线电流源激励下无耗异向介质层覆盖导体圆柱电磁特性的研究

研究了线电流源平行放置在无耗异向介质层覆盖的无限长导体圆柱附近模型的电磁特性.首先，给出了电磁模型描述，求出此电磁模型的精确解.其次，利用精确解进行数值计算，得到几何参数和电磁参数不同情况下电磁模型的近场图形，并通过方向性系数和归一化辐射阻抗研究其远场特性.利用异向介质的负折射特性，在异向介质层的近场图形内得到明显的“焦点”，这个焦点在普通介质层中是不存在的.该电磁模型同前人方法进行了比较，验证了本电磁模型的有效性. 关键词： 线电流源 异向介质 方向性系数 辐射阻抗     

光波辐射下正多边形异向介质柱聚焦特性分析

基于时域有限差分方法,建立了光波辐射下正三角形、正四边形和正六边形无耗异向介质柱的二维电磁模型.分析了其近场特性,并对模型的有效性进行了验证.仿真结果表明:即便对于εr和μr都很小且无耗的正多边形异向介质柱,当平面光波沿正多边形顶角角平分线方向传播时,在异向介质柱内不会发生电磁聚焦.     

线电流源激励下正多边形双负介质柱的聚焦特性

 利用时域有限差分法分别分析了线电流源激励下,正三角形、正四边形和正六边形双负介质柱的近场分布。发现对于介电常数和磁导率都很小且无耗的双负介质柱,当激励位于正多边形顶角角平分线的外延长线上时,在双负介质柱内部不会出现 焦点。将该电磁模型同Arslanagic等人介绍的方法进行了比较,证明了电磁模型的有效性。     

加载异向介质的太赫兹频段高方向性喇叭天线

设计了一款工作在太赫兹波段的类三角形异向介质单元,对传输系数进行处理得到单元的等效介电系数和有效磁导率,数据结果表明该单元在太赫兹波段出现双负频段.通过将该类三角形异向介质应用于太赫兹波段的喇叭天线,利用零折射率特性制作天线填充介质实现电磁波的定向辐射,减小天线的后向波能量损耗,达到提高天线方向性系数和增益的目的.实验结果表明:加载类三角形异向介质单元后的喇叭天线的后向波辐射减弱、前向辐射效率增强,方向性系数提高了46.86,增益提高了2.77dB.     

远近场混合源的波束解卷积定位

提出了定位远近场混合源的波束解卷积技术,针对非相干远近场混合声信号的线列阵观测结果,推导了其常规波束形成(CBF)空间谱中固有的广义二维卷积数学关系,利用Richardson-Lucy算法实现波束能量聚焦以获得近场目标的精确空域参数估计,通过混合源协方差矩阵向近场流形的正交补空间投影操作提取远场分量,并分析得到其内在的一维卷积关系,然后通过角度域波束解卷积进行远场信号的波达估计。仿真分析表明,所提方法提升了CBF谱的空域分辨力,通过投影映射隔离近场分量后实现了混合源的分离。与现有方案相比,所提算法针对远场信源可实现10 dB的背景噪声级抑制。     

有耗介质层上多导体传输线的电磁耦合时域分析方法

目前,针对空间电磁场作用有耗介质层上传输线的电磁耦合,仍缺乏有效的数值分析方法.因此,本文提出一种高效的时域混合算法,很好地解决了有耗介质层上传输线电磁耦合建模难的问题.首先,对经典传输线方程进行改进,推导了适用于有耗介质层上多导体传输线电磁耦合分析的修正传输线方程.然后,结合时域有限差分方法和相应插值技术,求解修正传输线方程,获得多导线及其端接负载上的电压和电流响应,并实现空间电磁场辐射与多导线瞬态响应的同步计算.最后,通过相应计算实例的数值模拟,与CST软件的仿真结果进行对比,验证了时域混合算法的正确性和高效性.     

合成近场校准基阵的方法研究

本文利用单根的束控线阵来构造一个虚拟近场校准基阵,使换能器能在它的近场直接测量辐射声场特性。通过利用改进的矩阵稳定技术,解决了求解束控线阵的束控系数时最小二乘解的稳定性和实用性问题。借助计算机仿真,研究了特定线阵条件下,合成近场测量基阵（Ｓ－ＮＦＣＡ）的近场空间的平面波均匀性与线阵间的间距、线阵的工作频率之间的关系。并以小球源为阵元合成一虚拟平面ＮＦＣＡ,用它对一平面喇叭换能器的近场测量得到远场指向性,并与远场实测结果比较,得到了相当吻合的结果。     

