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The finite-difference time-domain method is herein combined with polynomial-chaos expansions for the study of axially-symmetric structures featuring material uncertainties. By exploiting the problem's periodicity, we reduce the high computational burden of fully 3D simulations, and reliably extract the necessary statistical information from a single simulation.

A finite-difference time-domain (FDTD) method for simulating wave propagation in Havriliak-Negami (H-N) dispersive media is presented. In an H-N medium, the time-domain polarization relation involves a fractional differential operator, whose analytical evaluation is troublesome. Moreover, if the fractional differential operator is approximated by a sum of fractional time derivatives, the numerical treatment of the polarization relation results in significant memory storage demands, because fractional derivatives are not local. However, by an appropriate approximation of the Grunwald-Letnikov definition of fractional derivatives, an FDTD scheme with reasonable memory requirements is feasible. In particular, the coefficients that appear in the Grunwald-Letnikov definition are approximated by sums of decaying exponentials. As a result, the FDTD scheme is based on recursive equations, while a limited number of auxiliary vectors need to be stored. The proposed scheme has been applied successfully to the simulation of the excitation of an H-N medium by a wideband Gaussian electromagnetic pulse.

We present here a 3-D E-B linear eigenmode formulation for the analysis of periodic structures, including lossy periodic waveguides, photonic crystals, and metamaterials. The proposed finite-element formulation utilizes proper H (curl) vector basis functions for the electric field and H (div) vector basis functions for magnetic flux density. The resulting eigenmode problem is linear, and it enforces periodic boundary conditions in a simple manner via appropriate field transformations and does not exhibit spurious modes. Moreover, it can provide accurate complex-k dispersion diagrams, in terms of both propagation and attenuation constant, while it is also able to deal with either propagating or evanescent modes, including those with oblique propagation directions.

The consistent modeling of microwave components with random fluctuations regarding their electromagnetic material parameters is performed in this paper via a 3-D stochastic finite-difference time-domain method. The featured formulation provides precise numerical results with a considerably smaller computational cost compared to the extensively resource-consuming Monte Carlo approach. Numerical simulations of microstrip transmission lines with media uncertainties are conducted in order to verify the accuracy of the proposed technique along with its effectiveness to investigate the operation of complicated microwave structures in a stochastic environment.

The propagating modes on a magnetically biased curved graphene microstrip are thoroughly examined in the present work by means of an efficient finite element modal solver. The influence of the anisotropic surface conductivity due to the magnetic bias is carefully addressed and investigated through a multitude of numerical results, including the effective index and the electric field distribution of the dominant modes.

Aiming at the minimization of numerical dispersion effects in finite-difference time-domain (FDTD) simulations, we investigate the potential of introducing frequency-dependent coefficients in the approximations of differential operators. The proposed methodology exploits some previous results on optimized finite-differences, but initially avoids the necessity for selecting a specific optimization point. Herein, the operators' coefficient formulae are approximated by a low-order polynomial, which is transformed into a finite-difference counterpart using the Helmholtz equation. This modifies the initial spatial stencil and improves performance over all frequencies, which can be further controlled with the least-squares method, leading to various new FDTD schemes of augmented reliability.

In this paper a new technique for mutual coupling reduction of uniplanar MIMO antennas is presented. The only necessary information needed, is the transmission coefficient of a plane wave which is propagating over a specific structure, of a new proposed geometry.

In this paper, the design of novel, polarization-insensitive metamaterial absorbers that operate in the microwave regime is presented. The proposed unit cell comprises an electric ring resonator and a pair of crossed wires imprinted on the opposite faces of a dielectric substrate. The performance of the structure can be further enhanced by adding multiple layers of the optimized surface along the propagation direction, while keeping the dimensions of the device relatively small compared to the wavelength. Finally, unit cells designed for different resonant frequencies can be placed adjacent to each other in order to create multiple absorption bands. The results of the proposed concept appear to be very promising for various EMI/EMC applications.

The consistent and cost-effective modeling of large-scale EMC structures, is presented in this paper via a novel parametric hybrid method. The proposed scheme blends a stencil-optimized time-domain algorithm in curvilinear coordinates and a modified finite-element time-domain approach to divide the problem into smaller flexible regions. Constructing a class of curvilinear 3-D operators, the framework assigns weights to each spatial increment and approximates spatial derivatives through interpolating polynomials. An important asset is that the two techniques are updated independently and interconnected by versatile boundary conditions. Numerical results from different realistic EMC setups verify our method and reveal its universal applicability.

The determination of the field and losses inside a transmission line simulated by a long conducting and nonmagnetic cylinder is discussed. The excitation pulse current flows in coils surrounding the cylinder, with their length being small enough compared to the cylinder's length. A /spl pi/ shaped solenoid is further analysed, in order to have a more homogenised field inside the conducting cylinder. The results can be applied in eddy current nondestructive materials evaluation and in the examinations of the materials strength, under external influence for transmission technology purposes.

A spatial domain technique for the reconstruction of the electromagnetic properties of unknown scatterers is presented. The new methodology combines the finite element method (FEM) and the Polak-Ribiere nonlinear conjugate gradient optimization algorithm. The forward scattering problem is treated via the FEM, while inversion is implemented by minimizing a cost function, consisting of a standard error term and a regularization term. The first one is related to the scattered near-field measurements, obtained by illuminating the scatterer with plane waves from various directions. A sensitivity analysis, performed by an elaborate finite element procedure, provides the direction required for correcting the scatterer profile. Significant reduction of the computation time is obtained by introducing the adjoint state vector methodology.