Traianos V. Yioultsis

A Novel Adaptive Beamforming Technique Applied on Linear Antenna Arrays Using Adaptive Mutated Boolean PSO

The present work introduces a new optimization technique suitable for adaptive beamforming of linear antenna arrays. The proposed technique is a new PSO variant called Adaptive Mutated Boolean PSO (AMBPSO) where the update formulae are implemented exclusively in Boolean form by using an e-ciently adaptive mutation process. The AMBPSO aims at estimating the excitation weights applied on the array elements considering that a desired signal and several interference signals are received by the array at respective directions of arrival. In order to exhibit the robustness of the technique, the optimization process does not take into account the interference correlation matrix. A certain power level of additive Gaussian noise is also considered by the technique. The AMBPSO has been applied in several cases of uniform linear antenna arrays with difierent spacing between adjacent elements and difierent noise power level and therefore seems to be quite promising in the smart antenna technology.

Metamaterial-Based Electrically Small Multiband Planar Monopole Antennas

We present electrically small multiband planar antennas for wireless applications, enhanced by negative permeability metamaterial rings. The presence of the rings results in a significant reduction of the operation frequency. Two different feeding methods (microstrip and coplanar) are considered, resulting in efficient, entirely planar structures.

Theoretical analysis of thermally tunable microring resonator filters made of dielectric-loaded plasmonic waveguides

Microring resonator filters, which are made of dielectric-loaded surface plasmon polariton waveguides and operate in the telecom spectral range, are thoroughly analyzed by means of vectorial three dimensional (3D) finite element method (FEM) simulations. The filters’ functional characteristics, such as the resonant frequencies where the transmission minima occur, the free spectral range, the extinction ratio, and the minima linewidth associated with the quality factor of the resonances, are investigated for different values of the key structural parameters, namely, the ring radius and the gap separating the bus waveguide from the ring. The rigorous 3D-FEM simulations are qualitatively complemented by a simplified model. Apart from the harmonic propagation simulations, the uncoupled microring is treated as an eigenvalue problem, and the frequencies of the resonances are compared with those of the transmission minima. Furthermore, the possibility of exploiting the thermally tuned microring resonator filter ...

Inverse scattering using the finite-element method and a nonlinear optimization technique

A new spatial-domain technique for the reconstruction of the complex permittivity profile of unknown scatterers is proposed in this paper. The technique is based on a combination of the finite-element method (FEM) and the Polak-Ribiere nonlinear conjugate gradient optimization algorithm. The direct scattering problem is explicitly dealt with by means of the differential formulation and it is solved by applying the FEM. The inversion methodology is oriented to minimizing a cost function, which consists of a standard error term and regularization term. A sensitivity analysis, which is carried out by an elaborate finite-element procedure, results in the determination of the direction required for correcting the profile. Significant reduction of the computation time is obtained by introducing the adjoint state vector methodology. The efficiency of the presented inversion technique is validated by applying it to the inversion of synthetic scattered far-field measurements, which are corrupted by additive noise.

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.

On the Direct Evaluation of Weakly Singular Integrals in Galerkin Mixed Potential Integral Equation Formulations

Weakly singular integrals over coincident triangles, arising in the Galerkin discretization of mixed potential integral equation formulations, are calculated using a direct evaluation method. The proposed method utilizes a series of coordinate transformations, together with a re-ordering of the integrations, in order to reduce the dimensionality of the original four-dimensional (4D) weakly singular integrals into 1D numerical integrations of smooth functions. The final formulas can be easily evaluated with a standard Gaussian quadrature rule, resulting in a scheme with great accuracy and efficiency properties. Numerical results for the comparison of the proposed method with both singularity subtraction and singularity cancellation methods, often used for the evaluation of multidimensional singular integrals, are presented, indicating the superior overall performance of the direct evaluation scheme.

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.

Interfacing Dielectric-Loaded Plasmonic and Silicon Photonic Waveguides: Theoretical Analysis and Experimental Demonstration

A comprehensive theoretical analysis of end-fire coupling between dielectric-loaded surface plasmon polariton and rib/wire silicon-on-insulator (SOI) waveguides is presented. Simulations are based on the 3-D vector finite element method. The geometrical parameters of the interface are varied in order to identify the ones leading to optimum performance, i.e., maximum coupling efficiency. Fabrication tolerances about the optimum parameter values are also assessed. In addition, the effect of a longitudinal metallic stripe gap on coupling efficiency is quantified, since such gaps have been observed in fabricated structures. Finally, theoretical results are compared against insertion loss measurements, carried out for two distinct sets of samples comprising rib and wire SOI waveguides, respectively.

A Robust Method for the Computation of Green's Functions in Stratified Media

Closed-form Greens functions for unbounded planar stratified media are derived in terms of cylindrical and spherical waves. The methodology is based on a two-level approximation of the spectral-domain representation of Greens functions. This robust, efficient, and fully numerical approach does not call for an analytical extraction of ldquoproblematicrdquo behaviors, such as the quasi-static terms and the surface wave poles, prior to the spectrum fitting. Instead, the spatial-domain Greens functions derived in this paper provide an accurate description of both the near-field singularity in the vicinity of the source and the far-field dominant behavior of the surface waves.

Consistent Study of Graphene Structures Through the Direct Incorporation of Surface Conductivity

An efficient 3-D FDTD formulation for the precise analysis of electromagnetic phenomena in graphene structures is presented in this paper. The new concept considers the surface nature of graphenes conductivity and encompasses it directly into the integral form of Maxwells equations, avoiding the necessity to discretize the materials transverse dimension via a subcell process. Hence, the proposed technique is much easier to implement and combined with existing FDTD codes, as it requires less computational time. Numerical verification involving comparisons with closed-form solutions and the results of other schemes, exhibits the high accuracy of the algorithm.