多层圆筒屏蔽特性计算中的矩阵法

采用矩阵方法分析多层圆筒屏蔽体对内部场的屏蔽特性。对位于屏蔽空间中心的激励沿径向传播的电磁波进行了研究。考虑了多层圆筒屏蔽体的直径、各个屏蔽材料层的媒质参数以及屏蔽层的厚度。分析了在屏蔽空间内到达多层圆筒屏蔽体的电磁波以及穿过多层圆筒屏蔽体的电磁波。利用麦克斯韦方程在各个圆筒屏蔽层的分界处的边界条件,建立相应的矩阵方程。利用矩阵之间的关系,给出了任意多层圆筒屏蔽体的反射系数、屏蔽系统及屏效的完整形     

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.     

An Empirical Study of ELF and VLF Shield Cans

An empirical ELF and VLF study of 1/16- and 1/32-inch thick shield cans (stainless steel, aluminum, copper, Polyform, annealed Hipernom, and annealed and unannealed coldrolled steel, mumetal, and Co-Netic) was performed to investigate shielding effectiveness versus frequency characteristics with emphasis on the effects of machining (notches, slots, holes, and louvers), dropping, annealing, and welding. The erratic nature of many of the shielding functions is explained by the resonance theory of shielding, and the shielding characteristics of certain shield cans are compared. The paper also presents a brief introduction duction to the automated testing system used during this study to obtain the numerous shielding effectiveness versus frequency curves that were required.     

Shielding Density: An Electromagnetic Shielding Design Concept for Weight Sensitive Applications

Laboratory data obtained in a shielding study led to the conclusion that the size or shape of a shielding enclosure does not significantly affect its shielding effectiveness at frequencies below cavity resonance. A figure-of-merit expression for weight-sensitive applications was devised and termed shielding density. It relates shielding effectiveness per unit weight per unit area. This concept is useful in providing the shield designer with answers to three specific questions concerning maximum shielding effectiveness, lightest shielding enclosure, and optimum shielding density.     

低频脉冲磁场中磁屏蔽体磁饱和效应的试验研究

针对磁屏蔽体在低频脉冲磁场环境中可能存在的磁饱和问题,利用试验方法开展了磁饱和效应研究,证实了常规工程屏蔽体可在低频脉冲磁场环境中达到磁饱和状态,并通过观测屏蔽效能的变化获得了磁饱和规律,同时分析了磁饱和效应对屏蔽效能的影响及其与屏蔽体的材料磁导率、壳体厚度、外形尺寸等参数的关系.研究表明：磁屏蔽体屏蔽效能在磁饱和效应影响下,呈现出明显的动态变化特点,具有与屏蔽壳体磁导率类似的变化趋势;壳体厚度2 mm以内、长宽高为2 m×2 m×2 m左右的屏蔽体在上升时间为300μs、持续时间为1.2 ms的磁场环境中,达到磁饱和状态的磁化场强度约为10 mT,其磁饱和难易程度与磁导率及外形尺寸负相关,与壳体厚度正相关.试验研究结果与理论分析结论一致,可为磁屏蔽体的科学合理设计提供参考,具有较高的工程应用价值.     

多层平板屏效计算中的等效传输线法

讨论了多层平板屏效计算中的等效传输线法。定义了不同传输线间的反射系数与透射系数,求出了反射系数和透射系数表达式。文中透射系数的定义是对传线基本理论的有益补充。     

屏蔽完整性保证措施(上)

屏蔽材料对电场波,磁场波和电磁波提供的屏蔽效能是从理论上获得的.但是,实际情况下,由于屏蔽机箱,机柜,房间或运载器并不是完全封闭的,都有这种或那种必要的穿孔或孔洞,造成完整性损失,故常常达不到这种屏蔽效能.本文讨论屏蔽完整性损失,屏蔽完整性保证措施及如何恢复屏蔽完整性的实际应用.     

Radiation properties of a pigtail-terminated coaxial transmissionline

The method of moments is used to study the shielding effectiveness of a pigtail-terminated shielded wire (coaxial line) above a perfectly conducting ground plane. Numerical computations are performed in the RF frequency range, i.e. 25-1000 MHz. Pigtail wires on either or both ends of the shield are considered. Results are compared to those from the ungrounded case (floating shield). Results show that the presence of pigtail wires as ground connections at RF frequencies is undesirable and may further enhance radiation. The impedance terminations have a small effect on the shielding effectiveness of the antenna structure     

截止波导在电磁屏蔽中的应用

电磁屏蔽效能取决于屏蔽体薄弱环节,只有屏蔽体的所有组成部分都能达到屏蔽效能要求,才能满足屏蔽体的整体屏蔽要求。在工程实践中,由于功能要求,电子设备的屏蔽体会留有各种各样的孔洞或缝隙,如通风散热孔等,解决这些问题最有效的方法就是截止波导。     

高频状态下的圆孔屏蔽效能经验公式

电子设备电磁屏蔽效能的好坏直接影响着屏蔽体内电子器件工作的稳定性,而电磁屏蔽体上的孔洞造成的电磁泄漏是一个不容忽视的问题。针对这个问题,对有孔洞的屏蔽体的电磁泄漏进行了仿真计算,定量分析孔洞大小对屏蔽外壳的屏蔽性能的影响。分析结果表明在屏蔽体壁厚保证的情况下,孔洞的直径越小,屏蔽效能越高。当孔洞的半径在10mm以下时,屏蔽效能在30dB以上,满足实际工程要求的屏蔽效能要求。并由此得到屏蔽效能经验公式。     

Comparison of test methods for the characterization of shielding ofboard-to-backplane and board-to-cable connectors

We present the work carried out in the SOBITS project executed within the framework of the European Measurement and Testing program. The goal of the project was to compare different methods for characterization of the shielding performance of board-to-backplane and backplane-to-cable connectors used in telecommunication and computer systems. Four measurement methods have been selected. Dedicated test samples have been designed and developed in order to enable the measurement of the shielding characteristics of the connectors under test with all methods. Different shielding configurations were considered, in order to determine the influence of the quality of the shield. Single-ended as well as differential excitations have been considered. Among the test samples, a nonshielded version was designed for each signal/ground configurations. This sample was used as a reference. The definition of the quantities that describe the shielding performance is different for each of the measurement methods. Conversion formulas between these quantities have been formulated. They were verified by measuring a reference device that is easy to measure by all methods. Thus we were able to compare the relative shielding effectiveness of the various shielded board-to-backplane and backplane-to-cable connectors for all four measurement methods. The relative shielding effectiveness of a shielded connector was calculated with respect to the nonshielded version. We also compared the different shielding performance results obtained with each measurement method individually     

Measuring Connector Shielding Effectiveness During Vibration

This paper describes the equipment and technique used to measure shielding effectiveness* of threaded electrical connectors during vibration. The vibration fixture is a modified coaxial trough of which the connector is a part. The connector is mechanically loaded with a short section of cable. A known current is applied on the shield of this short cable section and across the receptacle/plug interfaces. RF leakage into the interior of the cable is determined by forming the interior wiring of the cable into a closed loop and measuring the current in this loop. The logarithmic ratio of the current in the shield to the current in the interior loop is the shielding effectiveness of the cable-connector combination. The cable shield is composed of brass convolute with a braid covering and provides, by itself, 100 dB isolation. This isolation was demonstrated by testing the shielding effectiveness (S.E.) of a base-line specimen which substituted a brass fitting for the connector. It is demonstrated that this method of testing connectors can be used to determine S.E. under vibration. The method is proposed as a standard method of qualification testing of connectors.     

Theoretical and experimental near-field characterization ofperforated shields

The shielding performance of an infinitely large, periodically perforated plane shield is investigated, both theoretically and experimentally. Attention is focused upon the near-field characterization by using two loop antennas at both sides of the shield. The electromagnetic coupling between the two loops is analyzed numerically using a plane-wave spectral representation of the radiated field and the method of moments. Comparison of these numerical results with near-field measurements yields a very good agreement up to high frequencies. The predicted classical far-field shielding effectiveness drastically overestimates the near-field shielding performance of the screen     

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.     

Cable Shielding Effectiveness Testing

This paper discusses an improved method of measuring the effectiveness of cable shielding and describes the results of tests on single- and multi-branched cables. Effects of significant shielding parameters of cables are also reported. These are the the effect of number of shield braid layers, braid material, braid angle, optical coverage, cable length, and wire size. The test method permits measurement of long specimens using high currents with a uniform current distribution along the cable shield. Measurements were made in the frequency range 0.5 to 100 MHz. The method is offered as a standard technique for measuring the shielding effectiveness* of shielded cables.     

Low-Frequency Shielding Properties of Conducting Cylindrical Shells of Arbitrary Cross Section

The shielding properties of general, two dimensional, finitely conducting shells with vanishingly thin walls are analyzed by the integral-equation method. For the circular cylinder case, a field plot throughout the shield interior reveals that, at the 'no shielding' frequencies found by Schieber, fields are attenuated greatly, except near the center. The influence of slots in the shell walls is also assessed for both circular and rectangular cylinders. Due to focusing effects, the field at the shield center is even stronger than the incident field at certain resonant frequencies for the slotted cylindrical shells.     

垂直入射均匀平面波的多层平板屏蔽效能分析

本文基于电磁场方法,详细导出多层平板屏蔽体对垂直入射均匀平面波的屏蔽效能计算公式。此外,提出计算屏蔽效能时,集肤深度和屏蔽体厚度的约束关系,给出用作计算机机箱的几种常用材料的屏蔽效能计算实例。     

Design of multilayered cylindrical shields using a geneticalgorithm

The design of infinitely long multilayered cylindrical shields with circular cross-section are considered and a method based on the genetic approach is proposed. An analytical method for the calculation of the shielding effectiveness of a cylindrical shield, consisting of homogeneous layers is presented for the case of an obliquely incident plane wave. By making use of this method, a genetic algorithm is implemented for the design of multilayered cylindrical shields in order to achieve a prespecified shielding effectiveness for a given band of frequencies or a range of angles of incidence     

Electromagnetic shielding of thermoformed lightweight plastic screens

The paper describes an efficient subcell model for the simulation of thermoformed lightweight shield by the three-dimensional (3-D) finite-difference time-domain (FDTD) method. The shield is made of a foil of polycarbonate, having the thickness of about 100 mm, coated with a two-layer thin film of tin and nickel. The model developed allows to simulate the metallic coating with nonuniform thickness and is applied to compute the shielding effectiveness of the thermoformed screen. The numerical results of the developed 3-D FDTD procedure are validated by comparison with experimental data.