摘要:

A three-objective Pareto genetic algorithm is applied to the design of practical impedance-modulated feeds for log-periodic monopole arrays (LPMAs) mounted on arbitrary structures. Traditional design techniques do not permit the synthesis of feed structures for arrays on complex grounding structures. The multimodality of the search space of the feed-network design parameters precludes the use of standard gradient-search methods. The use of Pareto genetic algorithms allows for the study of tradeoffs between competing performance goals, and permits the generation of a database of feed designs that represent optimal performance tradeoffs from which one can select a posteriori. Numerical results are presented for the design of an LPMA mounted on a realistic wing.

ISSN:0920-5071    

DOI:10.1163/156939399X01032

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Expressions are derived for the electromagnetic fields due to a circular cylindrical axially-directed electric current source, a radial electric ring source, and magnetic frill in free space and above an infinite ground plane. The expressions are valid everywhere in space including the source region. The fields are given in cylindrical coordinates, and the results are applicable, among others, to the analysis of coaxially-fed cylindrical antennas.

ISSN:0920-5071    

DOI:10.1163/156939399X01041

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The dominance of creeping wave mode of the backscattered field from a coated sphere is investigated. The diffracted field is entirely associated with the creeping wave on the coated sphere which consists of infinite number of TE and TM modes. The magnitude of each creeping wave mode is determined by two factors: the attenuation constant and the diffraction coefficient. These two factors are the functions of the size parameters such as the thickness of coating and the radius of the conducting sphere, and the coating material. The contribution of the different creeping wave mode to the total diffracted field is compared and the change of the dominant creeping wave mode with the changes in the thickness of coating is also presented.

ISSN:0920-5071    

DOI:10.1163/156939399X01050

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The inverse scattering problem for a stratified bianisotropic slab is considered in the frequency domain. The constitutive parameters are reconstructed by minimizing a suitable objective functional. Exact and explicit expressions for the gradients are derived in a compact form through the matrix trace operator.

ISSN:0920-5071    

DOI:10.1163/156939399X01069

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Coupled equations for the electric and magnetic fields describing pulse-envelope propagation in nonlinear, dispersive single-mode fibers in the frequency domain are presented. These equations contain frequency-dependent susceptibilities and conserve energy and momentum, thus enabling calculation of nonlinear, dispersive, and backscattering effects. Modeling of stimulated Raman scattering is improved by using a modified Sellmeier series. Two different pseudo-spectral algorithms are developed for simulation of signals propagating in one direction only and in both directions, respectively. Numerical results for the former are compared to those computed with the (generalized) nonlinear Schrödinger equation using the Lorentzian approach for describing stimulated Raman scattering. We show that significant differences between the models occur for bandwidths beyond 2.5 THz.

ISSN:0920-5071    

DOI:10.1163/156939399X01078

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

In this paper, the computation of the resultant electromagnetic (EM) field distribution inside a dielectric prolate spheroidal human head model due to the radiation from a mobile phone dipole is conducted. By means of the dyadic Green's function technique and the principle of scattering superposition, the EM fields are expressed in terms of appropriate spheroidal vector wave functions. The coupled coefficients of scattered EM fields in human head are obtained numerically and the specific absorption rate (SAR) distributions inside the spheroidal head models are calculated. The various SAR distributions versus the distance between the source and the head, at different operating frequencies and handset inclination angles are presented and discussed.

ISSN:0920-5071    

DOI:10.1163/156939399X01087

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Methods previously developed for designing linear array monopulse antennas with relatively simple feed network requirements are extended to allow the suppression of jamming signals in both sum and difference modes.

ISSN:0920-5071    

DOI:10.1163/156939399X01096

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Two classes of formulations are prevalent on the perfectly matched layer (PML) concept for the reflectionless absorption of electromagnetic waves. In the first, additional degrees of freedom modify the curl and div operators of Maxwell's equations. This results in the so-called non-Maxwellian PML. The original Berenger formulation belongs to this class. The non-Maxwellian PML can be systematically derived by an analytic continuation of the coordinate space to a complex variables coordinate space (complex-space) which permits the extension of the PML to general geometries and media. In the second class of PML formulations, the additional degrees of freedom are entirely incorporated into modified constitutive tensors and the usual Maxwell's equations are recovered. This results in a Maxwellian PML. Interestingly enough, for all cases where the non-Maxwellian PML was derived, a Maxwellian PML was also later derived. This suggests a duality between the formulations and the possibility of a fundamental reason behind the existence of the Maxwellian PML. In this work, we review the PML concept using the language of differential forms to (i) explain the deeper reason allowing for the ubiquitous presence of the Maxwellian PML; (ii) to provide the general framework which unifies the various PML formulations; and (iii) to show that, in principle, many other classes (hybrid) of PML formulations can be derived in the frequency-domain. This is done by introducing a novel, geometrical interpretation of the PML in terms of a change on the metric of space and exploring the metric independence of Maxwell's equations unfolded by such a language.

ISSN:0920-5071    

DOI:10.1163/156939399X01104

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The method of decomposition of the time reversal operator has been introduced with the aim of focusing acoustic waves on a selected target. It is based on the derivation of the invariants of the time reversal operator, which is linked to the transfer matrix of the array of transducers insonifying a time invariant scattering medium. The theoretical and numerical results concern independent point-like scatterers. Here, we apply numerically the same technique to electromagnetic waves, and all the interactions between the various scatterers are taken into account. This method may be very useful to detect and localize buried objects. With this goal, we have investigated the role of polarization and of the geometrical or electromagnetic parameters, as well as the ability of accurately focusing on a scatterer. The sensitivity of the results to the initial set of data, namely the number of measurements or the accuracy, is also studied.

ISSN:0920-5071    

DOI:10.1163/156939399X01113

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

ISSN:0920-5071    

DOI:10.1163/156939399X01122

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor