摘要:

Anapproximate and efficient formulation is presented for calculating the scattered field from a three-dimensional narrow crack in a ground plane illuminated by a plane wave. This formulation is based on a quasi-static approximation supplemented with terms that include the effect of traveling waves. It is shown that for shallow cracks, only the first dominant traveling-wave mode needs to be included to render an accurate solution. However, when a crack is deep enough, the second mode must be considered. The accuracy of the calculation is demonstrated by several numerical examples.

ISSN:0920-5071    

DOI:10.1163/156939399X01573

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The gyromagnetic uniaxial chiral medium, which is a generalization of the well-studied reciprocal chiral material, can be created by mixing three sets of small metal helices in a host gyromagnetic medium (e.g., magnetically biased ferrite), with the axes of two sets perpendicular and parallel to a fixed direction, respectively, and the other set distributed in random orientations and locations. Based on the concept of characteristic waves and the method of angular spectral expansion, vector wave functions are rigorously developed to represent the electromagnetic fields in a source-free gyromagnetic uniaxial chiral medium. The analysis reveals that the solutions of the source-free vector wave equation for the gyromagnetic uniaxial chiral medium, which are composed of two transverse waves and a longitudinal wave, can be represented in sum-integral forms of the cylindrical vector wave functions. Addition theorem of the vector wave functions for gyromagnetic uniaxial chiral medium can be directly obtained from its counterpart for isotropic medium. To apply the present formulations for practical applications, an extended mode-matching method is proposed to study the two-dimensional electromagnetic scattering of a gyromagnetic uniaxial chiral cylinder with arbitrary cross section and a conducting circular cylinder with an inhomogeneous coating thickness. To check the convergence of the present cylindrical vector wave functions for multiple-body problem, electromagnetic scattering of two circular cylinders of gyromagnetic uniaxial chiral media is also investigated. Excellent convergence properties of the cylindrical vector wave functions in these application examples are numerically verified, which

ISSN:0920-5071    

DOI:10.1163/156939399X01582

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Cylindrical waves are fundamental building blocks in constructing the solution of two-dimensional scattering problems. The reflection problem of a cylindrical wave of any integer order from a generic plane surface of discontinuity for the electromagnetic constants is faced making use of the plane-wave decomposition approach. The resulting numerical problem, involving the quadrature of highly oscillating functions is solved by means of suitable techniques. Numerical results are presented for some cases of practical interest. A comparison with a particular case for which a closed-form solution is available is also presented.

ISSN:0920-5071    

DOI:10.1163/156939399X01591

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

In this paper the Genetic Algorithm (GA) is used to design and optimize a dual-band antenna system covering the UHF and VHF bands. The system contains two stacked vertical wire dipoles, one for each of the bands, above a perfectly-conducting ground plane. Unlike the case of the GA design of a single-band antenna [1, 2], the mutual coupling between the two dipoles introduces some complications in the design of dual-band antennas. Specifically, in the process of broad-banding one dipole by optimizing its loading and matching network for its own frequency band, it becomes necessary to take into account the influence of the second dipole operating in a different frequency band. It is, therefore, essential to carry out the analysis and the synthesis of the entire coupled system concurrently. This paper proposes a design procedure for the dual-band antenna system that makes initial choices for the antenna configuration on the basis of a systematic screening of the potential candidates, and then goes on to apply the GA in conjunction with the Numerical Electromagnetics Code (NEC4) [3] to meet the specifications for the two operating in a simultaneous manner.

ISSN:0920-5071    

DOI:10.1163/156939399X01609

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The present paper reports the results of an extensive numerical analysis of electromagnetic fields in random dielectric materials. The effective permittivity of the mixture is calculated by FDTD simulations of such a sample in a TEM-waveguide. The mixture consists of a homogeneous free space background in which circular cylinders are embedded in random positions. Clustering of the inclusions is allowed. The choice of two-dimensional spheres allows us to make much more use of the numerical capacity than what would be the case in three-dimensional inclusions. Various dielectric contrasts are studied from permittivity ratio 2 to 50. Numerical results are compared with classical mixing rules, and they show moderate agreement with the Bruggeman rule rather than the Maxwell Garnett approach. However, another key finding of this paper is that obtained mixing results are not symmetric with respect to inclusions and background. This makes the exact validity of the symmetric Bruggeman formula questionable.

ISSN:0920-5071    

DOI:10.1163/156939399X01618

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The fields scattered by a target buried beneath the ground and observed above the ground are perturbed from their values with no target present. These perturbed fields are evaluated using a Sommerfeld approach for all possible cases of transmitting antenna; i.e., VMD, HMD, VED and HED. For frequencies in the 1 to 30 KHz range, the scattered fields have a quasi-static behavior. The exact expressions for the Sommerfeld integrals are evaluated numerically, and compared with their quasi-static approximations. The scatter from the buried target is generated by the induced electric and magnetic dipole moments from the incident dipole field at the dipole source and propagate to the target. The induced dipole moments then reradiate to an observer. All components of E and H are considered. Several examples are given for various combinations for the transmitting antenna, buried object and layer properties.

ISSN:0920-5071    

DOI:10.1163/156939399X01627

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

In this article, an adaptive multiscale algorithm is presented for the analysis of scattering by a thin, perfectly conducting plate. This algorithm employs the conventional moment method and a special matrix transformation, which is derived from solving the first-kind Fredholm equation of the first-kind by the multiscale technique. Pulse functions and Dirac delta functions are used for expansion and testing functions, respectively, utilizing the conventional moment method. The impedance matrix and the source terms of the matrix equation are computed using a five-point average scheme. By use of the matrix transformation, the currents, source terms and impedance matrix can be arranged in the form of different scales resulting in an adaptive multiscale method. By going from one scale to another scale, the initial estimate for the solution can be predicted according to the properties of the multiscale technique. AMMM can reduce automatically the size of the linear equations so as to improve the efficiency of the conventional moment method. Several numerical results are presented, which demonstrate that AMMM is a useful method to analyze the scattering from perfectly conducting plates.

ISSN:0920-5071    

DOI:10.1163/156939399X01636

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

This paper presents a new technique for the synthesis of multiple wide band nulls by controlling the complex current excitations in linear arrays. The complex current excitations are determined using the minimax approximation approach to suppress multiple wide band sectors while maintaining the sidelobe level and the main beam width almost unchanged. This method estimates the required number of pattern nulls to realize the prescribed suppressed sectors and locates the positions of the remaining nulls to control the optimum sidelobe level for the perturbed pattern in the minimax sense. The results show the formation of single and multiple wide band nulls at arbitrary prescribed directions. Also, this method can impose arbitrary symmetric and asymmetric nulls in the sidelobe region and hence, overcome the limitations of the linearized amplitude-only or phase-only solutions.

ISSN:0920-5071    

DOI:10.1163/156939399X01645

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

A genetic algorithm (GA) approach to reconstruction of the locations and shapes of two-dimensional impenetrable objects from scattered field data is presented. Local shape function method is used in the forward model, while a GA is used to seek for a solution minimizing the discrepancy between the measured and the estimated scattered fields in the inverse process. Some GA operators appropriate for the 2-D objects are proposed to achieve a high convergence rate. Several numerical results are presented to demonstrate the effectiveness of the GA approach.

ISSN:0920-5071    

DOI:10.1163/156939399X01654

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

A new technique based on the Galerkin Method of Moments (MoM) is presented to analyze discontinuities in microwave planar circuits. The technique is formulated to compute the admittance matrix of N -port-discontinuities. Each port is connected via a line to an excitation source. To assure the adaptation source-line, the normalized fundamental mode is used as a source. An integral equation is derived by applying the boundary conditions of the electromagnetic fields of the sources and of the circuit. The admittance matrix is obtained from the solution of the integral equation using the Galerkin method. This technique is implemented to compute a number of discontinuities in coplanar wave guides and finlines. Computed results show good agreement with the published data.

ISSN:0920-5071    

DOI:10.1163/156939399X01663

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor