Óscar Rubiños-López

A Bifocal Ellipsoidal Gregorian Reflector System for THz Imaging Applications

Current terahertz imagers rely on reflector systems for the beam quality and imaging speed because the cross-range span that the system can cover is limited by the beam aberrations when the antenna scans. We present the design of a Bifocal reflector system that can rapidly scan a terahertz beam for standoff imaging applications while increasing the field of view of previous designs up to 50%. The design is based in a confocal Gregorian system where the nominal reflector surfaces are substituted by shaped surfaces to reduce the beam aberrations, while not increasing the manufacture cost of the reflector antenna. We also provide a set of useful design formulas for the design of this kind of reflector systems. The beam patterns obtained by the proposed designs are numerically calculated with the commercial software GRASP and compared with those obtained with previous approaches to the same problem, showing the better performance of the proposed solution.

Junction modeling for piecewise non-homogeneous geometries involving arbitrary materials

This paper presents a new method for the electromagnetic analysis of metallic and/or dielectric structures that are in contact with other metallic and/or dielectric structures. The method is based on modeling each junction edge by sets of fictitious ordinary Rao-Wilton-Glisson (RWG) functions. Unlike other approaches, the proposed method can be easily implemented in existing MoM (Method of Moments) codes, capable of handling open and/or closed, disconnected, metallic and/or dielectric bodies. This is because the boundary conditions at junction edges are imposed to the MoM matrix after it has been generated by the existing MoM method. Additionally, the presented junction method is applicable to any Surface Integral Equation (SIE) formulation. A simulation example is analyzed in order to assess the accuracy and iterative performance of several MoM implementations based on different SIE formulations.

Comparison of iterative solvers for electromagnetic analysis of plasmonic nanostructures using multiple surface integral equation formulations

The electromagnetic behavior of plasmonic structures can be predicted after discretizing and solving a linear system of equations, derived from a continuous surface integral equation (SIE) and the appropriate boundary conditions, using a method of moments (MoM) methodology. In realistic large-scale optical problems, a direct inversion of the SIE–MoM matrix cannot be performed due to its large size, and an iterative solver must be used instead. This paper investigates the performance of four iterative solvers (GMRES, TFQMR, CGS, and BICGSTAB) for five different SIE–MoM formulations (PMCHWT, JMCFIE, CTF, CNF, and MNMF). Moreover, under this plasmonic context, a set of suggested guidelines are provided to choose a suitable SIE formulation and iterative solver depending on the desired simulation error and available runtime resources.

A computational method for modeling arbitrary junctions employing different surface integral equation formulations for three-dimensional scattering and radiation problems

This paper presents a new method, based on the well-known method of moments (MoM), for the numerical electromagnetic analysis of scattering and radiation from metallic or dielectric structures, or both structure types in the same simulation, that are in contact with other metallic or dielectric structures. The proposed method for solving the MoM junction problem consists of two separate algorithms, one of which comprises a generalization for bodies in contact of the surface integral equation (SIE) formulations. Unlike some other published SIE generalizations in the field of computational electromagnetics, this generalization does not require duplicating unknowns on the dielectric separation surfaces, except duplications for a very small fraction of unknowns corresponding to junction edges, which introduces a negligible computational cost. Additionally, this generalization is applicable to any ordinary single-scatterer SIE formulations employed as baseline. The other algorithm deals with enforcing boundary conditions and Kirchhoff’s Law, relating the surface current flow across a junction edge. Two important features inherent to this latter algorithm consist of a mathematically compact description in matrix form, and importantly, from a software engineering point of view, an easy implementation in existing MoM codes which makes the debugging process more comprehensible. A practical example involving a real grounded monopole antenna for airplane-satellite communication is analyzed for validation purposes by comparing with precise measurements covering different electrical sizes.

Fast Far Field Computation of Single and Dual Reflector Antennas

The physical optics (PO) method has been widely used for the analysis of the electromagnetic behavior of single and dual reflector antennas. An extensive work has been done by the authors of this paper in order to increase the speed for obtaining far field patterns from single and dual geometries and also in order to increase the accuracy of the method. This paper reviews these contributions and improves the existing published work with the physical interpretation of the radiation from a single patch and the computer implications when using acceleration techniques such as OpenMP.

