摘要:

A variational principle for the scattering amplitude is applied to obtain a relationship, in the Fourier domain, between the scattering potential and the scattered field recorded over a measurement line outside the potential. This relationship leads to a novel iterative reconstruction algorithm for recovering the distribution of the index of refraction from measurements of the scattered field. The proposed technique uses and builds upon the well-knownfiltered backpropagationalgorithm of diffraction tomography. Computer simulations are presented that compare the reconstructions obtained with the proposed algorithm to the ones obtained with Born's approximation.

ISSN:0920-5071    

DOI:10.1163/156939396X00180

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A time-domain method to determine two of the physical parameters L, C, R, and G of a nonuniform transmission line, from the knowledge of the remaining two parameters and the transient response of the line, is presented. The method is based on the recently introduced compact Green functions approach. It is shown that any pair of the parameters can be reconstructed if L and C are considered in terms of the characteristic impedance Z and the wavefront speed c. The impedance is assumed to be continuous along the nonuniform line, but may be discontinuous at the end. It is shown that a discontinuity at the end both simplifies the reconstruction and makes the reconstruction less sensitive to noise. The parameters R and G, which both represent losses, are the most difficult to reconstruct. It is found that reconstructions involving c as well as reconstructions of R and G fail under certain conditions, at certain points along the line. The method is tested numerically for all combinations of parameters, and the failure to determine the parameters at some points is illustrated. The influence of noise is also illustrated. The present method should be useful as a fast analysis tool for time domain reflectometer measurements.

ISSN:0920-5071    

DOI:10.1163/156939396X00199

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Electromagnetic sources in axially chiral uniaxially anisotropic media are decomposed into two parts. The resulting field can also be separated into two components, each with constant axial impedence (i.e. ratio of axial electric field to axial magnetic field). It is seen that, for certain combinations of the electric and magnetic fields due to the decomposed sources, the governing differential equations are reduced from the fourth order to the second order and can be solved through affine transformations. The TE/TM decomposition theory, previously derived for simple isotropic and nonchiral uniaxial media, is seen to be a special case of the present theory, corresponding to zero and infinite axial impedances. As a consequence, it is demonstrated that the decomposed fields with constant axial impedance see the original axially chiral uniaxial medium as two nonchiral uniaxial media. These two media can be characterized as affine-isotropic because they can be transformed to isotropic media through certain uniaxial affine transformations.

ISSN:0920-5071    

DOI:10.1163/156939396X00207

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

In recent times electromagnetic scattering calculations are often done by local methods such as Finite Elements or Finite Differences with approximate local absorbing boundary conditions because of their computational efficiency. The exact formulation of the field problem by combining a local with a global method (boundary integral) has the drawback that fully populated matrices have to be handled, making the method often computationally intensive. But there are many cases in which such hybrid-techniques have advantages as compared with pure local methods. We present an FEM/BEM-hybrid approach and show its application to scattering problems with several separated finite inhomogeneous regions and to combined scattering/radiation problems. The calculation of radiation problems (e. g. antennas nearby inhomogeneous bodies) is especially attractive because radiation structures can be modeled by well-established integral equation techniques. Particular attention is paid to the evaluation of the coupling integrals for the BEM with singular integral kernels. The matrix solution procedure is optimized by a new renumbering technique for the FE-mesh. Numerical results are presented.

ISSN:0920-5071    

DOI:10.1163/156939396X00216

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

This paper discusses the problem of a spherical anisotropic scatterer exposed to a uniform electrostatic field. It has been common in previous literature to treat this problem similar to the case of isotropic sphere for which it is known that the internal field is uniform throughout the volume of the sphere. In this paper, a proof is given for the uniformity of the field inside the anisotropic sphere, which (to the authors' knowledge) has not been proven before. This result gives rigorous justification to those analyses in which the polarizability matrices of anisotropic inclusions are determined and the effective material parameters of anisotropic mixtures are calculated. In the present article, the proof is also generalized for bianisotropic spheres within electromagnetostatic field.

ISSN:0920-5071    

DOI:10.1163/156939396X00225

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Based on a spectral domain technique, asymptotic expressions for Green's functions in infinitely extended bianisotropic media are determined. This is achieved by considering the spectral representation of the Green's dyadic, which can be represented as a rational function in the spectral variables. Extraction of pole singularities and the spectral behavior at infinity leads to the far field-and the source point asymptotics in the spatial domain, respectively. The extraction procedure and its peculiarities are discussed and explicit formulas and sample results for diagonally bianisotropic media show the applicability of the approach.

ISSN:0920-5071    

DOI:10.1163/156939396X00234

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

In this paper, we investigate the oscillating electromagnetic processes in a circular and regular waveguide of a moving boundary. An analytical solution for this problem is not possible in the general case. Following the suggestion in [1,2] we have developed an analytical method for the case of axisymmetric waves in the waveguide. Also the case of linearly moving boundary is treated.

ISSN:0920-5071    

DOI:10.1163/156939396X00243

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

We consider the 2D-problem of the reflection of a plane wave from arrays of dielectric and magnetic fibers of arbitrary cross section. The gratings are placed in air or embedded in a dielectric slab which may be backed by a perfectly conducting plane. We use a T-Matrix method to compute the field scattered by these structures. Comparison with an integral representation is made. The efficiency of the T-Matrix approach is illustrated through the examples of some absorbing structures.

ISSN:0920-5071    

DOI:10.1163/156939396X00252

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The generalized admittance matrix (GAM) approach to the analysis of cylindrical cavities coupled to external waveguides by rectangular apertures is discussed. Green's dyad of the admittance type under the longitudinal section electric (LSE) field approximation is derived and GAM is obtained by Galerkin's method. Several numerical results are presented and a criterion for the optimal mode selection is discussed. A reduced formulation having improved efficiency is derived and its use is critically discussed. Comparisons with results of mode-matching, finite-element method and with measurements are shown. The analysis is successfully applied to waveguide bandpass filter design.

ISSN:0920-5071    

DOI:10.1163/156939396X00261

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

ISSN:0920-5071    

DOI:10.1163/156939396X00270

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor