Diffusive electromagnetic penetration into metallic-enclosures

The physics of shielding by a metallic enclosure is discussed. Simple engineering formulas in the frequency and time domains are deduced from a rigorous formulation to calculate the cavity fields for certain canonical enclosure shapes. These simple formulas are generalized so as to be applicable to enclosures of arbitrary shape. The great difference in shielding effectiveness between enclosures and planar slabs is pointed out.     

Geometrical Effects on Shielding Effectiveness at Low Frequencies

A frequent approach to computing the magnetic shielding effectiveness of enclosures is to consider the effect of a plane wave impinging on a sheet of infinite extent. This permits an analysis based on a transmissionline characterization. However, when the wavelength is large compared to the dimensions of the enclosure, other analytical approaches provide better results. It has been shown that the current distribution on a box-like object scattering in the Rayleigh region tends to concentrate at the edges and corners of the box. This leads to concentrations of the magnetic field in the vicinity of edges and corners both inside and outside the enclosure. Since the effects of the current concentrations are localized, the magnetic shielding problem can be simplified by assuming a uniform current distribution on the exterior of the enclosure. Under this assumption the socalled ?circuit approach? can be applied. The box-like enclosure is characterized as a series of shorted turns which shield a sensor within the enclosure. Based on the geometry, the mutual and leakage impedances between the source and sensor are used to compute the magnetic shielding effectiveness. This approach yields valid results for shields constructed of either wire mesh or sheet metal. It can also be extended to account for degradation due to bad bonds. A comparison of results, both transient and steady state, of the circuit approach and scattering theory show close agreement for spherical enclosures.     

混响室法欠模状态屏蔽体屏蔽效能测试技术研究

欠模状态下屏蔽壳体内部场分布不均匀,屏蔽效能测量结果与位置相关,因此需要利用探头或单极子天线进行多位置测量,过程繁琐。文中利用长线天线来测量欠模状态下屏蔽壳体屏蔽效能,并与单极子天线测量结果进行比较。在暗室中测量天线自由空间反射系数S₁₁;在混响室中利用不同天线测量屏蔽壳体的屏蔽效能,并利用S₂₂分析腔体谐振特性。测量结果表明,长线天线宽频带匹配特性优于单极子天线,测量的屏蔽效能能够更好地反映屏蔽体谐振特性,为屏蔽壳体的屏蔽效能测试评估提供了指南,具有工程应用价值。     

FDTD modeling of common-mode radiation from cables

Radiation from cables attached to printed circuit boards and shielding enclosures is among the primary concerns in meeting FCC Class A and B limits. The finite-difference time-domain (FDTD) method can be employed to model radiation from printed circuit boards and shielding enclosures with complex geometries, but difficulties in modeling wires and cables of arbitrary radii are encountered. Modeling the wire by setting the axial component of the electric field to zero in the FDTD method results in an effective wire radius that is determined by the mesh discretization. Neglecting the wire radius in applications, such as electromagnetic interference (EMI) or printed circuit board modeling, may result in gross errors because near-field quantities are typically sensitive to wire thickness. Taflove et al. (1988) have developed a subcellular FDTD algorithm for modeling wires that has been shown to work well for plane wave scattering. The method uses a quasistatic field approximation to model wires with a well defined radius independent of the mesh dimensions. The wire model is reviewed and investigated for application to common-mode radiation from cables attached to printed circuit boards, where the source is often a noise voltage at the connector. Also investigated is energy coupling to attached cables through enclosure apertures resulting in common-mode radiation from the cable. The input impedance for a center-fed dipole antenna, as well as a monopole connected to a conducting half-sheet, is computed with FDTD methods and compared to moment method input impedance results. A simulation of a shielding enclosure with an attached cable demonstrates the utility of FDTD analysis in modeling common-mode radiation     

New figures of merit for the characterization of the performance of shielding enclosures

Evaluation and measurement of shielding performance of enclosures and protection structures in general are based on the comparison between the local field values with and without the shield. Apart from some practical problems arising in specific, but relevant, situations like enclosures of small dimensions, such an approach appears rather incomplete and sometime deceitful. Despite this, shielding effectiveness (SE) is a well-established parameter, and it seems that for decades, the attention has been focussed more on how to evaluate and measure the so-called SE, rather on what the goal of any shielding structure is and why each evaluation and measurement should be performed. The main drawback is that SE is a local quantity and its knowledge does not help in the prediction of the real mitigation of undesired effects achieved by means of any shielding structure; undesired effects are mainly due to an integral of an electric or magnetic field, and/or to spatial variations of electric and magnetic field components. In the past, proposals were advanced toward an improved definition and measurement of electromagnetic SE; the proposed new figures of merit were based on the energy (power) penetrating the enclosure, perceived as the key factor for the shielding problem. However, it seems more adequate and correct the direct reference to the mechanism of birth of induced effects, as stemming from Maxwell equations. For these reasons, two new figures of merit are proposed for the comparison of enclosures and shields performance.     

Use of Reverberation Chambers to Determine the Shielding Effectiveness of Physically Small, Electrically Large Enclosures and Cavities

With the proliferation of small electric devices in recent years, along with various other applications, there is a growing need to test and determine the shielding properties or shielding effectiveness (SE) of physically small (but electrically large) enclosures or cavities. In this paper, we discuss how a reverberation chamber technique can be used to measure the SE of such enclosures. The approach consists of placing the small enclosure inside a reverberation chamber and using frequency stirring to excite the reverberation chamber. A small surface probe (i.e., a monopole) is mounted on the inside wall of the small enclosure to measure the power level inside the small enclosure. We present measured data from various other reverberation chamber approaches obtained from various enclosure configurations. The data from these other reverberation chamber approaches are used to validate the proposed approach. We also compared measured data to theoretical calculations of the SE for two small enclosures with circular apertures. These various comparisons illustrate that the proposed technique is a valid approach for determining the SE of physically small (i.e., cubic enclosure dimensions of the order of 0.1 m and smaller), but electrically large enclosures (that support several modes at the lowest frequency of interest).      

New Technique for the Determination of the Integrity of Shielded Enclosures by the Measurement of the Perpendicular Magnetic Field

A solution of electromagnetic field equations with appropriate boundary conditions shows that at the surface of a material with infinite conductivity, no parallel electric field or perpendicular magnetic field exists. As conductivity decreases, there are large relative increases in the parallel electric field and perpendicular magnetic field at the surface. A reduction in permeability lowers the parallel electric field while increasing the perpendicular magnetic field. For most practical shielded enclosures of room size, the shielding effectiveness is determined largely by the quality of the seams between panels, along door edges, or filter mountings. Hence an investigation is presented of results of an evaluation of a method developed by the government for finding faults in a shielded enclosure by detecting changes in the perpendicular component of magnetic field along the surface of the conducting wall emphasizing continuity of all types of seams of shielded enclosures.     

Shielding effectiveness of a rectangular enclosure with arectangular aperture

An analysis is presented of the shielding effectiveness (SE) of a rectangular enclosure with an aperture in one face. The enclosure is treated as a length of rectangular waveguide and the aperture as a length of coplanar strip transmission line. Theoretical values of SE agree with measurements for a range of enclosures, apertures and frequencies. The variation of SE with position in the enclosure is also correctly predicted     

A proposed new definition and measurement of the shielding effect of equipment enclosures

This paper describes the rationale behind a new proposed measurement of the screening effect of an equipment enclosure that takes into account the contents of the enclosure. The method uses a set of representative contents for enclosures. The representative contents are equipped with surface field probes to measure the power entering the contents. The ratio of this power to the incident power density is used to derive a quantity with the dimensions of area, termed here the shielding aperture. The measurement technique is described and examples of measurements are given along with computed comparisons with the conventional shielding effectiveness of the enclosures used.     

Corrosion of a zinc-coated steel enclosure at the contactinterfaces of gasket joints

A typical enclosure for information technology (IT) equipment is made of various sub-assemblies such as enclosure cases, gaskets and I/O connectors, and the shielding effectiveness of the enclosure is largely determined by junctions in the enclosure for electrically connecting different shielding panels or components. Inter-metallic junctions may get corroded and the shielding effectiveness of the enclosure may deteriorate. The identification of the corrosion mechanisms responsible for the shielding degradation of zinc-coated steel enclosures is the primary focus of this study. A series of preliminary experimental investigations were conducted to help in identifying the corrosion mechanisms. Factors contributing to the corrosion process are discussed. This work provides a basis for on-going effort in experimentally quantifying and modeling the corrosion and corrosion-induced shielding-degradation behavior of electromagnetic shielding enclosures     

An EMI estimate for shielding-enclosure evaluation

A relatively simple, closed-form expression has been developed to estimate the EMI from shielding enclosures due to coupling from interior sources through slots and apertures at the enclosure cavity modes. A power-balance method, Bethe's (1944) small-hole theory, and empirically developed formulas for the relation between radiation, and slot length and number of slots, were employed to estimate an upper bound on the radiated EMI from shielding enclosures. Comparisons between measurements and estimated field strengths suitably agree within engineering accuracy     

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.     

Fast and Rigorous Analysis of EMC/EMI Phenomena on Electrically Large and Complex Cable-Loaded Structures

A fast and comprehensive time-domain method for analyzing electromagnetic compatibility (EMC) and electromagnetic interference (EMI) phenomena on complex structures that involve electrically large platforms (e.g., vehicle shells) along with cable-interconnected antennas, shielding enclosures, and printed circuit boards is proposed. To efficiently simulate field interactions with such structures, three different solvers are hybridized: (1) a time-domain integral-equation (TDIE)-based field solver that computes fields on the exterior structure comprising platforms, antennas, enclosures, boards, and cable shields (external fields); (2) a modified nodal-analysis (MNA)-based circuit solver that computes currents and voltages on lumped circuits approximating cable connectors/loads; and (3) a TDIE-based transmission line solver that computes transmission line voltages and currents at cable terminations (guided fields). These three solvers are rigorously interfaced at the cable connectors/loads and along the cable shields; the resulting coupled system of equations is solved simultaneously at each time step. Computation of the external and guided fields, which constitutes the computational bottleneck of this approach, is accelerated using fast Fourier transform-based algorithms. Further acceleration is achieved by parallelizing the computation of external fields. The resulting hybrid solver permits the analysis of electrically large and geometrically intricate structures loaded with coaxial cables. The accuracy, efficiency, and versatility of the proposed solver are demonstrated by analyzing several EMC/EMI problems including interference between a log-periodic monopole array trailing an aircraft's wing and a monopole antenna mounted on its fuselage, coupling into coaxial cables connecting shielded printed circuit boards located inside a cockpit, and coupling into coaxial cables from a cell phone antenna located inside a fuselage.     

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.     

Accurate Analysis of Reverberation Field Penetration Into an Equipment-Level Enclosure

The paper focuses on the reverberation chamber method for the shielding properties evaluation of equipment-level enclosures. The enclosure under test is numerically modeled by an in-house finite-difference time-domain code that is able to predict the field inside the enclosure and the voltage captured by the probe placed inside it. The chamber fields have been modeled by applying the plane-wave superposition. The code is validated by measurements in our reverberation chamber. Subsequently, the effect of probe positioning and length on the induced voltage is analyzed. Finally, the enclosure shielding effectiveness is evaluated by applying two different definitions, and a statistical analysis is carried out, thus allowing an estimation of the measurement uncertainty.      