流体-孔隙介质圆柱界面波传播特性

研究流体-孔隙介质圆柱界面波传播特性,分析孔隙介质孔隙率等参数对频散曲线的影响。理论上建立了无限大流体包裹孔隙介质圆柱界面波的模型,利用孔隙介质弹性波动理论,通过数值模拟计算得到流体-孔隙介质圆柱的频散曲线及时域波形,并分析了孔隙介质为开孔和闭孔状态下孔隙介质圆柱半径、孔隙率及渗透率对频散曲线的影响。结果表明,时域上斯通利波可以被明显区分开,孔隙介质圆柱半径的变化改变了圆柱尺度,孔隙率的变化改变了孔隙介质的纵、横波波速,因此对于斯通利波频散曲线的影响较大。而渗透率的变化既不改变圆柱的尺度也不改变孔隙层的纵、横波速度,因此对斯通利波频散曲线影响较小。      

1.7—2.7GHz宽频带小单元异向介质设计及其介质参数提取

提出一种单元电尺寸小、工作频带宽、损耗小、结构简单的异向介质设计方案，在1.7—2.7 GHz上所设计的异向介质结构单元电尺寸小于0.035，相对带宽达到45.5%，在整个工作频带上单个结构单元传输损耗小于0.75 dB.对由上述异向介质单元构成的半无限大异向介质平板的电磁波反射、透射特性进行了数值仿真分析，并提取出了电磁波在该异向介质平板中传播时的波数、相速、折射率以及该异向介质平板的有效介电常数和有效磁导率等一系列电磁特性参数，仿真与计算结果表明复波数的实部、相速以及折射率的实部在1.7—2.7 GHz的范围上为负值，并且在相同频带上，有效介电常数和有效磁导率的实部同时为负值，从而有效地验证了“后向行波效应”、“负折射效应”、“双负效应”等异向介质特有的电磁特性，对上述异向介质的存在性给予有力证明. 关键词： 异向介质 宽频带 小单元 介质参数     

各向异性有耗介质板介电系数和电导率的反演

简述了各向异性介质FDTD方法，并用FDTD方法分析了三维各向异性有耗介质板的瞬态后向散射.根据各向异性介质板后向散射与入射电磁波极化方向有关的特点，利用其后向RCS的谐振特性和后向散射场的时域波形特点反演有耗介质板的横向介电系数和电导率.数值模拟结果表明本反演方法可行，且方便、快捷. 关键词： 各向异性板 FDTD方法 有耗介质 谐振 反演     

Properties of Near and Far Fields for the Electric Line Source Illumination of a Lossy Metamaterial Covered Dielectric Cylinder

The electric field properties of the electric line source radiation in the presence of a lossy metamaterial covered dielectric cylinder are investigated. Firstly, the electromagnetic model configuration is given. The exact solution of the electromagnetic is presented. Secondly, the near field and the far field properties are investigated according to the electromagnetic and geometrical parameters. The different “focus” phenomenon, the directivities and the normalized radiation resistance are discussed and important conclusions are obtained. During all the numerical computation, the lossy conventional material covered dielectric cylinder is utilized as the reference.     

Properties of near and far fields for the electric line source illumination of a lossless metamaterial covered conductor cylinder

This work concentrated on an ELS (electric line source) radiating parallel to a lossless metamaterial covered infinitely long conductor cylinder. First, the exact analytical solution of the electromagnetic model is derived. Second, the numerical results based on the solution are given. Under different geometric and electromagnetic parameters, the patterns of the near field are obtained; the directivity and normalized radiation resistance are presented to discuss the properties of the far field. Because of the negative refraction of the metamaterial, it can be seen that there is a distinct “focus” in the metamaterial layer from the near field pattern, which does not exist in conventional material layer. The presented electromagnetic model is compared with the former, simpler model through the directivity and its validity is proven. PACS 78.70.Gq; 81.05.Zx; 84.40.Ba     

Modeling of arbitrary cross-sectional cylinder using N circular cylinders for the scattering calculations

Exact solution of the electromagnetic wave scattering by N dielectric cylinders is presented by using matrix formulation. To check this present method, two comparisons between exact solutions for a single circular conducting and dielectric cylinder and this model composed of N=25 circular cylinders are made. Numerical results of conducting and dielectric square cylinder has been also checked with well-known result (B.E.M). The scattering patterns and the near field distributions in space are presented for the concave, convex and dielectric circular cylinder with conducting reflector.     

