Flux Distribution in a Linear Magnetic Shield

The two-dimensional flux distribution in a magnetic shield of constant permeability and conductivity is presented in a series of magnetic vector-potential contour plots. The plots show the leakage-flux tubes in the shield and in the shielded region. Three- dimensionless parameters representing the shielding 1) thickness, 2) skin depth, and 3) permeability are introduced as the flux-distribution variables. The induced eddy currents cause the resulting fields in the shield to be elliptically polarized and to break into a characteristic cell structure which increases in complexity with rising frequency.     

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.     

Magnetostatic simulations for design of superconducting magneticshields

A magnetostatic approach to numerical simulations of magnetic field attenuation by superconducting shields is demonstrated on simple geometries. Comparisons to published measurements and analytic calculations show that results are accurate for simulation of a shield in the shape of a cylindrical tube. The capabilities of the method are shown by simulations which close the cylinder with end caps having access ports or gaps. With end caps having cylindrical ports, the simulated attenuation transits smoothly between the analytical results for semi-infinite tubes of the two radii. Radial gaps between solid end caps and the cylinder allow little flux leakage for time-varying fields, but significant leakage for static fields     

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.     

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     

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.     

Magnetocardiography using HTS rf SQUIDs with coplanar resonators

We have employed HTS rf washer SQUIDs with coplanar resonators for magnetocardiography measurements. The white field noise of such a magnetometer (rf washer SQUID, 3.5 mm in diameter, flip-chip mounted on a flux concentrator 13 mm in diameter) inside a 4 layer μ-metal shielding was 16 fT/√Hz. MCG measurements were performed with a magnetometer in the Berlin magnetically shielded room (BMSR) and with a first order gradiometer in a standard shielded room in Jülich (JMSR). The results are compared regarding the fragmentation score, which is supposed to be a risk marker for sudden cardiac death. Despite different environments, score values determined with the magnetometer and the gradiometer were almost identical.     

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.     

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.     

Effectiveness of Earphone Shielding for Artifact Control in Auditory Evoked Potentials

Control of the electromagnetic artifact produced by earphones is a major problem in the acquisition of surface-recorded auditory evoked responses, especially short latency responses (0-10 ms). Several investigators have used magnetically shielded earphones to reduce the artifact. In this paper, a method for measuring shielding effectiveness is described, and results obtained with a TDH-39 earphone are presented. Two layers of shielding were found to provide 10-15 dB of shielding effectiveness; the resulting artifact is less than 0.1 ?V for acoustic stimuli below 120 dB sound pressure level (SPL). The shielding also affects the response of the transducer, especially at low frequencies. It is concluded that shielding is effective in controlling artifact for high-frequency transient stimuli. However, for low-frequency stimuli, for which the response begins while the stimulus is present, results obtained with shielded earphones should be interpreted with caution.     

Nonuniform Layer Model of a Millimeter-Wave Phase Shifter

The electromagnetic wave propagation of millimeter waves in dielectric waveguides with thin surface plasma layers is characterized. The phase and attenuation of a 94-GHz wave are computed for various surface plasma layer thicknesses as a function of earner density levels. The electron/hole pairs generated in the vicinity of the dielectric waveguide surface by photo excitation are assumed to have an exponential profile due to either carrier diffusion or the exponential absorption of the optical field. Field computations made for a uniform plasma layer are compared with those of the nonuniform plasma to illustrate the effects of the exponential tails of the carrier profiles on both the phase and attenuation of the millimeter wave. The thin plasma layers slightly affect the field profile of the transverse electric modes (fields polarized parallel to the plasma layer). The transverse magnetic fields are highly distorted at plasma densities greater than 10/sup 16/ cm/sup -3/.     

Radiation from cylindrical leaky waves

Formulas are derived for the far-infrared radiation pattern of cylindrical leaky waves propagating on a planar surface. The formulas can be used to predict the radiation pattern of a general class of leaky-wave antennas, consisting of a finite-size source which excites a radially propagating leaky wave on some planar surface. Leaky-wave antennas consisting of antenna elements embedded in dielectric layers (microstrip elements) fall into this category. Using the equivalence principle, formulas are derived for both transverse electric (TE) and transverse magnetic (TM) leaky waves with arbitrary propagation constants. The formulas allow for radiation from cylindrical apertures of arbitrary size, so that the effect of truncating the supporting planar surface with an absorbing material can be determined. Particular attention is devoted to the case of a leaky wave for which the real and imaginary parts of the complex propagation constant are equal, since this type of wave has been shown to be responsible for broadside radiation in certain leaky-wave antennas comprised of dielectric layers     

Extension of the 3-D range migration algorithm to cylindrical andspherical scanning geometries

A near field three-dimensional (3-D) synthetic-aperture radar (SAR) algorithm specially tailored for cylindrical and spherical scanning geometries is presented. An imaging system with 3-D capability can be implemented by using a stepped-frequency radar which synthesizes a two-dimensional (2-D) aperture. Typical scanning geometries commonly used are planar, cylindrical, and spherical. The 3-D range migration algorithm (RMA) can be readily used with a planar scanning geometry. This algorithm is extremely accurate, preserves the phase, and corrects for the wavefront curvature. The RMA cannot be directly applied with nonplanar scanning geometries. However, as an alternative solution, we propose to backpropagate the backscattered data onto a planar aperture in the vicinity of the measurement aperture and then apply the 3-D RMA. The proposed imaging algorithm is validated both numerically and experimentally     

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     

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     

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     

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.     

An overview of near-field antenna measurements

After a brief history of near-field antenna measurements with and without probe correction, the theory of near-field antenna measurements is outlined beginning with ideal probes scanning on arbitrary surfaces and ending with arbitrary probes scanning on planar, cylindrical, and spherical surfaces. Probe correction is introduced for all three measurement geometries as a slight modification to the ideal probe expressions. Sampling theorems are applied to determine the required data-point spacing, and efficient computational methods along with their computer run times are discussed. The major sources of experimental error defining the accuracy of typical planar near-field measurement facilities are reviewed, and present limitations of planar, cylindrical, and spherical near-field scanning are identified.     

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。