A simple theory for optimizing finite width ELF magnetic fieldshields for minimum dependence on source orientation

The effectiveness of single layer, finite width, planar extremely low frequency (ELF) magnetic field shields is strongly dependent upon the orientation of the field sources. Since source information is difficult to obtain, the issue of designing shields which are independent of source orientation is important. Here, a simple analytic model for shielding by multiple layer, finite width, planar shields constructed from perfect electric and perfect magnetic material is presented. This is augmented by a study of conditions for which the perfect material approximation is valid. The simple model is used to determine strategies for designing shields which are independent of source orientation. It is found that two layer perfect electric/magnetic shields perform significantly better than single layer shields     

Design of Shielded Cables Using Saturable Ferromagnetic Materials

Previously, a numerical solution was evolved that may be used to analyze the effect of material saturation on the low frequency shielding characteristics of a thin ferromagnetic cable shield. Numerical solutions are difficult to employ in design problems and so a simplified theory has also been developed. This supplemental theory, described here, yields simple equations useful in the design of ferromagnetic cable shields to provide protection against some anticipated intense low frequency environment.     

A sheet impedance approximation for electrically thick multilayered shields

This paper describes an extension of the sheet impedance concept to treat inhomogeneous or multilayered shields that may be thick in terms of material shield wavelengths. For shields with magnetic materials, a simple relation between the equivalent electric and magnetic currents representing the shield is obtained. This allows the magnetic current to be treated as a dependent unknown and the electric current to be found as the solution of a single surface integral equation shown to be a perturbation of that for a perfect electric conducting (PEC) surface. By using the proper interior equivalent problem, it is shown that the method produces accurate and stable results for shielding by a rectangular box.     

Low-Frequency Shielding Effectiveness of Nonuniform Enclosures

This paper presents a quasi-static approximate solution to the magnetic shielding of several nonuniform enclosures using the integral form of Maxwell's equations and insight gained from other approaches. The solution is called quasi-static as the assumptions made are from physical arguments based on low-frequency cases where the enclosure size is much less than a wavelength. The integral form of Maxwell's equations is used to obtain a first order correction to the static solution to obtain induced currents in the time-varying case. A cylindrical shell immersed in an axial magnetic field is used to illustrate the method, which is then extended to derive a formula for a similarly excited rectangular enclosure. These shields are seen to behave like a low-pass filter. Although the enclosure dimensions are small compared to the wavelength, the skin depth effects in the walls cannot be neglected even for relatively thin material as usually encountered in an enclosure. These skin effects are included in the analysis and experimental checks performed on a variety of enclosure sizes and materials, excited by a Helmholtz coil show agreement within two decibels over the 4-octave frequency range examined. No one can say whether this method offers a better solution to the shielding problem, as all solutions are approximate, but the author attempts to present an alternative formulation that aids in understanding the physical processes involved in the shielding effectiveness of an enclosure and fills some of the gaps between the plane-wave analysis and circuit approaches presently used.     

Losses in the PEMC Boundary

Perfect electromagnetic conductor (PEMC) was recently introduced to generalize both the perfect electric conductor (PEC) and the perfect magnetic conductor (PMC), which are used as boundary material. In this paper, an extension of PEMC boundary conditions is made to a more realistic model with small losses (“good electromagnetic conductor”). It is shown that the antisymmetric boundary impedance dyadic defining the ideal PEMC boundary cannot alone carry any losses and a symmetric component must be added that actually dominates the lossy behavior of the boundary. As an example, a slab of low-loss gyrotropic wave-guiding material is studied and shown to represent the lossy counterpart of the PEMC boundary. The effect of losses is demonstrated by considering plane-wave reflection from an imperfect PEMC plane.     

Geometrical aspects of magnetic shielding at extremely lowfrequencies

The magnetic shielding effectiveness for closed and open shield structures is studied at extremely low frequencies. Analytical solutions are used for simple geometries, while more complex structures are evaluated using a finite-element method. Both highly conductive and ferromagnetic materials are studied, and their different shielding behavior is shown. Ferromagnetic shields give good results for small and closed shields and they also give a large field attenuation at close range to the source for open shield geometries. Highly conductive materials, on the other hand, are found to be suitable for large shield sizes. The attenuation is, however, reduced in the close vicinity of the source. Comparisons of numerical results with analytical calculations and measurements confirmed the high accuracy of the finite-element model     

Interaction of Magnetic Fields and Ferromagnetic Shields

An analytic procedure is developed for the purpose of determining the effectiveness of ferromagnetic shields in shielding against magnetic fields. The basic approach is to separate the magnetization relation of a ferromagnetic material into regions in which each region is characterized by a constant permeability. Maxwell's equations are then solved in each time-varying geometric region (which correspond to the regions of the magnetization relation) and the solutions are matched at interfaces. This procedure permits solutions for nonlinear shielding problems to be readily obtained using linear techniques.     

Electromagnetic Pulse Transmission Through a Thin Sheet of Saturable Ferromagnetic Material of Infinite Surface Area

A numerical solution is presented for determining the shielding properties of a thin sheet of saturable ferromagnetic material of infinite surface area. Several examples are given to indicate the behavior of ferromagnetic shields in an intense electromagnetic environment. These examples illustrate that only a few numerical results are required to determine the electric field transmitted through the material for a given incident pulse of any amplitude.     

不完全屏蔽腔体辐射耦合电场增强效应防护方法

针对武器装备机箱内部电磁辐射防护的技术需要,从不完全屏蔽腔体辐射耦合电场增强效应形成机理出发,对孔缝耦合及贯通导体耦合导致的屏蔽腔体内部局部电场增强效应防护方法进行了研究.仿真计算了屏蔽腔体内部加载吸波材料、腔体分区隔断以及贯通导体安装金属导管等防护方法对不完全屏蔽腔体电磁耦合的影响,研究结果表明:在屏蔽腔体内部加载吸波材料能够有效降低由于腔体谐振产生的电场增强效应,相同的吸波材料放置在强场位置防护效率会更高;采用分区隔断的屏蔽腔体能够提高腔体的谐振频率,大幅降低腔体内大部分位置的电磁耦合能力;贯通导体通过金属导管进入屏蔽腔体能够有效降低贯通导体的电磁耦合能力,削弱屏蔽腔体内部的电场增强效应,屏蔽腔体内部及外部的金属导管长度越长,其防护效果越明显.     

边界条件微扰法计算非理想波导的传播常数

考虑到非理想导体波导壁金属损耗对传播常数的影响,对理想金属边界条件进行一阶近似微扰,得到了非理想波导的传播常数。和传统微扰法相比,能够在截止频率附近较为精确地计算衰减常数,也适用于导体损耗较大的情况;同时能给出非理想导体波导壁对相位常数的影响。将结果与Ansoft HFSS软件的仿真结果进行比较,两者具有较好的一致性。同时给出了边界条件微扰法解与解析解及传统微扰法解在一定条件下的联系,表明边界微扰法比解析法实现简单,比传统微扰法具有更广泛的适用性。     

A sheet impedance approximation for electrically thick materialshields

This paper describes a simple extension of the sheet impedance concept to treat electromagnetic (EM) shields that may be thick in terms of material shield wavelengths. For magnetic shields, a simple relation between the equivalent electric and magnetic currents representing the shield is obtained, and the electric current is found as the solution of a single surface integral equation that is shown to be a simple perturbation of that for a perfect electric conducting (PEC) surface. Finally, it is shown that the computation of the small interior fields of good shields requires the use of the proper PEC interior equivalent problem     

Shielding effect of hollow chiral sphere

The low-frequency shielding effect of a spherical layer is studied. The layer is made of a chiral material and it is electromagnetically characterized with three material parameters: permittivity, permeability, and chirality. Due to chirality, there is magnetoelectric coupling. The electric and magnetic shielding effects are derived and are shown to be functions of the three material parameters and also the relative thickness of the layer. Illustrations display the effects of the various parameters on the shielding, which is different for the magnetic and electric fields. Among the special effects is that the shielding increases rapidly as the chirality parameter approaches the refractive index of the shell. This makes chiral shells in principle effective shields, and in the future they may offer an alternative to conducting materials for novel shielding applications     

An Equivalent Surface Source Method for Computation of the Magnetic Field Reduction of Metal Shields

A numerical method for computation of the resultant quasi-static magnetic field in the vicinity of parallel wires and metal shields is presented. The primary magnetic field source is time-harmonic currents in wires. This field is modified by conducting magnetic and/or nonmagnetic shields. The material is assumed to be linear under the applied source field. The shielding effectiveness can be estimated by a comparison between the primary and the resultant field. The reaction magnetic field is expressed by a sum of fields caused by equivalent single- and double-layer sources distributed on the shield surface. Integral equations for unknown distributions of these equivalent sources are derived from the Green's second identity implemented inside and outside the shields. These equations are coupled integral equations, and are solved by the moment method. Numerical results of the resultant (shielded) magnetic field obtained with the proposed method are compared with the results of: 1) analytically solvable problems; 2) measurements; and 3) two different numerical methods.      

Influence of Conductor Shields on the Q-Factors of a TE /sub 0/ Dielectric Resonator

Two approaches due to the complex frequency and to the perturbation theory are described to compute accurately the Q-factors of the circularly-symmetric TEO modes for dielectric rod resonators placed between two parallel conductor plates and in a conductor cavity. These techniques allow us to estimate separately the Q-factors due to radiation, conductor, and dielectric losses from only the computation of resonant frequencies by means of the mode-matching method. Validity of the theories is verified by experiments. The influence of the conductor shields on the Q-factors is discussed from the computed results. A possibility of realizing high-Q dielectric resonators is suggested.     

