Showing papers by "Giuseppe Vecchi published in 2006"

TOPICA: an accurate and efficient numerical tool for analysis and design of ICRF antennas

Abstract: The demand for a predictive tool to help in designing ion-cyclotron radio frequency (ICRF) antenna systems for today's fusion experiments has driven the development of codes such as ICANT, RANT3D, and the early development of TOPICA (TOrino Polytechnic Ion Cyclotron Antenna) code. This paper describes the substantive evolution of TOPICA formulation and implementation that presently allow it to handle the actual geometry of ICRF antennas (with curved, solid straps, a general-shape housing, Faraday screen, etc) as well as an accurate plasma description, accounting for density and temperature profiles and finite Larmor radius effects. The antenna is assumed to be housed in a recess-like enclosure. Both goals have been attained by formally separating the problem into two parts: the vacuum region around the antenna and the plasma region inside the toroidal chamber. Field continuity and boundary conditions allow formulating of a set of two coupled integral equations for the unknown equivalent (current) sources; then the equations are reduced to a linear system by a method of moments solution scheme employing 2D finite elements defined over a 3D non-planar surface triangular-cell mesh. In the vacuum region calculations are done in the spatial (configuration) domain, whereas in the plasma region a spectral (wavenumber) representation of fields and currents is adopted, thus permitting a description of the plasma by a surface impedance matrix. Owing to this approach, any plasma model can be used in principle, and at present the FELICE code has been employed. The natural outcomes of TOPICA are the induced currents on the conductors (antenna, housing, etc) and the electric field in front of the plasma, whence the antenna circuit parameters (impedance/scattering matrices), the radiated power and the fields (at locations other than the chamber aperture) are then obtained. An accurate model of the feeding coaxial lines is also included. The theoretical model and its TOPICA implementation have been fully validated against measured data both in vacuo and in plasma-facing conditions for real-life structures.

Fast Analysis of Large Finite Arrays With a Combined Multiresolution—SM/AIM Approach

Abstract: We present a synthesis of the sparse matrix/adaptive integral method (SM/AIM) and the multiresolution (MR) approach for the analysis of electrically large finite arrays, with planar or 3-D radiating elements; the two methods were separately introduced previously. The use of the MR has the effect of a preconditioner and speeds up the convergence rate of the SM/AIM of almost two orders of magnitude, with a total reduction of the numerical complexity with respect to the standard MoM of almost three orders of magnitude

Synthetic-Functions Decomposition of Large Complex Structures

Abstract: The synthetic function expansion (SFX) technique, previously introduced by the authors has been successfully applied to the analysis of various types of antennas, inter-antenna coupling and antenna farm problems. The technique begins with breaking a large structure into smaller portions; the emphasis here is on the aspects specific to the applications where a (large) conducting body has to be "cut" into several "pieces"; the body may be solid (like a satellite body, a ship or an aircraft) or thin (e.g. a reflector or a plate), and can either be excited by an antenna placed on the structure (as in satellite problems, or antenna siting issues) or near it, or by an incident wave. The aim of the analysis is to discuss how to manage the cases when long cuts are present for the considered structures, and to address some "optimum" issues in the breakdown of the structure in smaller blocks

A compact antenna reconfigurable in the band 1.7GHz - 2.7GHz

Abstract: This article describes a novel structure that is reconfigured via switches. The switch-reconfigurable antenna is first optimized by means of a genetic algorithm (GA), coupled to a MoM-based EM. After selection of the optimal shape (including switch positions), the optimal switch set states are obtained by exhaustive search. Optimization via GA have received widespread attention recently and have been successfully applied in automated antenna design (F.J. Villegas et al., 2004) when tight specifications and strong constraints are to be satisfied; to our knowledge this is the first attempt to apply GA in the design of reconfigurable structures as done here. This work can be seen as an evolution of the concept outlined in A. Cavallero et al., (2004).

Hybrid spatial-spectral analysis of periodic structures

Abstract: For the MoM analysis of complex printed periodic structures, the use of sub-domain basis functions is essential. In this work, triangular mesh and Rao-Wilton-Glisson (RWG) functions are assumed. The aim of this work is to propose a hybridization of the standard spectral- and spatial-domain MoM approaches, through a convenient manipulation of the periodic Green's function (PGF).

