Alona Boag

New designs of ultra wide-band communication antennas using a genetic algorithm

The paper investigates new designs of loaded wire antennas with broad-band characteristics. A two-step design procedure is proposed for the design. First, different unloaded antennas configurations are examined with a view to identifying candidates that exhibit the lowest voltage standing wave ratio (VSWR) and highest gain characteristics. Multi-armed antennas with straight wire segments that branch off symmetrically from a central stem are found to be excellent potential candidates. Next, the antenna with the best performance is loaded with resonant tank circuits and a matching network is designed. The loads can enhance the antenna characteristics to yield high gain and low VSWR by modifying the current distribution and can force it to radiate nearer to the horizon in the elevation plane with an almost omnidirectional pattern in the azimuth plane. An efficient optimization procedure based on the genetic algorithm is employed to simultaneously determine the load components, their locations, and the parameters of the matching network. Several examples of four- and eight-arm antennas with bandwidth of 7.5:1 and 15:1 illustrate the effectiveness of the design procedure.

Analysis of two-dimensional electromagnetic scattering from nonplanar periodic surfaces using a strip current model

A method-of-moments solution is presented for the problem of two-dimensional transverse magnetic (TM) scattering of a plane wave from a nonplanar periodic surface separating two contrasting homogeneous media. The moment solution uses fictitious spatially periodic and properly modulated electric current strips to simulate the field scattered by the surface and the field penetrating the surface. The fields radiated by the current strips are represented in terms of Floquet modes, and the problem is reduced to a consideration of the fields over a single period. The simulated fields are forced to obey the continuity conditions for the tangential components of the electric and magnetic fields at a selected set of points on the interface within a single period. The procedure is simple to implement, rapidly converging, and applicable to arbitrary smooth profiles. Perfectly conducting media are treated as reduced cases of the general procedure for penetrable media. Results for sinusoidal surfaces are given and compared with available data. The efficiency of the method is demonstrated. >

Analysis of electromagnetic scattering using a current model method

Abstract Fictitious current models have been used extensively in recent years to facilitate a computational scheme for a variety of scattering problems. This paper reviews the basic philosophy behind this new approach and outlines the relevant formalism. A number of cases which shed light on various specialty aspects of the general technique are also described in some detail. The wide applicability and demonstrated success of the method are recorded.

Optimal excitation of multiapplicator systems for deep regional hyperthermia

A method is proposed for determining the excitation amplitudes and phases of the elements of electromagnetic multiapplicator systems used for optimizing the specific absorption rate (SAR) distribution around a deep-seated tumor. In this method, the ratio of the power dissipated in the tumor to a weighted summation of the powers supplied to the surrounding regions is optimized. The optimization procedure is combined with a recently proposed technique for analysis of various electromagnetic scattering and interaction problems. The general principle is applied to a two-dimensional problem of a piecewise homogeneous cylinder heated by an array of electric-current filaments placed outside the cylinder. Numerical simulations are performed to check the effectiveness of the approach. The results demonstrate that improved SAR distributions can be achieved using this optimization method. The extension to three-dimensional configurations is discussed.<<ETX>>

On the use of SVD-improved point matching in the current-model method (EM scattering)

An approach which uses the singular value decomposition (SVD) to improve the accuracy of the numerical solution obtained with fictitious current models is introduced. In this approach, the SVD is essentially facilitating a systematic way to optimally reduce the generalized inverse matrix used in the solution to a submatrix of smaller rank. This reduction strikes a balance between the fulfillment of the boundary conditions at the matching points and that between them. Clearly, the boundary conditions errors at the matching points are no longer strictly zero. However, the previously discernible errors between the matching points are markedly suppressed. The approach is efficacious not only when the impedance matrix is inherently singular or highly ill conditioned, but also when this matrix is entirely well conditioned. It can be generalized and implemented in any method of moments code which uses point matching for testing. The approach has been incorporated into an existing solution based on the current-model method for the problem of scattering from periodic sinusoidal surfaces, and is shown to render the solution more accurate. >