Target identification of coated objects

We consider the three-dimensional electromagnetic inverse scattering problem of determining information about a coated object from a knowledge of the electric far-field patterns corresponding to time harmonic incident plane waves at fixed frequency. We assume that the obstacle is either a perfect conductor coated by a thin dielectric layer or a dielectric coated by a thin layer of a highly conducting material, i.e., the coated portion of the boundary is modeled by either an impedance boundary condition or a conductive boundary condition. No a priori assumption is made on the connectivity of the scattering obstacle nor on the extent of the coating, i.e., the object can be fully coated, partially coated, or not coated at all. We present an algorithm based on the linear sampling method for reconstructing the shape of the scattering obstacle together with an estimate of either the surface impedance or surface conductivity. Numerous numerical examples are given showing the efficaciousness of our method.     

A circuital approach to estimate the magnetic field reduction ofnonferrous metal shields

The reduction of magnetic fields is a topic of concern to the electric utility industry and the electromagnetic compatibility community. One technique to reduce the magnetic fields is to use metal plates and enclosures for shielding, in addition, knowledge or the loading effect of a conducting structure in close proximity to power cables is needed for estimating power loss. Until now there has not been a simple and efficient method to calculate the low frequency magnetic shielding of, nor power loss caused by, two-dimensional shields. This paper presents a method for the rapid numerical estimation of the magnetic shielding efficiency and power loss of nonferrous metal structures. The method was validated analytically and experimentally     

Low-Frequency Shielding of a Circular Loop Electromagnetic Field Source

The problem of low-frequency shielding of a loop axially perpendicular to a plane shield of infinite extent is analyzed by 1) the thin shield work of S. Levy, 2) solution of the vector wave equation, and 3) application of the transmission theory of shielding of Schelkunoff. Experimental data are obtained and compared with results of parts 1) and 3) in the frequency range 100 Hz to 50 kHz. The first analytical technique is not general, and the limits of applicability of the results are discussed. In the second solution, which is general, expressions are derived for the total electric and magnetic fields on both sides of and within the shield. The resulting expression for shielding effectiveness is not solved because of its complexity. The results of the third theory are adapted to the problem. The shielding effectiveness expression S = R + A + B is computer evaluated for the six shields considered (1/16-inch and 1/8-inch thick aluminum, copper, and steel). Although some approximations are made, this analytical method is the most useful in predicting the insertion loss of the shield, since the theory includes those parameters neglected in the first analytical technique.     

碳纤维增强复合材料薄层高效建模方法研究

作为金属的替代品，碳纤维增强复合材料已被越来越广泛地用于飞行器、舰船和导弹等目标的壳体制造，而碳纤维增强复合材料的屏蔽效能对目标体的电磁安全起着重要作用.采用常规的全波分析方法计算碳纤维增强复合材料的屏蔽效能存在两大难题:复杂的材料属性和薄层带来的电大多尺度问题.为实现对碳纤维增强复合材料薄层的高效建模，通常采用均匀化的方法来等效具有复杂材料属性的复合材料，采用嵌入式薄层模型技术解决电大多尺度问题.本文将从均匀化方法和嵌入式薄层模型技术两个方面来探讨碳纤维增强复合材料薄层高效建模方法，并概述自主研发的大规模并行全波分析软件JEMS-FDTD及其集成的嵌入式薄层模型技术.最后，通过计算实例说明集成了嵌入式薄层模型的JEMS-FDTD软件在对碳纤维增强复合材料进行建模的正确性和高效性，并通过模拟仿真壳体为碳纤维增强复合材料飞机的屏蔽效能，说明集成了嵌入式薄层模型的JEMS-FDTD软件的实际应用价值.     

含短贯通导体金属腔体电磁耦合规律研究*

为研究平面波辐照下含短贯通导体金属腔体电磁耦合规律,利用电磁仿真软件CST 搭建仿真耦合模型,从频域角度进行了仿真研究,并利用矢量网络分析仪、功率放大器、GTEM 室和电场测试探头搭建实验系统进行了验证。通过仿真研究了贯通导体长度、贯通孔尺寸和腔体内屏蔽效能监测点位置等因素对屏蔽效能的影响。结果表明:仿真和实验的结果具有一致性;贯通导体腔体外长度越长,屏蔽效能越低,腔体内长度越长,腔体谐振频点越低;贯通孔尺寸越大,腔体屏蔽效能越低;监测点位置离贯通导体越近,屏蔽效能越低。同时研究了吸波材料和开口金属环对含短贯通导体金属腔体的防护效果。此研究具有实用意义,能够指导电子设备的设计和安装。     

Analysis of a Wire Antenna in the Presence of Sphere

An integro-differential equation is formulated for the problem of a thin-wire antenna in the presence of a sphere, and this equation is solved by the method of moments. The analysis is valid for an arbitrarily oriented and positioned wire relative to the sphere which may be either a perfect conductor or may be any simple homogeneous material. Input admittance of the wire antenna near a conducting sphere is determined from computed wire current and is corroborated by measured values of admittance. Also presented are calculated radiation patterns for the antenna/sphere structure.