Time-Domain Investigation on Cable-Induced Transient Coupling Into Metallic Enclosures

A hybrid time-domain method is proposed for characterizing electromagnetic interference (EMI) signals coupled into some composite structures with metallic enclosures, braided shielded cable, printed circuit boards, and even lumped active devices. In order to rapidly capture the induced interior EMI, the finite-difference time-domain, modified node analysis, and multiconductor transmission lines methods are combined together and implemented successfully. Numerical investigation is carried out to demonstrate the frequency-dependent transfer impedance of the coaxial cable, the induced voltage at the place of active loaded element in the transmission line network, and the enclosure shielding effectiveness of these composite enclosures. The captured transient response information is useful for further designing electromagnetic protection of the inner circuits against the impact of voltage or current surge caused by nonintentional as well as intentional electromagnetic interference.      

Shielding Characterization of Metallic Enclosures With Multiple Slots and a Thin-Wire Antenna Loaded: Multiple Oblique EMP Incidences With Arbitrary Polarizations

Shielding effectiveness (SE) of metallic rectangular enclosures with thin slots and a thin-wire antenna loaded by an impedance, illuminated by multiple electromagnetic pulses (EMPs) simultaneously, is investigated using a hybrid finite-difference time-domain (FDTD) method. In order to enhance the simulation efficiency of the FDTD algorithm, accurate formulas for handling multiple thin slots, a thin-wire antenna, and a lumped network are integrated together. Numerical results show that for real metallic enclosures, their shielding performance is very sensitive to the variations in direction and polarization angle of the incident EMPs. However, at a given frequency, the resistance and inductance loaded at the terminal of a thin-wire antenna have little effect on the SE level. For two, three, as well as more EMP incidences, common-frequency interferences will result in strong inner field resonance in the enclosure.      

Accurate Characterization of Shielding Effectiveness of Metallic Enclosures With Thin Wires and Thin Slots

In many electrical and electronic systems, metallic enclosures are used to provide electromagnetic shielding. These enclosures normally contain thin wires, thin slots, and frequency-selective slots (FSS) that degrade the shielding effects. In this paper, integrated FDTD formulations are developed that can model both subcellular thin slots and thin wires simultaneously. The formulations are shown to be capable of accurately predicting shielding effectiveness and inner field distributions of a metallic enclosure in both the frequency- and time-domains when subject to a high-power electromagnetic pulse.      

高频有损斜开孔腔体屏蔽效能研究

针对带孔屏蔽腔体工作时受复杂电磁干扰问题,基于电磁拓扑理论和BLT 方程,考虑开孔倾斜、偏 心、任意平面波照射、任意观测点位置等情况,提出用于计算斜开孔腔体屏蔽效能的解析模型。同时将腔体材料考 虑为有损导体,对BLT 方程中散射矩阵和传播矩阵进行修改;并利用电压和电场分布关系,考虑腔体内多种传播模 式,将模型的可计算频段拓展为0~3 GHz。最后,设计五组试验,对比CST 仿真值、Robinson 拓展算法计算值与文中 所提模型计算值,验证所提模型的准确性与优化性。为实际工程中孔缝腔体屏蔽问题的预测与计算提供方法。     

Wideband Pulse Responses of Metallic Rectangular Multistage Cascaded Enclosures Illuminated by an EMP

A generalized mathematical procedure is developed for investigating shielding performance and inner wideband pulse responses of metallic rectangular multistage cascaded enclosures with multiple rectangular apertures, which is illuminated by an electromagnetic pulse (EMP). The mathematical methodology is based on the integral equation technique combined with the method of moments, by which all unknown aperture fields are solved according to a set of linear algebraic equations of infinite order. Numerical computations are performed to show frequency- and geometry-dependent shielding effectiveness and inner pulse waveforms of single- and double-stage cascaded enclosures, respectively, with computational accuracy and convergence rate also checked. It is found that using appropriate metallic multistage cascaded enclosures, high shielding performance can be achieved, which can provide an effective protection for certain electronic and communication systems from the interference of an EMP.