Christos S. Antonopoulos

Optimal Modeling of Infinite Graphene Sheets via a Class of Generalized FDTD Schemes

The accurate and fully 3-D analysis of graphene surface conductivity models by means of a frequency-dependent finite-difference time-domain method is introduced in this paper. For the infinite sheet to be consistently simulated, the novel technique uses a set of periodic boundary conditions that lead to a unit cell excited with a spectral scheme in terms of a total-field/scattered-field formulation. On the other hand, graphene itself is modeled through a subcell approach and a complex surface conductivity concept defined by quantum mechanical equations. This conductivity model is next converted to a volume one in order to permit a realistic time-domain study. Numerical outcomes, addressing a variety of applications, reveal a promising coincidence with those acquired from analytical closed-form expressions.

Design and Optimization of Uniplanar EBG Structures for Low Profile Antenna Applications and Mutual Coupling Reduction

We present a series of designs for uniplanar electromagnetic bandgap structures for antennas and microwave circuits. The proposed, easy to fabricate configurations, are based solely on metallic surfaces on layer interfaces, without the use of vias or other kinds of vertical connections. Their use in low profile antenna applications and mutual coupling reduction of planar radiating elements is investigated through an efficient and versatile technique, since the only information needed is the reflection phase and the dispersion diagram of the unit cell.

Computational Investigation and Design of Planar EBG Structures for Coupling Reduction in Antenna Applications

The uniplanar cross-like compact EBG structure is thoroughly assessed by means of a computational FEM-based eigenvalue analysis. A careful inspection of the dispersion diagram reveals the existence of a nearly-TEM mode that compromises the bandgap behavior, a fact that is also confirmed in coupling performance investigations. The structure is then modified and optimized to maximize the resulting bandgap zone, for mutual coupling reduction in WiMAX MIMO arrays.

A Family of Ultra-Thin, Polarization-Insensitive, Multi-Band, Highly Absorbing Metamaterial Structures

The systematic design of size-conflned, polarization- independent metamaterial absorbers that operate in the microwave regime is presented in this paper. The novel unit cell is additionally implemented to create e-cient multi-band and broadband structures by exploiting the scalability property of metamaterials. Numerical simulations along with experimental results from fabricated prototypes verify the highly absorptive performance of the devices, so developed. Moreover, a detailed qualitative and quantitative analysis is provided in order to attain a more intuitive and sound physical interpretation of the underlying absorption mechanism. The assets of the proposed concept, applied to the design of difierent patterns, appear to be potentially instructive for various EMI/EMC conflgurations.

A general form of perfectly matched layers for for three-dimensional problems of acoustic scattering in lossless and lossy fluid media

The concept of perfectly matched layer (PML) has been proven very effective in absorbing electromagnetic waves in lossless media. An extension of this method and a complete three-dimensional (3-D) scheme properly suited for finite-difference, time-domain (FDTD) modeling of acoustic propagation and scattering in unbounded problems are presented in this paper. This generalized PML is constructed in such a way that it performs significant absorption of traveling waves in acoustics for both lossless and lossy media. Theoretically, no reflections occur when propagating waves encounter the lossy medium-PML interface, no matter what the angle of incidence is, introducing at the same time the possibility for further wave attenuation via the stretched coordinates idea. Numerical results support the suggested PML theory as well as reveal the proper modifications, which lead to the achievement of the optimum absorbing-boundary condition.

A fully explicit Whitney element-time domain scheme with higher order vector finite elements for three-dimensional high frequency problems

In this paper we propose a new explicit Whitney Element-Time-Domain method for the solution of Maxwells equations in three dimensions. This scheme could be considered a generalization of the well known Finite Difference Time Domain (FDTD) method for unstructured meshes. However, an entirely different methodology is introduced, based on the use of higher order tangential and normal vector finite elements. The exploitation of higher order approximations in an elaborate leapfrog scheme permits the use of the primary mesh only, thus avoiding the implementation of dual mesh. The new method is, finally, applied to a typical high frequency electromagnetic field formulation.

Numerical modeling of an indoor wireless environment for the performance evaluation of WLAN systems

A site-specific numerical model, based on the finite-difference time-domain method, is developed in this paper for the indoor radio channel. The scenario of interest is concerned with wave propagation in a typical office environment, for which several simulations are performed considering different placements of the transmitting antenna. Both the 2- and 5-GHz bands are examined, where contemporary wireless local area networks operate. Important channel characteristics are evaluated via the estimation of received power levels, as well as the examination of small-scale fading and time dispersion

A finite difference time domain scheme for transient eddy current problems

In this paper, we present a general explicit finite difference time domain (FDTD) algorithm for transient electromagnetic problems at low frequencies. A detailed theoretical analysis is given, based on a nonstandard finite difference scheme, which is studied in terms of stability and consistency. The study reveals a whole class of finite difference diffusion schemes with different properties. Some of the existing methods are special cases of this general framework and an optimal algorithm is proposed. The application of finite difference methods to eddy current problems requires the introduction of a hybrid methodology, which combines the explicit differencing scheme for the diffusion equation with a boundary element method (BEM) for the open regions. The resulting algorithm involves a simple time stepping iteration, without any system solution, thus being remarkably robust.

A Wideband ADI-FDTD Algorithm for the Design of Double Negative Metamaterial-Based Waveguides and Antenna Substrates

An enhanced 3-D alternating-direction implicit finite-difference time-domain method for the systematic analysis of double negative metamaterial-based waveguide and antenna devices is presented in this paper. Being fully frequency-dependent, the new scheme introduces a set of generalized multidirectional operators which incorporate the suitable Lorentz-Drude model and subdue inherent lattice deficiencies for broad spectra. So dispersion errors, as time-steps exceed the Courant limit, are drastically minimized yielding fast and accurate solutions for propagating and evanescent waves. The technique is applied in the design of microwave structures, realized via the prior model or networks of thin wires and split-ring resonators. Numerical results certify its merits, without requiring elongated simulations and excessive overheads

Field calculation in single- and multilayer coaxial cylindrical shells of infinite length by using a coupled T- Omega and boundary element method

A method is presented for the calculation of the electromagnetic field in systems of single-layer or multilayer coaxial cylindrical shells of infinite length excited by an oscillating current source arbitrarily oriented inside the first shell. The electric vector potential T and the magnetic scalar potential Omega are used for the evaluation of the quantities of the problem. The Helmholtz equations for T and Omega are transformed into integral equations by the use of the Greens function method. Applying the boundary element method, three systems of simultaneous equations have to be solved to give the sought field quantity. >