Electromagnetic shielding
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In electrical engineering, electromagnetic shielding is the practice of reducing or redirecting the electromagnetic field (EMF) in a space with barriers made of conductive or magnetic materials. It is typically applied to enclosures, for isolating electrical devices from their surroundings, and to cables to isolate wires from the environment through which the cable runs ( ). Electromagnetic shielding that blocks radio frequency (RF) electromagnetic radiation is also known as RF shielding.
EMF shielding serves to minimize
Materials used
Typical materials used for electromagnetic shielding include thin layer of metal, sheet metal, metal screen, and metal foam. Common sheet metals for shielding include copper, brass, nickel, silver, steel, and tin. Shielding effectiveness, that is, how well a shield reflects or absorbs/suppresses electromagnetic radiation, is affected by the physical properties of the metal. These may include conductivity, solderability, permeability, thickness, and weight. A metal's properties are an important consideration in material selection. For example, electrically dominant waves are reflected by highly conductive metals like copper, silver, and brass, while magnetically dominant waves are absorbed/suppressed by a less conductive metal such as steel or stainless steel.[2] Further, any holes in the shield or mesh must be significantly smaller than the wavelength of the radiation that is being kept out, or the enclosure will not effectively approximate an unbroken conducting surface.
Another commonly used shielding method, especially with electronic goods housed in plastic enclosures, is to coat the inside of the enclosure with a metallic ink or similar material. The ink consists of a carrier material loaded with a suitable metal, typically copper or nickel, in the form of very small particulates. It is sprayed on to the enclosure and, once dry, produces a continuous conductive layer of metal, which can be electrically connected to the chassis ground of the equipment, thus providing effective shielding.
Electromagnetic shielding is the process of lowering the electromagnetic field in an area by barricading it with conductive or magnetic material.
EMI (Electromagnetic Interference) shielding is of great research interest and several new types of nanocomposites made of ferrites, polymers, and 2D materials are being developed to obtain more efficient RF/microwave-absorbing materials (MAMs).
Example of applications
One example is a shielded cable, which has electromagnetic shielding in the form of a wire mesh surrounding an inner core conductor. The shielding impedes the escape of any signal from the core conductor, and also prevents signals from being added to the core conductor. Some cables have two separate coaxial screens, one connected at both ends, the other at one end only, to maximize shielding of both electromagnetic and electrostatic fields.
The door of a microwave oven has a screen built into the window. From the perspective of microwaves (with wavelengths of 12 cm) this screen finishes a Faraday cage formed by the oven's metal housing. Visible light, with wavelengths ranging between 400 nm and 700 nm, passes easily through the screen holes.
RF shielding is also used to prevent access to data stored on
NATO specifies electromagnetic shielding for computers and keyboards to prevent passive monitoring of keyboard emissions that would allow passwords to be captured; consumer keyboards do not offer this protection primarily because of the prohibitive cost.[7]
RF shielding is also used to protect medical and laboratory equipment to provide protection against interfering signals, including AM, FM, TV, emergency services, dispatch, pagers, ESMR, cellular, and PCS. It can also be used to protect the equipment at the AM, FM or TV broadcast facilities.
Another example of the practical use of electromagnetic shielding would be defense applications. As technology improves, so does the susceptibility to various types of nefarious electromagnetic interference. The idea of encasing a cable inside a grounded conductive barrier can provide mitigation to these risks.
How it works
Electromagnetic radiation consists of coupled
Similarly, varying magnetic fields generate eddy currents that act to cancel the applied magnetic field. (The conductor does not respond to static magnetic fields unless the conductor is moving relative to the magnetic field.) The result is that electromagnetic radiation is reflected from the surface of the conductor: internal fields stay inside, and external fields stay outside.
Several factors serve to limit the shielding capability of real RF shields. One is that, due to the
In the case of high-
Magnetic shielding
Equipment sometimes requires isolation from external magnetic fields.
Because of the above limitations of passive shielding, an alternative used with static or low-frequency fields is active shielding, in which a field created by electromagnets cancels the ambient field within a volume.[12] Solenoids and Helmholtz coils are types of coils that can be used for this purpose, as well as more complex wire patterns designed using methods adapted from those used in coil design for magnetic resonance imaging. Active shields may also be designed accounting for the electromagnetic coupling with passive shields,[13][14][15][16][17] referred to as hybrid shielding,[18] so that there is broadband shielding from the passive shield and additional cancellation of specific components using the active system.
Additionally, superconducting materials can expel magnetic fields via the Meissner effect.
Mathematical model
Suppose that we have a spherical shell of a (linear and isotropic) diamagnetic material with
See also
- Electromagnetic interference
- Electromagnetic radiation and health
- Radiation
- Ionising radiation protection
- Mu-metal
- MRI RF shielding
- Permalloy
- Electric field screening
- Faraday cage
- Anechoic chamber
- Plasma window
References
- ^ a b "Medical Device EMI Shielding - Cybershield". www.cybershieldinc.com. Retrieved 2023-05-02.
- ^ "Understanding EMI/RFI Shielding to Manage Interference". Ceptech. Retrieved 2020-04-23.
- ^ Seale, Wayne (2007). The role of copper, brass, and bronze in architecture and design; ‘‘Metal Architecture,’’ May 2007
- ^ Radio frequency shielding, Copper in Architecture Design Handbook, Copper Development Association Inc., http://www.copper.org/applications/architecture/arch_dhb/fundamentals/radio_shielding.html Archived 2020-08-07 at the Wayback Machine
- S2CID 257867862.
- ^ "Metal shields and encryption for US passports". Newscientist.com. Retrieved 18 November 2012.
- USENIX Security Symposium.
- S2CID 238583775.
- ^ "MuMETAL" (PDF). Magnetic Shield Corp. 2012. Catalog MU-2. Retrieved 26 June 2016.[permanent dead link]
- ^ "Trademark Status & Document Retrieval". tsdr.uspto.gov. Retrieved 2017-08-02.
- ^ "Interference Technology Magazine Whitepaper on Ferromagnetic Nanocrystalline Metal Magnetic Shield Coatings". Archived from the original on March 15, 2010.
- ^ "NMR Magnet Shielding: The seat of the pants guide to understanding the problems of shielding NMR magnets". Acorn NMR. 22 January 2003. Retrieved 27 June 2016.
- S2CID 221538013.
- S2CID 230524109.
- S2CID 195833040.
- S2CID 218718690.
- S2CID 218719330.
- Bibcode:2017APS..DNP.EA034R.
- ISBN 978-0471309321.
External links
- All about Mu Metal Permalloy material
- Mu Metal Shieldings Frequently asked questions (FAQ by MARCHANDISE, Germany) magnetic permeability
- Clemson Vehicular Electronics Laboratory: Shielding Effectiveness Calculator Archived 2017-07-09 at the Wayback Machine
- Shielding Issues for Medical Products (PDF) — ETS-Lindgren Paper
- Practical Electromagnetic Shielding Tutorial
- Simulation of Electromagnetic Shielding in the COMSOL Multiphysics Environment