Scattering by a rotating circular cylinder with finite conductivity

The effect of motion on the signal scattered by a rotating circular cylinder with finite conductivity is investigated. The problem is solved by means of the "instantaneous rest-frame" hypothesis. The analysis shows that a surface current must be taken into account to calculate the jump in the tangential magnetic field at the cylindrical surface. This holds even in the case of finite conductivity. For a perfectly conducting cylinder, the influence of the motion on the fields is negligible. This is shown by considering the limit of high but finitesigma.     

Quasi-static image theory for the bi-isotropic sphere

Image theory for the static point charge and the conducting sphere, produced by Kelvin's inversion theory, is extended to the bi-isotropic sphere, including the chiral sphere as a special case. Image expressions for the bi-isotropic sphere can be derived in a manner similar to that of the dielectric sphere except that the quasi-static problem now involves both electric and magnetic scalar potentials, coupled through the interface conditions at the spherical surface. The image is a combination of electric and magnetic line charges along the axis connecting the point charge and the center of the sphere, and their expressions are obtained through what can be labeled as finite Mellin transformation. The expressions derived can find application in more complete quasi-static analyses of interactions of bi-isotropic spherical particles in artificial bi-isotropic media     

FDTD 结合支持向量机反演各向异性材料电磁参数

本文首先介绍了太赫兹波导和3D打印技术的发展现状。3D打印作为一项新兴的技术,以数字模型文件为基础,运用粉末状金属或塑料等可粘合材料通过逐层打印的方法构造实体,打破了传统THz波导技术的局限性。本文介绍的3D打印THz波导利用聚合树脂作为打印材料,打印完成的THz波导在其传输通路上镀500nm的金,金的厚度足以支持THz传播。利用这种方法可以打印出直波导、三维弯曲面、三维Y劈和U型波导等多种结构。3D打印THz波导除传输损耗略高外,其传输模式及其特性与传统的金属波导基本一致,这种额外的传输损耗归咎于商业3D打印机的精度。     

随机分布树干层中球的电磁散射

本文用随机分布的有限长多层介质圆柱模拟树干层,用互易原理得到了位于粗糙地面上有限长多层介质圆柱与球目标的二阶散射场近似解析表达式.在L、C、X波段下,研究了位于此环境中金属球目标与周围树干相互作用的电磁散射,计算结果表明,在低频段时树干对目标影响大,低频段可以穿透植被层并应用于目标探测和识别.     

Three-dimensional electromagnetic scattering from inhomogeneousobjects by the hybrid moment and finite element method

The hybrid moment and finite element methods are used to obtain 3-D scattering and/or absorption from inhomogeneous, arbitrarily shaped objects. The surface of the object is approximated by triangles and the volume of the object is approximated by tetrahedrons. The electrical parameters are assumed constant in each tetrahedron. The Galerkin testing procedure is used. To avoid contaminations of spurious mode, a divergenceless vector basis function is used in finite elements. The calculated internal field and scattered field for a homogeneous sphere, a layered sphere, and a lossy prolate spheroid are compared with Mie series solutions and other numerical techniques. The accuracy and rate of convergence of the solution are discussed     

The Measurement of Conductivity and Permittivity of Semiconductor Spheres by an Extension of the Cavity Perturbation Method

A technique based on cavity perturbation theory is described with which one can determine the microwave conductivity and dielectric permittivity of a small sphere of completely arbitrary conductivity. These properties follow from the measured frequency shift and quality change occurring when the sample is inserted into a region of maximum electric field in a cavity resonator. The range of validity of the quasi-static internal field approximation is discussed, and curves are provided for extending the measuring technique beyond this range. The extended theory is valid for the entire conductivity range from zero to infinity. Measurements on several samples of known conductivity and permittivity in which the approximation is not satisfied are seen to agree with the theory. For highly conductive materials, the present method is closely related to the "eddy current loss" measuring technique discussed by others. The two methods are compared from the point of view of perturbation theory in order to determine their relative merits. Because the measuring technique employs a spherical sample, it may be applied profitably to materials with nonisotropic carrier nobilities and to semiconducting materials for which contact fabrication techniques are poorly known.     

Extended boundary condition integral equations for perfectly conducting and dielectric bodies: Formulation and uniqueness

The equivalence theorem is used to derive novel generalized boundary condition (GBC) integral equations for the tangential components of the electric and magnetic fields on the interfaces of a finite number of dielectric or conducting scatterers. Closed surface, plane, and line extended boundary conditions (EBC) equivalent to the GBC are introduced. The GBC integral equations can now be replaced by any of these EBC integral equations whose solutions are unique and easy to obtain numerically using the moment method. A perfectly conducting sphere and a dielectric sphere in the electrostatic field of two equal and opposite point charges are presented as simple examples of the general procedure.     

Realistic conductivity geometry model of the human head forinterpretation of neuromagnetic data

In this paper, the computational and practical aspects of a realistically-shaped multilayer model for the conductivity geometry of the human head are discussed. A novel way to handle the numerical difficulties caused by the presence of the poorly conducting skull is presented. Using our method, both the potential on the surface of the head and the magnetic field outside the head can be computed accurately. The procedure was tested with the multilayer sphere model, for which analytical expressions are available. The method is then applied to a realistically-shaped head model, and it is numerically shown that for the computation of B, produced by cerebral current sources, it is sufficient to consider a brain-shaped homogeneous conductor only since the secondary currents on the outer interfaces give only a negligible contribution to the magnetic field outside the head. Comparisons with the sphere model are also included to pinpoint areas where the homogeneous conductor model provides essential improvements in the calculation of the magnetic field outside the head.     

Magnetostatic Surface Waves in Ferrite Slab Adjacent to Semiconductor (Short Papers)

Magnetostatic surface waves propagating along the ferrite slab adjacent to a semiconductor are discussed in this paper. Our numerical results indicate that the conductivity of the semiconductor plays an important role in the determination of the dispersion relation in the case of nondrifting carriers. The backward wave appears for a finite value of the conductivity.     

Microwave Properties of Thin Films with Apertures

The theory of microwave transmission through thin-film screens of finite conductivity which contain circular, elliptical, and rectangular apertures and which are placed in the transverse plane of an X-band rectangular waveguide is developed. The theory is an extension of Bethe's work in which only screens with infinite conductivity were considered. Experimental verification of the theory is achieved by use of vacuum deposited thin-film screens, and the results are compared with the transmission properties of identical screens of thick foils of the same material. Significant differences are noted in the real and imaginary components of impedance presented by the thin-film and the foil screens with identical apertures. The finite conductivity of the screen is low enough to support an appreciable tangential component of electric field at the film surface.     

The transient electric field response of an array of parallel wires on the Earth's surface

The transient electric field response of a step-function current excited cable lying on the surface of a homogeneous and isotropic earth is considered. Attention is focused upon that portion of the response at the earth's surface where the field is either constant or decaying with time. This information appears to have application in the measurement of the earth's conductivity using a single cable with finite length and grounded at both ends. Then, by superposition, the response of an array of equally spaced parallel cables is also studied.     

Absorption enhancement of silicon solar cell with Ag nanoparticles by surface plasmons resonance

The absorption enhancements of silicon layer in silicon solar cells with three kinds of Ag nanoparticles including sphere, cylinder and cuboid are studied by the finite difference time domain （FDTD） method, respectively. The results show that the light absorption of silicon is significantly improved due to the localized surface plasmon （LSP） reso- nance. The relations of the absorption enhancement with the parameters of nanoparticles are thoroughly analyzed. The optimal absorption enhancement can be achieved by adjusting the relevant parameters. Among the three types of Ag nanoparticles, i.e., sphere, cylinder and cuboid, the silicon with the cubical Ag nanopaticles shows the most efficient absorption enhancement at optimal conditions, its maximum absorption enhancement factor is 1.35, and that with the spherical Ag nanopaticles gets the lowest absorption enhancement. The work is useful for the further theoretical study and design for the plasmonic thin-film solar cell.     

