Equivalent dipole moment method to characterize magnetic fields generated by electric appliances: extension to intermediate frequencies of up to 100 kHz

A previously proposed simple method to characterize magnetic fields near electric appliances was extended to intermediate frequencies of up to 100 kHz. The method consists of identification of the magnetic dipole moment that is equivalent to a magnetic field source of an electric appliance and simple estimation of the magnetic field distribution around the appliance. In addition, frequency characteristics of the magnetic field were taken into account by considering the harmonic components in the magnetic-field waveform for both power frequency and intermediate frequency ranges. For the application of the method, a wide-frequency range (from power frequency to 100 kHz) magnetic-field measuring instrument was developed and applied to appliances that generate intermediate frequency magnetic fields, i.e., an induction heating cooker, a TV set, and a metal detector. The results revealed that the method is adequate to quantify the magnetic field near the electric appliances at frequencies of up to 100 kHz.     

On the shielding effectiveness of enclosures

This paper deals with new definitions of shielding effectiveness, in particular for high-frequency and transient electromagnetic fields. They are practicable and supposed to better characterize the shielding ability than the commonly used definitions. From the ratio of the time-averaged input power of the unshielded load to that one of the shielded load, in the limiting case of a vanishing load the electromagnetic shielding effectiveness is derived. This is a simple combination of the commonly used and easily measurable electric and magnetic shielding effectiveness. A similar procedure is then employed for the transient case, where in the limiting case of a vanishing load the ratio of the absorbed energies turn into the transient shielding effectiveness. Numerical results are shown for closed as well as for nonclosed cylindrical shields.     

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.      

A hybrid FD-MoM technique for predicting shielding effectiveness of metallic enclosures with apertures

In this paper, a hybrid technique combining the finite-difference (FD) method and the method of moments (MoM) in the frequency domain is proposed to predict the shielding effectiveness of rectangular conducting enclosures with apertures under external illumination. The interior and exterior regions of the enclosure are analyzed separately by employing the field equivalence principle. Internal electromagnetic fields are discretized using the (FD) method, while external fields are formulated by the MoM. Enforcement of continuity of the tangential magnetic field over the aperture surface gives the desired equation to solve for electromagnetic fields everywhere. Numerical results for the shielding effectiveness of a rectangular cavity with apertures calculated by the new hybrid technique are presented and validated by comparing with experimental data.     

一种SQUID传感器基线漂移和工频干扰联合抑制新方法

基于超导量子干涉器件的低频微弱磁场测量系统,在无磁屏蔽环境下工作时容易受到大地磁场和输电线路干扰,测量数据中混叠有严重的基线漂移和工频谐波干扰。为了提高微弱磁场信号检测性能,本文提出了一种基线漂移和工频干扰联合抑制新方法。该方法首先利用噪声辅助的多维经验模式分解部分重构信号,消除基线漂移带来的非平稳性影响。然后根据工频干扰的短时平稳特性,采用最小二乘法对工频干扰进行抑制。数值仿真和不同环境下实测数据处理结果表明,与自适应滤波方法相比,本文方法对于基线漂移和工频干扰具有更好的处理效果,满足系统在野外无磁屏蔽条件下的应用要求。      

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.     

Shielding Circuits from EMP

It is generally impractical to filter low-frequency electromagnetic pulse (EMP) signals from victim circuits. Twisting signal pair conductors is helpful but often results in insufficient isolation. The remainder must be provided by shielding. Highly permeable ferritic materials have generally been found to provide maximum shielding from low-frequency magnetic fields. It is shown that this may not be the case when the signal source is relatively distant from the shield. With large separation, there appears to be a greatly increased mismatch between the wave impedance at the shield and the intrinsic impedance of the metal. This results in much greater reflection of the impinging wave than occurs for the same signal strength with small source to shield separation. The mismatch is greatest with a highly conductive shield material. All common highly permeable materials have low relative conductivity. High permeability does not improve the shielding effectiveness at low audio frequencies because no significant attenuation occurs as the wave passes through the shield. It is concluded that materials such as copper or aluminum are logical choices for shielding circuits from distant, high-intensity, low-frequency EMP.     

Magnetic Shielded Enclosure Design in the DC and VLF Region

A review of magnetic, shielding concepts and equations as applied to ideal shield configurations has provided a basis for the analysis of the shielding effectiveness of practical shielded enclosures to dc and VLF magnetic fields. The permeability of the shield material is considered as a function of the induction, and a significantly improved method of estimating the induction and permeability of the shield is presented. The effects of multiple shell geometry are given by the equations of this analysis, which are indeterminable with a transmission line analysis. The degrading effects of other departures from ideal shield materials and configurations are analyzed qualitatively; equations for estimating the magnitude of those effects are developed where possible.     

磁场环境对磁保持继电器影响及防护

磁保持继电器广泛应用在航空、航天、电子、邮电等军用及民用电子装备中。分析了磁场对磁保持继电器的影响机理,分析不同量级、不同方向磁场对磁保持继电器的影响程度,研究不同厚度、不同尺寸屏蔽方式防护效果,得出磁保持继电器电磁屏蔽的方法,通过加大防护罩厚度或减少防护罩尺寸可有效改善防护罩的屏蔽效果,为磁保持继电器电磁防护的工程实施提供依据。     

Detailed FD-TD analysis of electromagnetic fields penetratingnarrow slots and lapped joints in thick conducting screens