Analysis of diffraction from echelette gratings, using a strip-current model

A method is presented for analyzing electromagnetic scattering from an echelette grating separating two contrasting homogeneous media and illuminated by a plane wave. The reduction of the general problem to a consideration of the fields over a suitably selected period, referred to as the unit cell, is facilitated by the Floquet theorem. The solution, in the p-polarization case (electric field parallel to the grooves), uses sets of spatially periodic and properly modulated fictitious electric-current strips to simulate the field scattered by the grating boundary surface and the field penetrated through the surface. In the s-polarization case (magnetic field parallel to the grooves), which is not examined in this paper, sets of fictitious magnetic-current strips, instead of electric ones, should be used. The fields radiated by the current strips are expressed in terms of Floquet modes and are adjusted to fit the continuity conditions for the tangential components of the electric and magnetic fields at a finite number of points on the grating surface within the unit cell. Special attention is given to the behavior of the fields at the corners. The procedure is simple to perform and is applicable to gratings of arbitrary cross section. Perfectly conducting gratings are treated as reduced cases of the general procedure. Results are given and compared with existing data. The efficiency of the suggested method is demonstrated.

Analysis of acoustic scattering from fluid cylinders using a multifilament source model

A solution is presented for the problem of two‐dimensional acoustic scattering from homogeneous fluid cylinders. The solution uses fictitious filamentary isotropic sources to simulate both the field scattered by the cylinder and the field inside the cylinder and, in turn, point‐matches the continuity conditions for the normal component of the velocity and for the pressure across the cylinder surface. The procedure is simple to execute and is general in that cylinders of arbitrary shape can be handled effectively. Perfectly rigid cylinders are treated as reduced cases of the general procedure. Results are given and compared with available analytic solutions, which demonstrate the very good performance of the procedure.

Analysis of diffraction from doubly periodic arrays of perfectly conducting bodies by using a patch-current model

A novel solution is presented for the problem of three-dimensional electromagnetic scattering of a plane wave from a doubly periodic infinite array of perfectly conducting bodies. A set of fictitious spatially periodic and properly modulated patches of magnetic current is used to simulate the scattered field. These patch currents are of dual polarization and have complex amplitudes. The electromagnetic field radiated by each of the periodic patch currents is expressed as a double series of Floquet modes. The complex amplitudes of the fictitious patch currents are adjusted to render the tangential electric field zero at a selected set of points on the surface of any of the scatterers. The procedure is simple to implement and is applicable to arrays composed of smooth but otherwise arbitrary perfectly conducting scatterers. Results are given and compared with an analytic approximation.

Analysis of three‐dimensional acoustic scattering from doubly periodic structures using a source model

A novel solution is presented for the problem of three‐dimensional acoustic scattering of a time‐harmonic plane wave from doubly periodic structures. The general problem is first reduced to a consideration of the fields over a suitably defined unit cell. Sets of fictitious doubly periodic and properly modulated patch sources are used to simulate the fields in the homogeneous regions crossed by the unit cell boundaries. Sets of fictitious point sources are used to simulate the fields inside the regions completely enclosed within the unit cell. The complex amplitudes of the fictitious sources are adjusted to satisfy the boundary conditions at a selected set of points on the boundaries between the regions. The suggested solution procedure is simple to implement and is applicable to doubly periodic structures composed of homogeneous regions of arbitrary shape. Structures comprising acoustically rigid and soft boundaries can also be handled by the procedure. The method has been tested for accuracy by studying the cases of scattering from arrays of spherical scatterers and from doubly periodic sinusoidal surfaces.

Analysis of electromagnetic scattering from doubly periodic arrays of penetrable bodies using a patch-dipole current model

A novel solution is presented for the problem of three-dimensional electromagnetic scattering of a time-harmonic plane wave from a doubly periodic infinite array of disjoint finite-size penetrable bodies. A set of fictitious doubly periodic and properly modulated patches of magnetic current of cross polarization is used to simulate the scattered field. A set of fictitious elemental lectric dipoles of cross polarization is used to simulate the field inside the penetrable bodies. The complex amplitudes of the two sets of fictitious sources are adjusted to render the tangential components of the electric and magnetic fields continuous at a selected set of points on the surface of any of the scatterers. The suggested solution procedure is simple to implement and is applicable to doubly periodic arrays composed of homogeneous bodies of smooth, but otherwise arbitrary, shape. The accuracy of the method has been demonstrated by means of several accuracy checks. It has also been shown that in the limiting case of widely spaced spherical scatterers the numerical solution agrees well with an approximate analytic solution.