An approach for valid covariance estimation via the Fourier series

The use of kriging for construction of prediction or risk maps requires estimating the dependence structure of the random process, which can be addressed through the approximation of the covariance function. The nonparametric estimators used for the latter aim are not necessarily valid to solve the kriging system, since the positive-definiteness condition of the covariance estimator typically fails. The usage of a parametric covariance instead may be attractive at first because of its simplicity, although it may be affected by misspecification. An alternative is suggested in this paper to obtain a valid covariance from a nonparametric estimator through the Fourier series tool, which involves two issues: estimation of the Fourier coefficients and selection of the truncation point to determine the number of terms in the Fourier expansion. Numerical studies for simulated data have been conducted to illustrate the performance of this approach. In addition, an application to a real environmental data set is included, related to the presence of nitrate in groundwater in Beja District (Portugal), so that pollution maps of the region are generated by solving the kriging equations with the use of the Fourier series estimates of the covariance.

Fast and accurate simulation of coaxial-fed antennas using full-wave and asymptotic computational methods

In this paper, we explain how to accurately simulate, using the method of moments (MoM) and the physical optics (PO) approximation, antennas with a coaxial probe feed. Special attention is given to properly modeling with MoM the connection between the antenna and the coaxial cable. Results were validated by comparison with measurements covering different electrical sizes of a grounded monopole antenna for airplane-satellite communication at the IEEE C-band. Good agreement, in radiation pattern and gain, is observed between MoM and PO. Equivalent circuit parameters are obtained through simulation, which allows to predict the communication system performance.

Comparison of iterative solver performances on multiple surface integral equation formulations for plasmonic scatterers

In computational electromagnetics, five surface integral equation (SIE) formulations are typically used to predict the electromagnetic behavior of plasmonic structures. These SIE formulations are discretized into a matrix form by the method of moments (MoM) approach. The derived linear system of equations needs to be typically resolved by iterative algorithms, since the direct inversion of the MoM matrix is unfeasible for large computational problems. This paper compares the performance of four different iterative solvers (GMRES, TFQMR, CGS, and BICGSTAB) for five different SIE-MoM formulations (PMCHWT, JMCFIE, CTF, CNF, and MNMF) used for the electromagnetic analysis of plasmonic structures.

Three-Dimensional Wedge Diffraction Correction Deduced by the Stationary Phase Method on the Modified Equivalent Current Approximation (Meca)

This paper presents a new method for computing fields diffracted by a wedge for the MECA formulation, which is valid not only for perfect electric conductors but also for lossy penetrable dielectrics. The method is based on the computation of a wedge correction matrix, which establishes a mapping function between fields incident at and diffracted by the wedge. The MECA method is based, in general, upon the oblique incidence of a plane wave at the interface between free space and lossy dielectric media. MECA reduces to the well-studied physical optics (PO) formulation in case of PEC (perfect electric conductor) scatterers. In this work, we consider a scenario involving diffraction caused by a plane wavefront incident on a wedge with flat faces and straight edge. The version of the stationary phase method for three-dimensional equivalent source distributions is employed to calculate the asymptotic contribution of the integration boundary along the edge of the diffraction wedge. This contribution of the critical boundary points is compared to the GTD (geometrical theory of diffraction) diffracted field in order to obtain the correction matrix by which the incident electric field vector is multiplied in MECA. As required to accomplish this comparison, the threedimensional incident electric field is previously resolved into an edgefixed coordinate system. Good agreement is demonstrated between full-wave method-of-moments (MoM) results and the results obtained by modifying MECA with our diffraction correction technique. Received 18 November 2011, Accepted 18 January 2012, Scheduled 17 February 2012 * Corresponding author: Hipolito Gomez-Sousa (hgomez@com.uvigo.es). 208 Gomez-Sousa, Martinez-Lorenzo, Rubinos-Lopez

A memory-hierarchy-based optimization of MECA (Modified Equivalent Current Approximation) for the analysis of electrically large dielectric and lossy structures

Asymptotic high frequency computational techniques are widely used in modeling scenarios with electrically large scatterers. A well-known technique of this type, Physical Optics (PO), has been extensively tested in several contexts [1], [2]. Two recent works [3], [4] present a new PO analysis of dielectric and lossy geometries. In this method, called MECA, the interface is discretized into planar triangular facets. Then, the Snell reflection coefficients establish the relation between incident and reflected waves. This modified PO method successfully models problems which may be prohibitive for full-wave simulation. For the present work, we developed an OpenMP parallel version of MECA in order to reduce even more the execution time. In our parallel code, we implemented memory-hierarchy-based optimization techniques [5], [6]. As seen in the results of this paper, these techniques can improve the computational performance when calculating the scattered fields.