The physics of electromagnetic-wave transmission through narrow slots and tapped joints in thick conducting screens is examined in detail by applying numerical models to compute both field distributions within the slots and joints, and fields transmitted to the shadow region. The primary modeling tool is the finite-difference time-domain (FD-TD) method, using a Faraday's-law contour integral approach to modify the basic FD-TD algorithm to properly model the slot physics, even when the slot gap width is much less than one space lattice cell. Finely sampled method-of-moments (MM) models are used to validate the FD-TD tool for relatively simple straight slots; FD-TD is then used to explore properties of more complicated lapped joints which are widely used for shielding purposes at junctions of metal surfaces. It is found that previously reported slot resonances occur in a more general sense for lapped joints as path-length resonances. In addition to greatly enhanced power transmission, path-length resonances can result in total fields within the joint exceeding the incident fields by more than one order of magnitude. Sample field distributions for this case are given     

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     

High-temperature superconducting magnetic shields formed by deepdrawing

A new method for the construction of high-temperature superconducting magnetic shielding structures has been demonstrated. With this process, a ceramic laminate of high-temperature superconducting powder and silver metal sheets is formed and then shaped into a cylindrical magnetic shield by deep drawing before being sintered. Two types of superconducting powders were used in this experiment, YBa₂Cu₃O₇ and Bi_1.8 Pb_0.4Sr₂Ca₂Cu₃O_x , which exhibited shielding factors of 1100 and 330, respectively     

低频脉冲磁场对屏蔽体耦合特性的试验研究

低频脉冲磁场对屏蔽体的耦合效应与低频磁场自身参数、屏蔽体尺寸、材料特性及屏蔽体结构有关,作用过程比较复杂,实际耦合强度和规律应以试验研究结果确定。目前关于时变磁场的研究成果主要集中在单频磁场、快沿磁场等方面,而且效应试验研究较少。针对此种情况,文章选取钢板、纯铁板和铜板等几种常用的金属材料,制作了相应的屏蔽体缩比模型,利用低频脉冲磁场模拟试验系统研究了两者的耦合规律,分析了电磁参数等因素对耦合特性的影响,在一定程度上弥补了目前研究成果的不足。     

FDTD方法分析THz波段金属平板的屏蔽特性

研究了室温下金属平板对THz波的透射屏蔽特性。考虑到THz波段金属电导率的频率色散特征,基于Z变换方法处理频域色散媒质本构关系到时域的转化,使FDTD仿真易于编程实现。仿真表明,对不同金属材料,其屏蔽效应随频率呈现出周期振荡,且金属平板厚度在百nm量级时对THz波的屏蔽效应＞60 dB。对不同厚度的金属平板,其屏蔽效应以dB为单位随厚度线性增大,有望利用此特性实现厚度μ＜m级薄层金属的厚度测量。     

A moment method solution for TMz and TEzwaves illuminating two-dimensional objects above a lossy half space

A moment method (MM) solution for analyzing the electromagnetic shielding and scattering properties of two-dimensional (2-D) objects over a lossy half space is presented. The materials of the objects can be metal, dielectric, or magnetic. Also, the lossy half space is included to simulate the effects of the earth ground or any flat homogeneous lossy surface. An MM based on a volume formulation and a special Green's function in the spectral domain is developed. Both TM _z and TE_z waves incident upon 2-D metal or lossy material structures are demonstrated for the shielding effects of those bodies in the presence of the lossy ground. Besides, the echo widths of a composite object either in free space or above the lossy half space are determined by using the MM. Some of the results are compared with those by other methods, and good agreements are obtained. The MM solution can be used to study the shielding and scattering problems for cylindrical structures located over a lossy ground     

Magnetic shielding effects in Ag-doped and pure YBa2Cu3O7 − x thin films probed by non-resonant microwave absorption

A non-resonant microwave absorption technique is used to study the magnetic shielding effects in Ag-doped and pure YBa₂Cu₃O_{7 − x}(YBCO) thin films for small modulating fields. While Ag-doped YBCO thin films show good shielding for small modulating fields (≈ 1 Oe) until 60 K, the pure YBCO films show significant shielding effect only up to 20 K. We interpret these results in terms of better grain alignment in Ag-doped YBCO films as compared to pure YBCO films, which can lead to better shielding effects.     

Impact of DC components in transport current and external magnetic field on hysteresis losses in an HTS tape

The high-temperature superconductor (HTS) development is presently intensive, and one can expect that HTSs will be used in different electric devices in the future. In some of these devices, e.g., a magnetostrictive actuator, the conductor will be exposed to both ac and dc transport currents and external magnetic fields. One of the most important parameters in the design of electric devices is the power loss. It is, therefore, of importance to develop models that are able to predict the power loss in devices based on HTSs. In this paper, we present an experimental characterization of the hysteresis losses in an HTS tape exposed to ac or dc transport currents in combination with external magnetic fields. The experimental results are compared to some existing models based on the critical state model. The power loss was measured on a multifilamentary Bi-2223 tape at constant temperature (70 K) and frequency (50 Hz).     

屏蔽门金属弹簧片的屏蔽效能分析

采用时域有限差分法(FDTD)对屏蔽门上的金属弹簧片进行建模分析，研究了其屏蔽效能特性。由于弹簧片上的缝隙是造成电磁泄漏的主要通道，因此首先对弹簧片的近似结构——薄金属板上的缝隙在不同形状下的屏蔽特性进行了研究，其次对弹簧片、安装槽和刀口组成的屏蔽组件进行了整体建模和仿真，得出弹簧片的屏蔽效能与缝隙长度、宽度、深度以及安装槽结构问的关系。结果表明该弹簧片结构无论对电场还是磁场都有非常高的屏蔽效能，这也为屏蔽门的优化设计提供了理论参考。     

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.