Understanding Thermal Insulation in Porous,Particulate Materials

Silica hollow nanosphere colloidal crystals feature a uniquely well‐defined structure across multiple length scales. This contribution elucidates the intricate interplay between structure and atmosphere on the effective thermal diffusivity as well as the effective thermal conductivity. Using silica hollow sphere assemblies, one can independently alter the particle geometry, the density, the packing symmetry, and the interparticle bonding strength to fabricate materials with an ultralow thermal conductivity. Whereas the thermal diffusivity decreases with increasing shell thickness, the thermal conductivity behaves inversely. However, the geometry of the colloidal particles is not the only decisive parameter for thermal insulation. By a combination of reduced packing symmetry and interparticle bonding strength, the thermal conductivity is lowered by additionally 70% down to only 8 mW m^?1 K^?1 in vacuum. The contribution of gaseous transport, even in these tiny pores (<200 nm), leads to minimum thermal conductivities of ≈35 and ≈45 mW m^?1 K^?1 for air and helium atmosphere, respectively. The influence of the individual contributions of the solid and (open‐ and closed‐pore) gaseous conductions is further clarified by using finite element modeling. Consequently, these particulate materials can be considered as a non‐flammable and dispersion‐processable alternative to commercial polymer foams.     

Fabrication and characterization of urchin-like polyaniline microspheres using lignosulfonate as template

An urchin-like conducting microsphere was fabricated by synthesizing polyaniline, PANI, with lignosulfonate, LGS. FESEM images showed that this special PANI structure was controllably formed because the pure PANI presented only a nanofiber formed mat, and the PANI/LGS mixture with the ANI/LGS ratio (%) at 36/1 and 18/1 formed spheres while the PANI nanofibers lied on spherical surface, and only at 9/1 led the PANI nanofiber to stand on sphere surface in the urchin-like structure. Taking the pure PANI as a reference, the urchin-like PANI/LGS microsphere has been found to have enhanced conductivity and thermal stability.     

Asymptotically dense spherical codes. I. Wrapped spherical codes

A new class of spherical codes called wrapped spherical codes is constructed by “wrapping” any sphere packing Λ in Euclidean space onto a finite subset of the unit sphere in one higher dimension. The mapping preserves much of the structure of Λ, and unlike previously proposed maps, the density of the wrapped spherical codes approaches the density of Λ as the minimum distance approaches zero. We show that this implies that the asymptotically maximum spherical coding density is achieved by wrapped spherical codes whenever Λ is the densest possible sphere packing     

理论分析毫米波螺旋线行波管慢波系统导体和介质损耗

该文基于夹持杆分层螺旋带模型和3维电磁场模型分析,详细研究了毫米波螺旋线行波管慢波系统的导体和介质损耗。螺旋带模型中介质损耗考虑为纵向传播常数的虚部,给出电磁场的解析解,导体损耗由螺旋线和管壳表面的面电流不连续性获得。3维电磁场模型分析通过本征模法,求解单周期结构的品质因数和周期储能,获得有限导电率导体和夹持杆陶瓷损耗角带来的慢波系统高频损耗。结果表明,毫米波段螺旋线的导体损耗和夹持杆的介质损耗远大于管壳导体损耗,介质损耗与陶瓷损耗角呈线性关系,对高频损耗的影响不可忽略。     

Image of a coil in relation to the gauging of distance of metal surfaces

The impedance of a circular loop in the presence of a plane metal surface is discussed, and it is shown to have certain features relevant to the use of coils as distance gauges. These features permit the elimination of those gauging errors that arise from the finite conductivity and permeability of the metal.     

Effect of conductivity uncertainties and modeling errors on EEGsource localization using a 2-D model

This paper presents a sensitivity study electroencephalography-based source localization due errors in the head-tissue conductivities and to errors in modeling the conductivity variation inside the brain and scalp. The study is conducted using a two-dimensional (2-D) finite element model obtained from a magnetic resonance imaging (MRI) scan of a head cross section. The effect of uncertainty in the following tissues is studied: white matter, gray matter, cerebrospinal fluid (CSF), skull, and fat. The distribution of source location errors, assuming a single-dipole source model, is examined in detail for different dipole locations over the entire brain region. We also present a detailed analysis of the effect of conductivity on source localization for a four-layer cylinder model and a four-layer sphere model. These two simple models provide insight into how the effect of conductivity on boundary potential translates into source location errors; and also how errors in a 2-D model compare to errors in a three-dimensional model. Results presented in this paper clearly point to the following conclusion: unless the conductivities of the head tissues and the distribution of these tissues throughout the head are modeled accurately, the goal of achieving localization accuracy to within a few millimeters is unattainable