单负材料异质结构中损耗诱导的场局域增强和光学双稳态

光学双稳态的阈值取决于非线性材料中的场局域程度, 场局域越强阈值越低. 而材料的损耗是影响场局域强弱的重要因素. 之前, 人们普遍认为, 增加损耗会削弱场局域, 不利于降低阈值. 本文研究了由磁单负材料和电单负材料组成的异质结构中光学双稳态现象, 发现随着损耗的增大, 其阈值可以呈现先降后升的非单调变化. 进一步研究表明, 异质结构界面处的电磁场强度随着损耗增大呈现先降后升的非单调变化, 即增加损耗也有可能增强场局域. 研究结果揭示了场局域程度与材料损耗之间的非单调依赖关系, 为设计开发非线性功能器件提供了新的思路.     

Calculation of the near fields for the scattering of electromagnetic waves by multiple infinite cylinders at perpendicular incidence

The near field solution for the scattering of a plane monochromatic electromagnetic wave by an ensemble of parallel infinite dielectric cylinders at perpendicular incidence is presented in this paper. The solution is given for the calculation of the electric and magnetic near fields and the Poynting vector. A MATLAB program has been developed to solve the near field formulas which is introduced and validated. The near to far field transition as well as formation and transport of photonic nanojets have been calculated for multiple cylinder scattering.     

Electromagnetic scattering from a perfect electromagnetic conductor circular cylinder coated with a metamaterial having negative permittivity and/or permeability

Electromagnetic scattering of an incident uniform plane wave from a perfect electromagnetic conductor (PEMC) circular cylinder, coated with a metamaterial is investigated theoretically. In the analysis the coating layer may be double-positive (DPS), double-negative (DNG), epsilon-negative (ENG) or mu-negative (MNG). It is assumed that both PEMC cylinder and the coating layer are infinite along the cylinder axis. Both parallel and perpendicular polarization cases are considered for the analysis. Comparison between the monostatic and bistatic echo widths, of a PEMC cylinder coated with metamaterial and coated with ordinary dielectric material (DPS), is presented. The numerical results are compared with the published literature, and comparison is found to be in good agreement.     

Image brightening in samples of high dielectric constant

An analytic solution is given for the electromagnetic problem of a lossy dielectric cylinder of infinite length, irradiated by a circularly polarized radiofrequency (RF) magnetic field; the NMR-active components of the field inside the cylinder are projected out by transforming the RF Hamiltonian to the rotating frame and retaining only those terms independent of time; it is noted that the resulting cartesian field components are required to be real. The squared magnitude of the NMR-active fields are then used to calculate the gradient-recalled images of the cylinder, for small tip angles of the magnetization; and the result is shown to predict almost quantitatively the intensity patterns of experimental proton images at 3.0 and 4.0T, in a cylindrical phantom of radius 9.25cm, filled with 0.05M aqueous NaCl. In particular, the artifactual brightening at the center of the recorded image is convincingly reproduced in a simulation, whose underlying model excludes wave propagation along the direction of the cylinder axis. Formation of the artifact is explained in terms of the focussing of the RF magnetic field at the center of the cylinder, as illustrated by contour plots showing the time evolution of the rotating flux. An extended electromagnetic model--having the dielectric cylinder enclosed in a long, shielded volume resonator (e.g., of bird cage type)--is then sketched. The mathematical details appear in Appendix A; and the simulated images are shown to be virtually indistinguishable from those of the simpler original model. The theory of the Q, or quality factor, of the dielectric cylinder--considered itself as a resonant object--is developed for the enclosed cylinder model, where flux containment by the shield permits an unambiguous treatment of both the stored energy and the radiative losses. This is extended to treat the Q of a lossy dielectric sphere without shielding. Further plots of flux contours are given for the sphere, excited at 208 MHz with a uniform circularly polarized field, as well as by a surface coil, and for the enclosed cylinder in the range 140-160 MHz. It is then argued that the center brightening artifacts in magnetic resonance images are due to the underdamped dielectric resonance of the sample, i.e., at Q >0.5, while the overdamped condition, Q < 0.5, leads to exclusion of flux from the center, i.e., to the classic skin effect. The term "dielectric resonance" is shown to require careful interpretation for mixed-mode excitation, such as occurs with a surface coil. An extended reciprocity formula for NMR reception, valid for an arbitrary electromagnetic Green's function, is also given in Appendix B.     

Closed solution to electromagnetic scattering of a plane wave by an eccentric cylinder coated with metamaterials

Closed series solution of electromagnetic scattering by an eccentric coated cylinder is achieved in matrix form. Diffracted field patterns are examined for an incident plane transverse magnetic (TM) wave. The solution is found by the classical separation of variables technique and the translational addition theorems. Wave transformation and orthogonality of the complex exponentials are employed to produce an infinite series. Numerical results are then shown by shortening the infinite series to a limited number of terms. Solutions are discussed for a dielectric or a metamaterial coating.