmetric (2,0) oscillation mode, an event with a highly organized line convection embedded within a larger rain system is studied. Measurements using two collocated 2DVD instruments and a C-band polarimetric radar clearly show the occurrence of mixed-mode drop oscillations within the line, which in turn is attributed to sustained drop collisions. Moreover, the fraction of asymmetric drops determined from the 2DVD camera data increases with the calculated collision probability when examined as time series. Recent wind-tunnel experiments of drop collisions are also discussed. They show mixed-mode oscillations, with (2,1) and (2,2) modes dramatically increasing in oscillation amplitudes, in addition to the (2,0) mode, immediately upon collision. The damping time constant of the perturbation caused by the collision is comparable to the inverse of the collision frequency within the line convection. Scattering calculations using an advanced method of moments numerical technique are performed to accurately and efficiently determine the pertinent parameters of electrically large oscillating raindrops with asymmetric shapes needed for radio wave propagation. The simulations show that the scattering matrix and differential reflectivity of drops are dependent on the particular oscillation modes and different time instants within the oscillation cycle. The technique can be utilized in conjunction with 3-D reconstruction of drop shapes from 2DVD data.

Higher Order Time-Domain Finite-Element Method for Microwave Device Modeling With Generalized Hexahedral Elements

A novel higher order and large-domain Galerkin-type finite-element method (FEM) is proposed for direct 3-D electromagnetic modeling in the time domain. The method is implemented in the time-domain finite-element method (TDFEM) analysis of multiport microwave waveguide devices with arbitrary metallic and dielectric discontinuities. It is based on the geometrical modeling using Lagrange-type interpolation generalized hexahedra of arbitrary geometrical-mapping orders, field expansion in terms of hierarchical curl-conforming 3-D polynomial vector basis functions of arbitrarily high field-approximation orders, time stepping with an implicit unconditionally stable finite-difference scheme invoking the Newmark-beta method, and mesh truncation introducing the waveguide port boundary condition. Numerical examples demonstrate excellent accuracy, efficiency, stability, convergence, and versatility of the presented method, and very effective large-domain TDFEM models of 3-D waveguide discontinuities using minimal numbers of large conformal finite elements and minimal numbers of unknowns. The results obtained by the higher order TDFEM are in an excellent agreement with indirect solutions obtained from the FEM analysis in the frequency domain (FD) in conjunction with the discrete Fourier transform and its inverse, as well as with measurements and with alternative full-wave numerical solutions in both time and FDs.

Electromagnetic scattering by oscillating rain drops of asymmetric shapes

Computational electromagnetic analysis of scatting by oscillating rain drops with asymmetric shapes is presented. Mixed-mode oscillations of drops are attributed to sustained drop collisions in events having a highly organized line convection embedded within a larger rain system. The scattering matrix and differential reflectivity of drops are dependent on the particular oscillation modes and different time instants within the oscillation cycle. The results also demonstrate the superiority of the higher order method of moments over the conventional discrete dipole approximation method in oscillating rain drop analysis.

RF excitation in 7 tesla MRI systems using monofilar axial-mode helical antenna

We present a novel RF exciter for traveling-wave magnetic resonance imaging (MRI) systems based on a monofilar axial-mode helical antenna. In specific, we present a bore-extended, subject-loaded monofilar helical antenna for 7-T (ultra-high magnetic field) MRI systems. By means of rigorous full-wave electromagnetic modeling, we confirm clear advantages of the novel exciter over the existing TW excitation methods, and its excellent performance in terms of circular polarization and uniformity of the RF magnetic field in simple phantoms inside MRI bores at 7 T.

Atmospheric particle scattering computation using higher order MoM-SIE method

Full-wave computational electromagnetics (CEM) approach based on the method of moments (MoM) for solving surface integral equations (SIEs) is proposed for atmospheric particle scattering analysis. The methodology is implemented as a higher order CEM technique. The paper demonstrates that the higher order MoM-SIE approach is much more efficient, accurate, general, and robust than the conventionally and almost exclusively used tools in atmospheric scattering analysis, namely, the T-matrix method and the discrete dipole approximation (DDA) method.

Spatially Large-Domain and Temporally Entire-Domain Electric-Field Integral Equation Method of Moments for 3-D Scattering Analysis in Time Domain

A novel spatially large-domain and temporally entire-domain method of moments (MoM) is proposed for surface integral equation (SIE) modeling of 3-D conducting scatterers in the time domain (TD). The method uses higher order curved Lagrange interpolation-generalized quadrilateral geometrical elements, higher order spatial current expansions based on hierarchical divergence-conforming polynomial vector basis functions, and temporal current modeling by means of orthogonal weighted associated Laguerre basis functions. It implements full temporal and spatial Galerkin testing and marching-on-in-degree (MOD) scheme for an iterative solution of the final system of spatially and temporally discretized MoM-TD equations. Numerical examples demonstrate excellent accuracy, efficiency, convergence, and versatility of the new MoM-MOD method. The results also demonstrate very effective large-domain MoM-TD SIE models of scatterers using flat and curved patches of electrical sizes of up to about 1.7 wavelengths at the maximum frequency in the frequency spectrum of the pulse excitation, higher order current expansions of spatial orders from 2 to 8 in conjunction with entire-domain Laguerre temporal bases, and minimal numbers of unknowns.

Improving traveling-wave RF fields inside magnetic resonance imaging bores by incorporating dielectric loadings

The next-generation magnetic resonance imaging (MRI) systems at ultra-high static magnetic fields (magnetic flux densities), B 0 > 3 T, and ultra-high Larmor frequencies, ƒ 0 > 127.8 MHz, utilize RF excitation magnetic fields, B 1 , in the form of traveling waves (TWs) in the MRI bore. Hence, the images of subjects are generated and received by far-field coils, namely, by excitation probes that essentially operate as antennas, in place of the traditional quasi-static, near-field RF coils used in 3-T clinical MRI scanners (e.g., birdcage coils). When compared to traditional, quasi-static, MRI systems, TW MRI systems can provide more homogeneous B 1 field distribution, better signal-to-noise ratio, larger field of view, more comfort for patients, etc. Moreover, it is possible to potentially benefit from the advantages of TW concepts also at relatively lower (but still considered high) field strengths (e.g., B 0 = 3 T; ƒ 0 = 127.8 MHz), in order to address challenges and enable substantial improvements of current clinical MRI scanners at 3 T.

Full-wave frequency-domain electromagnetic modelling of RF fields in MRI applications

We show that full-wave frequency-domain electromagnetic (EM) methods can be efficiently used in RF designs for magnetic resonance imaging (MRI) applications. We also compare the results obtained by the higher order surface integral equation (SIE)-based method of moments (MoM) and the finite element method (FEM), namely, HFSS, which are in excellent agreement. Finally, we present important details of MoM-SIE and FEM (HFSS) modeling, which enable rigorous analyses and cross-validation of the solutions.