Editing Electromagnetic shielding

{{Short description|Using conductive or magnetic materials to reduce electromagnetic field intensity}}
{{About|shielding materials|the atomic effect|Shielding effect}}
{{more citations needed|date=March 2010}}
[[Image:Electromagnetic shielding inside mobile phone.jpg|thumb|Electromagnetic shielding cages inside a disassembled [[mobile phone]].]]

In [[electrical engineering]], '''electromagnetic shielding''' is the practice of reducing or redirecting the [[electromagnetic field]] (EMF) in a space with barriers made of [[electrical conductor|conductive]] or [[Magnetism|magnetic]] materials. It is typically applied to enclosures, for isolating electrical devices from their surroundings, and to [[Electrical cable|cable]]s to isolate [[Electrical wiring|wire]]s from the environment through which the cable runs ({{crossreference|see [[Shielded cable]]}}). Electromagnetic shielding that blocks [[radio frequency]] (RF) [[electromagnetic radiation]] is also known as '''RF shielding'''.

EMF shielding serves to minimize [[electromagnetic interference]]. The shielding can reduce the [[Coupling (electronics)|coupling]] of radio waves, electromagnetic fields, and [[electrostatic field]]s. A conductive enclosure used to block electrostatic fields is also known as a ''[[Faraday cage]]''. The amount of reduction depends very much upon the material used, its thickness, the size of the shielded volume and the [[frequency]] of the fields of interest and the size, shape and orientation of holes in a shield to an incident electromagnetic field.

==Materials used==
[[File:Disassembled_laptop.jpg|thumb|A laptop case with visible copper electromagnetic interference (EMI) coating shield on the inside. Such coatings are usually deposited by using [[electroless plating]]. It is applied both to home appliances and [[medical device]]s.<ref name=":0">{{Cite web |title=Medical Device EMI Shielding |publisher=Cybershield |url=https://www.cybershieldinc.com/medical-device-emi-shielding/ |access-date=2023-05-02}}</ref>]]

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]].<ref>{{Cite web|url=https://ceptech.net/understanding-emi-rfi-shielding-to-manage-interference/|title=Understanding EMI/RFI Shielding to Manage Interference|website=Ceptech|language=en-US|access-date=2020-04-23}}</ref> 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.  [[Copper]] is used for radio frequency (RF) shielding because it absorbs [[radio wave|radio]] and other [[electromagnetic wave]]s. Properly designed and constructed [[Copper in architecture#Radio frequency shielding|RF shielding enclosures]] satisfy most RF shielding needs, from computer and electrical switching rooms to hospital [[CAT-scan]] and [[MRI]] facilities.<ref>Seale, Wayne (2007). "The role of copper, brass, and bronze in architecture and design". ''Metal Architecture''. May 2007</ref><ref>[http://www.copper.org/applications/architecture/arch_dhb/fundamentals/radio_shielding.html "Radio frequency shielding"]. ''Copper in Architecture Design Handbook''. Copper Development Association Inc. {{Webarchive|url=https://web.archive.org/web/20200807111854/https://www.copper.org/applications/architecture/arch_dhb/technical-discussion/fundamentals/radio_shielding.html |date=2020-08-07 }}</ref>

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).<ref>{{Cite journal |last1=Mohapatra |first1=Prajna P. |last2=Ghosh |first2=Sagnik |last3=Jain |first3=Ashish |last4=Aich |first4=Suman |last5=Dobbidi |first5=Pamu |date=2023-05-01 |title=Rare earth substituted lithium ferrite/carbon black ceramic composites for shielding electromagnetic radiation |url=https://www.sciencedirect.com/science/article/pii/S030488532300327X |journal=Journal of Magnetism and Magnetic Materials |language=en |volume=573 |pages=170678 |doi=10.1016/j.jmmm.2023.170678 |bibcode=2023JMMM..57370678M |s2cid=257867862 |issn=0304-8853|url-access=subscription }}</ref> EMI shielding is often achieved by [[electroless plating]] of copper as most popular plastics are non-conductive or by special conductive paint.<ref name=":0" />

==Example of applications==
[[Image:Coaxial cable cutaway.svg|thumb|right|Cross-section through a coaxial cable showing shielding and other layers]]
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&nbsp;cm) this screen finishes a [[Faraday cage]] formed by the oven's metal housing.  Visible light, with wavelengths ranging between 400&nbsp;nm and 700&nbsp;nm, passes easily through the screen holes.

RF shielding is also used to prevent access to data stored on [[RFID]] chips embedded in various devices, such as [[biometric passports]].<ref>{{cite web|url=https://www.newscientist.com/article/dn8227-metal-shields-and-encryption-for-us-passports.html |title=Metal shields and encryption for US passports |publisher=Newscientist.com |access-date=18 November 2012}}</ref>

[[NATO]] specifies electromagnetic shielding for computers and keyboards to prevent passive monitoring of [[Computer keyboard|keyboard]] emissions that would allow passwords to be captured; consumer keyboards do not offer this protection primarily because of the prohibitive cost.<ref>{{Cite conference|url=https://www.usenix.org/legacy/event/sec09/tech/full_papers/vuagnoux.pdf|title=Compromising Electromagnetic Emanations of Wired and Wireless Keyboards|date=August 2009|first1=Martin|last1=Vuagnoux|first2=Sylvain|last2=Pasini|conference=18th [[USENIX Security Symposium]]}}</ref>

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 ==

{{See also|Faraday cage}}

Electromagnetic radiation consists of coupled [[electric field|electric]] and [[magnetic field|magnetic]] fields.  The electric field produces [[force]]s on the [[electric charge|charge]] carriers (i.e., [[electron]]s) within the conductor. As soon as an electric field is applied to the surface of an ideal conductor, it induces a [[current (electricity)|current]] that causes displacement of charge inside the conductor that cancels the applied field inside, at which point the current stops.

