Fundamentals of Steady-State and Transient Electromagnetic Fields in Shielding Enclosures

A simplified theory of electromagnetic shielding by enclosures with conductive and/or magnetic wails is presented. The theory is based on a simplified type of boundary condition at the enclosure walls. Theoretical results are compared with exact solutions for steady-state and transient excitation and the approximation turns out to be extremely good in all cases of practical interest. For enclosures of arbitrary shapes, the problem is formulated as a single integral equation in the unknown current in the shield. Numerical solution of the equation is discussed, as well as a possible equivalent lumped-circuit representation.     

Shielding of Thin Generic Enclosures from Static Magnetic Fields

The present paper is concerned with the evaluation of the shielding factor of three-dimensional generic thin-walled enclosures embedded in a uniform magnetic field. To this purpose, a criterion based upon computing the outside surface flux has been adopted with success.     

Shielding Theory of Enclosures with Apertures

This paper develops the theory of EM radiation from metallic enclosures with apertures, excited by an internal source at frequencies below the fundamental resonance of the enclosure. The enclosure with an aperture is analyzed from two different points of view: as a cavity with a small aperture in a wall, and as a waveguide section short-circuited at one end and open at the other. Rectangular geometries are used throughout, since these are by far the most commonly encountered in practical enclosures and cabinets. Using the corresponding dyadic Green's functions, the fields generated inside the enclosure by some simple sources are determined. The fields radiated through small apertures in a cavity are determined using Bethe's theory of diffraction by small holes. The radiation from an open waveguide is calculated with the help of field-equivalence theorems, with assumptions applicable to the case of evanescent waves. The final step is to derive expressions for the " insertion loss" of the shield, defined as the ratio of the field strength at a point external to the shield, before and after the insertion of the enclosure. To do so, the effect of the shield upon the input impedance of the antenna is analyzed, and expressions obtained for the applicable cases. The resulting insertion-loss expressions are numerically evaluated for some representative cases, and graphically compared with a series of measurements performed to obtain experimental confirmation. Very good agreement is obtained in all cases, establishing the validity of the analysis.     

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.     

Survey of Techniques for Measuring RF Shielding Enclosures

An intensive literature survey has been conducted to review existing techniques for measuring the effectiveness of RF shielding enclosures. Prevalent methods of measuring E- and H-field components and for sensing the total field are discussed and evaluated. The procedures described in the literature are tabulated into groups determined by their similarities in method or theory and are compared by listing the advantages and disadvantages inherent in each. (A weighting system is devised to aid in evaluating each of the principal methods discussed.) Also discussed are the theoretical considerations underlying each of the methods listed, as applicable to the measurement of shielding effectiveness. Among these are the concepts of the resultant wave impedance of the E-M field at a point inside a shielded enclosure and the relationship of low-impedance and high-impedance fields in the near field regions of antenna systems. Techniques for extending the frequency and intensity of illuminating fields are also discussed, as well as the insertion loss method of determining leakage from a shield. Limitations and deficiencies of this latter procedure are noted.     

Proposed Recommended Practices for the Measurement of Shielding Effectiveness of High-Performance Shielding Enclosures

Uniform measurement procedures are proposed for determining the relative effectiveness of room-size high-performance shielding enclosures. Test procedures are designated to measure the effectiveness of a rectangular parallelepiped with edge dimensions ranging from 1.5 to 15 meters. They are formulated to enable use of commercially available equipment for conducting tests under nonideal conditions. The standard does not, however, specify construction details for shielding enclosures nor does it suggest methods for calculating their effectiveness.     

机箱最优冷却和屏蔽需遵循的设计规范和功率性能

针对具体的应用目标来对电子线路进行保护,对设计者来说意味着多方面的挑战.为了满足环境的要求,工程师必须考虑机械结构方面的限制、冷却要求、EMI/RFI控制、冲击/振动、功率分配、电缆管理、系统监控、高可用性(HA)、可靠性(MTBF)、可维护性(MTTR)以及难以计数的其它一些不明显的问题.     

关于电磁屏蔽

本文从场的角度阐述了电磁屏蔽原理,计算方法及实际中考虑屏蔽应注意的问题。提出了组合屏蔽方式。     

电磁屏蔽玻璃在电磁屏蔽方舱上的应用

文章介绍了电磁屏蔽玻璃国内生产现状、屏蔽效能理论计算方法、几种屏蔽玻璃屏蔽效能的实测值,指出了屏蔽玻璃在电磁屏蔽方舱上的应用原则.     

Surface Current Source Reconstruction for Given Radiated Electromagnetic Fields

A general analytic approach is presented for reconstructing: 1) the minimum energy source enclosed by a sphere, and 2) the surface current distribution on a sphere from the knowledge of the radiated fields. The surface current source is derived by adding proper non-radiating sources to the minimum energy source. In contrast to the minimum energy volumetric distribution, the surface current derived in this paper is practically realizable. Finally, we present a closed form formula for the reconstructed spherical surface current source. We will show that this spherical surface current is indeed the unique solution of the inverse source problem for square-integrable surface electric current on a sphere in a homogenous medium.      

Shielding Measurements of Equipment Enclosures in the Radiating Near Field

To evaluate the shielding performance of an equipment-level enclosure, a new measure termed shielding power ratio is proposed, which includes the average shielding power ratio (AS) and a statistically based enhancement factor. Two measurement environments, the anechoic chamber and the reverberation chamber, are considered and compared. To achieve a quick estimate of such measure, the reverberation chamber is proposed to measure the AS. The enhancement factor can be derived from the statistical analysis on both measurement results and simulation results. Based on the enhancement factor and the total radiated power, the maximum power density radiated from the enclosure can be estimated accurately. The paper is informing new developments in IEEE Standard 299.     

Shielding of ELF Magnetic-Dipole Fields by Ferromagnetic Cylindrical Shells

The shielding effectiveness of ferromagnetic cylindrical shells against static and extremely low-frequency ( ELF) magneticdipole fields has been measured and calculated under various conditions. The theory is specifically developed for initially non-uniform electromagnetic fields (no restriction on the dipole-shield separation distance). In addition, the theory takes into account the finite length of the shield where necessary. Satisfactory agreement between theory and experiment is obtained over the range of parameters tested. The results presented here agree in the uniform-electromagnetic-field limit with those derived previously by King for infinite cylinders.     

直接电流注入与自由场照射之间等价性的评估

介绍了直接电流注入技术在导弹EMC性能测试中的具体应用。文中一方面从理论上分析了直接电流注入技术及其模拟测试装置,评价了直接电流注入技术代替自由场照射的可行性;另一方面通过导弹自由场照射的理论计算及试验测试,评价了直接电流注入技术代替自由场照射的有效性。为导弹的高EMC安全裕度测试提供了一个简单、经济、有效的测试方法。     

轻型电磁屏蔽帐篷

1概述随着计算机与网络通信技术的快速发展,信息系统的电磁泄漏发射问题已引起各国的广泛关注。安方高科电磁安全技术(北京)有限公司研制出一种便携式电磁屏蔽帐篷,在帐篷内处理涉密信息安全可靠,成为真正的"信息保护伞"。     

电磁防护用品的屏蔽效能测试分析

设计了人体电磁防护用品屏蔽效能的测试方法和测试步骤,明确人体模型重要的测试部位.分析了电磁防护用品不同缝制方式对屏蔽效能的影响及简化的电磁防护用品模型的测试结果,认为不同的极化方式和照射方向对屏蔽服的防护效果都有影响.     

电磁屏蔽暗室中的接地设计

介绍了接地的目的及分类方法,对电磁屏蔽暗室的接地技术进行分析,提出了实用的设计方法和建议,并指出了合理设计和安装接地装置对电磁屏蔽暗室具有重要的意义。     

导电织物的电磁屏蔽性能研究

通过对金属纤维含量不同的混纺织物的电磁屏蔽效能测试,对导电织物的屏蔽性能进行了研究。分析了无限大金属网电磁屏蔽效能解析模型与金属纤维混纺织物的屏蔽效能之间的区别,并进一步研究了金属纤维密度和类型、织物的组织结构、金属纤维在纱线结构中的分布和经纬纱线的排列等因素对导电织物电磁屏蔽效能的影响。     

电磁屏蔽技术分析

随着电子产品的广泛应用以及电磁环境污染的加重,对其电磁兼容性设计的要求也越来越高,作为电磁兼容设计的主要技术-屏蔽技术的研究也就越显得重要了.从电磁屏蔽技术原理出发,讨论了屏蔽体结构、屏蔽技术分类、屏蔽材料的选择以及所要遵循的原则,在电子设备实施具体的电磁屏蔽时提供了重要的依据.     

电磁波屏蔽复合材料的研究进展

从电磁波屏蔽的机理出发,介绍导电型、导磁型以及导电导磁型电磁波屏蔽复合材料的发展现状,并展望了电磁波屏蔽材料未来的研究方向。