Low Frequency Magnetic Shielding Research Articles

Differences Between NSA 94–106 and IEEE Std. 299 Setups for Low-Frequency Magnetic Shielding Effectiveness Characterization: A Study Using Numerical Simulations

Differences Between NSA 94–106 and IEEE Std. 299 Setups for Low-Frequency Magnetic Shielding Effectiveness Characterization: A Study Using Numerical Simulations

Approximate Simulation of Low Frequency Magnetic Shielding of a Rectangular Shielding Box with All Walls Perforated Periodical Holes

Approximate Simulation of Low Frequency Magnetic Shielding of a Rectangular Shielding Box with All Walls Perforated Periodical Holes

Low-frequency Magnetic Shielding of Double-layer Conducting Plates with Periodic Apertures: Experimental Observation of Great Improvement of Shielding Effectiveness by Slightly Separating the Two Plates

Low-frequency Magnetic Shielding of Double-layer Conducting Plates with Periodic Apertures: Experimental Observation of Great Improvement of Shielding Effectiveness by Slightly Separating the Two Plates

Low-frequency Magnetic Shielding of a Cavity Formed by Two Imperfectly Conducting Sheets: Effect of Sheet-to-sheet Distance and Comparison with the Single-sheet Configuration

Low-frequency Magnetic Shielding of a Cavity Formed by Two Imperfectly Conducting Sheets: Effect of Sheet-to-sheet Distance and Comparison with the Single-sheet Configuration

Low-Frequency Magnetic Shielding of a Perfectly Electric Conducting Circular Disk: Approximately Analytical Formulation Considering Radius of Emitting Loop

This letter proposes an approximate analytical formulation for the shielding problem of a perfectly electric conducting disk against the low-frequency magnetic field produced by a circular current loop placed coaxially with the disk. First, this problem is related to a complementarity problem: The leakage of the magnetic field through a circular hole on a perfectly magnetic conducting plane of infinite extension. Then, for the complementarity problem, we develop the accurate analytical solution if only the distribution of the normal component of the unshielded magnetic field along the surface of the hole can be expressed as a polynomial function of radial distance. The analytical formulation is verified with finite element simulations and measured results.

Low-Frequency Magnetic Shielding of Planar Screens: Effects of Loop Radius and Loop-to-Loop Distance

For the magnetic shielding configuration of both emitting and receiving loops placed coaxially on either side of a planar screen, we investigate the effects of loop radius and loop-to-loop distance on shielding effectiveness by numerically solving the accurate integral expression. The results are in good agreement with finite element simulations and experimental results. Simple and reliable approximate formulas are also proposed for pure conductive screens and pure magnetic screens, respectively. It is shown that the effects are obvious when the loop-to-loop distance is smaller than the loop radius. Also, conductive screens and magnetic screens exhibit opposite and competing shielding effectiveness evolution behaviors. In addition, in the case of conductive screens with a smaller receiving loop compared to the emitting loop, the shielded loop-to-loop mutual inductance does not always decrease with the increase of the loop-to-loop distance but peaks at a distance between the screen thickness and half of the emitting loop radius.

Electromagnetic Simulation of Low-Frequency Magnetic Shielding of a Welded Steel Plate

In this article, we experimentally evaluated the effectiveness of magnetic shielding for welded steel plates with the control of parameters such as a gap and a spot spacing for automobiles at frequencies below 1000 kHz, and predicted the shielding effectiveness by an electromagnetic analysis. It is shown that the shielding effectiveness of welded steel plates is reduced by more than 20 dB compared to that of a single plate above 10 kHz. Moreover, the shielding effectiveness of the welded steel plates strongly depended on the spacing between the spot welds, and the mechanism was studied by analyzing the eddy currents in the plates. These results provide important data for automotive design to reduce human exposure to electromagnetic waves.

Low-frequency Magnetic Shielding of Planar Shields: A Unified Wave Impedance Formula for the Transmission Line Analogy

This article focuses on wave impedance calculations employed in the transmission line analogy for the low-frequency magnetic shielding problem: a planar shield against a loop current source placed parallel to the shield. We present a unified formula for wave impedance calculations that is distinct from traditional definitions of the transverse E -field to H -field ratio, which is related to the ratio of the H -field components to its longitudinal derivative. The formula is relatively simple in terms of mathematical complexity when applied to derive a concrete wave impedance expression. For a circular loop, the formula produces the same wave impedance expression as the traditional definition for field points on the central axis of the loop. For field points off the axis, the formula more accurately predicts the shielding effectiveness relative to traditional definitions. Based on the proposed formula, we also obtain wave impedance expressions for rectangular, elliptical, and regular polygon loops under quasi-static assumptions. The effectiveness of these expressions is verified through comparisons with finite element simulations.

Closed-Form LF Magnetic Shielding Effectiveness of Thin Planar Screens in Coplanar Loops Configuration

In this article, the low-frequency magnetic shielding effectiveness of a planar screen with magnetic and conductive properties in a coplanar-loops configuration (as required in the IEEE Standard 299) is evaluated through a new closed-form formula under the thin-sheet and small-loop approximations. The formulation clearly shows the separate contribution of the permeability and the conductivity of the screen to the shielding performance. The results obtained with the proposed closed-form formulas are compared with the exact ones derived through numerical integrations to assess the relevant accuracy and limits of validity.

Low-frequency magnetic field shielding effect of artificial joint-free REBCO coils

The screening current induced in superconductors can passively shield external magnetic fields and hence provide a noise free environment for precision sensors and instruments. This paper presents a novel design employing artificial joint-free REBCO (REBa2Cu3O7-x, RE: rare earths) coils which can economically extend the field shielding space for larger instruments, e.g. clinical equipment. We compared the shielding factors of single, double and multi-coils, and confirmed a coil with Helmholtz construction as the optimal architecture. Due to the zero resistance of the joint-free REBCO coils, the shielding effectiveness reaches 80 dB at the center of coils, shielding 99.99% of the external magnetic field of 10 μT at 1 × 10−5 Hz, and 40 dB (99%) in a greater spherical space of 40 mm in diameter. Additionally, the shielding effectiveness does not decrease in the external magnetic field of 1 nT. Such a low frequency magnetic shielding effect implies promising applications for the present work, such as preventing interference magnetic fields from precision instruments, which generally require field sensitivity as low as the fT · Hz−1/2 range.

Analytical Evaluation of the Low-Frequency Magnetic Shielding of Thin Planar Magnetic and Conductive Screens

A recent analytical formulation for the low-frequency magnetic shielding effectiveness of a thin metal screen against a circular loop source is extended to the case of a screen exhibiting both magnetic and conductive properties. The presence of magnetic properties introduces a series impedance in the equivalent network of the structure, in addition to the shunt admittance associated with the screen conductivity. However, the fields scattered by the two elements do not interact and hence can be treated independently; furthermore, the series impedance gives rise to integrals akin to those stemming from the shunt admittance. A generalized analytical formulation for a combined magnetic and conductive screen is thus obtained, that is favorably compared with both classical approximate formulations and exact results.

Low Frequency Magnetic Shielding Effectiveness of a Conducting Plate With Periodic Apertures

This article focuses on the shielding effectiveness (SE) prediction of the periodic aperture array on an infinite PEC plate against the low frequency (≤1 MHz) magnetic field resulting from a current loop parallel to the plate. Two different models of the penetration field through the plate are presented. From the two models, the same simplified expression for the field on the central axis is derived. The SE of a 1000 mm × 1000 mm aluminum plate with 2500 apertures against the magnetic field from a current loop is measured. The results show that the theoretical results are in good agreement with the measurement if the frequency is higher than a critical frequency. Above the critical frequency, penetration from the apertures is primary factor for field leakage, and the SE is constant with the increasing frequency. When the frequency is lower than the critical frequency, penetration from the aluminum is primary channel for field leakage, and hence the SE reduces with the decreasing frequency. In this case, the upper limit of the SE does not exceed the SE of the corresponding solid plate without apertures.

Shielding Effectiveness Analysis of the Conducting Spherical Shell With a Circular Aperture Against Low Frequency Magnetic Fields

In this paper, an analytical model is presented to calculate the low frequency magnetic shielding effective (SE) of the spherical shell with a circular aperture and finite conductivity. This model is obtained by the combination of two submodels: the finite conductivity shell without the aperture, and the perfect conductor shell with the aperture. Both the submodels have existing analytical solutions. The first submodel represents the diffusion effect of magnetic field penetration through the conducting shell, and the second one denotes the aperture effect of magnetic field leakage through the aperture. The total magnetic field is the superposition of these from the two submodels. Calculation results are provided for an aluminum spherical shell of radius 0.1m for frequencies between 10Hz and 1MHz. The results are in good agreement with these form 2D axisymmetric finite element simulations. It is shown that there is a critical frequency. Below this frequency, the diffusion effect is dominant and the SE enhances with the increase of frequency. Above this frequency, the aperture effect is dominant and the SE keeps unchanged with the variation of frequency. In addition, the phase shift characteristics are also analyzed for the two effects respectively, and are employed to elucidate the mechanism of the resonance phenomenon of the SE around the critical frequency. Further, the effect of aperture depth is investigated numerically, which shows that increasing the depth has similar effect on SE like reducing aperture radius.

도체 판의 자기장 차폐효과 분석: 확산과 슬롯 효과

This study analyzes the low-frequency magnetic shielding effectiveness (SE) of a metal plate, in terms of diffusion and slot effects, by conducting a numerical simulation and implementing an analytical solution. When the metal has a low conductivity, the SE is dominated by the diffusion effect. However, when the conductivity and frequency both increase, the slot has a major influence on the SE. These results can be used as guidelines in the shielding design and SE requirements of electromagnetic pulse protection facilities.

Measurement and Analysis of Low Frequency Magnetic Shielding Performance of Open-Cell Nickel Foam

Low frequency magnetic field in the range from 10 to 500 kHz is between very-low and mid-high frequencies, mainly coming from industrial equipments such as antenna, battery, inverter, etc. In some shielding applications simultaneously demanding for ventilation and cooling, high-performance shielding materials with a porous structure are necessary. However, the investigation of these materials in this band was rarely reported. Open-cell nickel foam has excellent electrical conductivity and magnetic permeability, which makes it a potential new kind of shielding material in the field of low frequency magnetic shielding. It is generally believed that, in this band, materials shield the magnetic field mainly through two mechanisms, i.e., flux shunting and eddy current cancellation. With the equation of shielding effectiveness (SE) for a cylindrical shell, SEs of copper foil and Permalloy were calculated. The results indicated that the effect of flux shunting can be ignored, which was further verified by the results of Helmholtz coil test. Thus, only eddy current cancellation mechanism was considered to establish a shielding model, in which the flat open-cell nickel foam was assumed to be a group of coils and the eddy current was assumed to flow along the conductive paths composed of the coils. The prediction of the model was verified by the measured results of a window test method. Both the experimental and theoretical results revealed that, SE is enhanced with the decrease of the average pore size of the foam because the number of conductive paths in the foam increases. In addition, the calculated results of SE indicated that the open-cell nickel foam has no obvious superiority compared to a fully dense nickel strip of the same areal density. However, the structure characteristics of open-cell foam make it definite to find applications simultaneously asking for shielding, ventilating and cooling.

Bulk superconducting tube subjected to the stray magnetic field of a solenoid

Hard type-II hollow superconductors are well suited for low frequency magnetic shielding. The properties and performances of superconducting magnetic shields subjected to homogeneous magnetic fields have been extensively discussed in the literature. In the present work, we investigate the magnetic shielding and the penetration of magnetic flux in a bulk high temperature superconducting tube subjected to the inhomogeneous fringe field of a solenoidal coil. Thanks to a bespoke microdisplacement measurement system, we measure the magnetic field distribution around the tube. We develop a full 3D finite element model based on an H formulation to understand the flux penetration mechanisms and predict the shape of the current loops. Using constitutive law parameters obtained from previous independent experiments, our model is found to be in excellent agreement with the measurements. We discuss how to assess the degree of inhomogeneity of the magnetic field and show that, in our case study, the field can be treated as the magnetic field of an equivalent magnetic dipole. We also show that some features of the flux penetration in inhomogeneous field can be also observed when the tube is subjected to an oblique homogeneous magnetic field, which offers a better understanding of the shielding current density distribution inside the shield. Finally, we discuss the magnetic field concentration occurring around the shield for different magnetic field configurations. In particular, we show that the extremities of the tube on the side not facing the magnetic field source experience the highest flux concentration.

Magnetic shielding analysis of a multiply-slotted metal plate coated with a ferrite sheet in a periodic line current source

This paper presents an analytical solution for low-frequency magnetic shielding analysis of a multiply-slotted metal plate coated with a ferrite sheet in the presence of a periodic line current source. The Fourier transform is used to represent the magnetic vector potentials in the spectral domain and the mode matching technique is enforced to obtain a modal solution in rapidly converging series form, leading to numerically efficient results. Shielding effectiveness is investigated in terms of the geometrical configuration of multiple slots, ferrite sheet, and periodic line current source. The proposed solution is verified by a commercial finite element solver with good agreement.

An Efficient Modeling Method for 3-D Magnetic Plates in Magnetic Shielding

This paper presents an efficient method for modeling 3-D magnetic plates used in low-frequency magnetic shielding. An extended formulation of partial element equivalent circuit is introduced. Both induced current and magnetization densities in the plates are approximated by an analytical function across plate thickness. The proposed method eliminates the need of plate meshing along its thickness, and reduces the number of unknowns significantly in 3-D shielding problems. The method is validated numerically with a small shielding structure, and experimentally with a large shielding structure. It is found that the method is reasonable in accuracy and efficient in computation, and is applicable to 3-D shielding structures having large thin conductive and magnetic plates.

Joint Modeling for Conductive Plates in Low-Frequency Magnetic Shielding

Low-frequency magnetic shielding is usually made with finite-size metallic plates. The discontinuity on the shield is unavoidable, and degrades the performance of shielding. In this paper an equivalent circuit method was proposed to model the joint of conductive plates, and to evaluate the magnetic fields around the joint. The circuit equation using total cell current was presented. The modeling method was validated experimentally. Shielding performance of a shield against various joint parameters was also investigated. It is found that reasonable shielding performance can be achieved if the joint is provided with an adequate mating surface area and multiple fixing points.

Effects of Electromagnetic Radiation From Digital Wireless Telephone Frequencies on a Double Relaxation Oscillation SQUID

In this paper, the effects of electromagnetic fields from the frequency band of digital wireless telephones on a superconducting quantum interference device (SQUID) were experimentally investigated. A first-order planar gradiometer based on the double relaxation oscillation SQUID (DROS) was operated without low-frequency magnetic shielding, and was exposed to radiated electromagnetic fields in the frequency range of 300 MHz-2 GHz. The experiments have been performed in a fully anechoic chamber to minimize reflected waves in the chamber. The flux-to-voltage transfer and the field sensitivity of the gradiometer have been observed under the irradiation of fields. Results show that the performance of the gradiometer was significantly affected at a typical power level of the digital wireless telephones for both horizontally and vertically polarized electromagnetic waves. The noise level of the DROS output voltage also increases as the strength of the external fields becomes higher.