Numerical and Experimental Characterization of a Wide-Band Conformal Base Station Antenna

Abstract: In this paper, a conformal 4times8 array antenna for a mobile communication base station is analyzed using the synthetic function expansion technique, a numerical approach previously introduced by the authors. While the numerical aspects of the problem were discussed in a previous paper, here the practical aspects like construction and measurements for a first prototype with a single polarization are stressed

A novel multi-resolution system of RWG functions

Abstract: In this paper, we showed a kind of multi-resolution (MR) basis for the analysis via the method of moments (MoM) of a generic conductor discretized with triangular cells. The proposed basis is derived from the classical Rao-Wilton-Glisson (RWG) basis (S.M. Rao et al., 1982): so we called the new basis functions "multi-resolution Rao-Wilton-Glisson" (MR-RWG) functions.

A far field wavelet expansion for the synthesis of contoured-beam array-fed reflector antennas

Abstract: This work is concerned with the design of "contoured-beam" antennas (also known as "shaped-beam" antennas) as used in satellite communication systems, i.e. antennas having the main beam shaped so that its footprint on ground closely follows the contour of specific geographical regions (e.g. Europe or the U.S.), with the objective of maximizing the radiation efficiency and reducing interferences with neighbouring regional systems. The reference solution for this work are "array-fed" reflectors, i.e. antennas fed by an array of horns or equivalent radiating elements, each producing a "component" beam toward a different direction upon reflection on the reflection dish. The idea behind this work is to find an efficient representation of the desired radiation pattern that concentrates the field information relevant to the design task in few, suitably constructed functions; if properly coded, this enables synthesis algorithms to converge faster. The proposed approach exploits the properties of the multi-resolution (MR) representation. We have generated a wavelet system suitable to the coverage problem of present interest. In this paper we consider array-fed offset reflector antennas. Two very different synthesis techniques are considered as application examples of the devised pattern representation: 1) a simple (and fast) field synthesis projection method; and 2) a power synthesis via an optimization based on a genetic algorithm (GA).

Efficient analysis of geometrically complex planar arrays

Abstract: In this paper, we present an efficient, physics-based preconditioning scheme for fast method-of-moments (MoM) methods for the analysis of geometrically complex planar arrays. The preconditioner derives from the generation of a multi-resolution (MR) basis and, unlike other preconditioners, requires a low memory occupation and computational cost for its generation and application. This feature makes the proposed preconditioner suitable for its use with fast integral techniques, as FMM and AIM, that present low computational complexity.

A wavelet-based approach to contoured-beam antennas

Abstract: In the design of contoured-beam antennas, several synthesis methods have been developed, but usually little importance has been given to the issue of finding an “efficient” representation of the antenna far field: this is the main scope of this work. A wavelet expansion of the far field is developed, and applied to the synthesis of array-fed reflector antennas for geographic coverage (e.g. Europe coverage). Two different synthesis techniques are considered as application examples of the devised pattern representation: a field-synthesis projection method, and a power synthesis optimization based on a genetic algorithm (GA) specifically adapted for antenna design. INTRODUCTION In this work we deal with the design of “contoured-beam” antennas as used in satellite communication systems [1], i.e. antennas having the main beam shaped so that its footprint on ground closely follows the contour of specific geographical regions (e.g. Europe), with the objective of maximizing the radiation efficiency and reducing interferences with neighbouring regional systems. As contoured-beam antennas we consider “array-fed” reflectors [1], [2], i.e. antennas fed by an array of horns or equivalent radiating elements, each producing a beam toward a different direction upon reflection on the reflection dish (see Figure 1.a). The synthesis is a key point in the design of all high performance antennas. While our final goal is the synthesis of contoured-beam antennas, we will not propose a new synthesis algorithm in the strict sense. In the many synthesis approaches that have been proposed usually little importance has been given to the representation of the antenna far field in the synthesis procedure: this will be instead our main concern here. The idea behind this work is to find an efficient representation of the desired radiation pattern that concentrates the field information relevant to the design task in few, suitably constructed functions; if properly coded, this enables synthesis algorithms to converge faster. The proposed approach exploits the properties of the multiresolution (MR) representation. In loose terms, a MR basis is divided into a number of subsets of functions (all linearly independent), and each subset possesses a different resolution. To synthesize the desired directivity two different synthesis techniques are considered: a field-synthesis projection method, and a power synthesis optimization based on a genetic algorithm (GA) [3], specifically adapted for antenna design. Preliminary results of the developed technique applied to the synthesis of circular aperture antennas are reported in [4].

Spectral Filtering for Space-Domain Analysis of Periodic Structures

Abstract: In the Method-of-Moments (MoM), applied to the Integral-Equation (IE) formulation for scattering or antenna problems, the solution (the unknown current) is approximated by a linear combination of a finite number of basis functions.

Spectral filtering for space-domain analysis of periodic structures

Abstract: In the Method-of-Moments (MoM), applied to the Integral-Equation (IE) formulation for scattering or antenna problems, the solution (the unknown current) is approximated by a linear combination of a finite number of basis functions.