Similarly, ''varying'' [[magnetic field]]s generate [[eddy current]]s 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 [[electrical resistance]] of the conductor, the excited field does not completely cancel the incident field. Also, most conductors exhibit a [[ferromagnetism|ferromagnetic]] response to low-frequency magnetic fields,{{citation needed|date=April 2022}} so that such fields are not fully attenuated by the conductor. Any holes in the shield force current to flow around them, so that fields passing through the holes do not excite opposing electromagnetic fields. These effects reduce the field-reflecting capability of the shield.

In the case of high-[[frequency]] electromagnetic radiation, the above-mentioned adjustments take a non-negligible amount of time, yet any such radiation energy, as far as it is not reflected, is absorbed by the skin (unless it is extremely thin), so in this case there is no electromagnetic field inside either. This is one aspect of a greater phenomenon called the [[skin effect]]. A measure of the depth to which radiation can penetrate the shield is the so-called [[skin depth]].

== Magnetic shielding ==
Equipment sometimes requires isolation from external magnetic fields.<ref>{{cite journal | last1 = Hobson | first1 = P. J. |display-authors=et al | year = 2022 | title =  Bespoke magnetic field design for a magnetically shielded cold atom interferometer | journal = Sci. Rep. | volume = 12 | issue = 1 | pages = 10520 | doi= 10.1038/s41598-022-13979-4 | pmid = 35732872 | pmc = 9217970 | arxiv = 2110.04498 | bibcode = 2022NatSR..1210520H | s2cid = 238583775 }}</ref>  For static or slowly varying magnetic fields (below about 100&nbsp;kHz) the Faraday shielding described above is ineffective.  In these cases shields made of high [[magnetic permeability]] metal [[alloy]]s can be used, such as sheets of [[permalloy]] and [[mu-metal]]<ref>{{cite web |url=http://www.magnetic-shield.com/literature/pdf/mu-2.pdf |title=MuMETAL |id=Catalog MU-2 |publisher=Magnetic Shield Corp. |date=2012 |access-date=26 June 2016 }}{{Dead link|date=March 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref><ref>{{Cite web|url=http://tsdr.uspto.gov/#caseNumber=73410422&caseType=SERIAL_NO&searchType=statusSearch|title=Trademark Status & Document Retrieval|website=tsdr.uspto.gov|language=en|access-date=2017-08-02}}</ref> or with nanocrystalline grain structure ferromagnetic metal coatings.<ref>{{cite web |url=http://www.nxtbook.com/nxtbooks/item/emcdirectory-design_2009/index.php#/134 |title=Interference Technology Magazine Whitepaper on Ferromagnetic Nanocrystalline Metal Magnetic Shield Coatings |url-status=dead |archive-url=https://web.archive.org/web/20100315010701/http://www.nxtbook.com/nxtbooks/item/emcdirectory-design_2009/index.php#/134 |archive-date=March 15, 2010 }}</ref>  These materials do not block the magnetic field, as with electric shielding, but rather draw the field into themselves, providing a path for the [[magnetic field lines]] around the shielded volume.  The best shape for magnetic shields is thus a closed container surrounding the shielded volume.  The effectiveness of this type of shielding depends on the material's permeability, which generally drops off at both very low magnetic field strengths and high field strengths where the material becomes [[magnetic saturation|saturated]].  Therefore, to achieve low residual fields, magnetic shields often consist of several enclosures, one inside the other, each of which successively reduces the field inside it. Entry holes within shielding surfaces may degrade their performance significantly.

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 [[electromagnet]]s cancels the ambient field within a volume.<ref>{{cite web |url=http://www.acornnmr.com/appnotes/shielding.htm |title=NMR Magnet Shielding: The seat of the pants guide to understanding the problems of shielding NMR magnets |publisher=Acorn NMR |date=22 January 2003 |access-date=27 June 2016}}</ref>  [[Solenoid]]s and [[Helmholtz coil]]s 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,<ref>{{Cite journal |last1=Packer |first1=M. |last2=Hobson |first2=P.J. |last3=Holmes |first3=N. |last4=Leggett |first4=J. |last5=Glover |first5=P. |last6=Brookes |first6=M.J. |last7=Bowtell |first7=R. |last8=Fromhold |first8=T.M. |date=2020-11-03 |title=Optimal Inverse Design of Magnetic Field Profiles in a Magnetically Shielded Cylinder |url=https://link.aps.org/doi/10.1103/PhysRevApplied.14.054004 |journal=Physical Review Applied |volume=14 |issue=5 |pages=054004 |doi=10.1103/PhysRevApplied.14.054004|arxiv=2006.02981 |bibcode=2020PhRvP..14e4004P |s2cid=221538013 }}</ref><ref>{{Cite journal |last1=Packer |first1=M. |last2=Hobson |first2=P.J. |last3=Holmes |first3=N. |last4=Leggett |first4=J. |last5=Glover |first5=P. |last6=Brookes |first6=M.J. |last7=Bowtell |first7=R. |last8=Fromhold |first8=T.M. |date=2021-06-02 |title=Planar Coil Optimization in a Magnetically Shielded Cylinder |url=https://link.aps.org/doi/10.1103/PhysRevApplied.15.064006 |journal=Physical Review Applied |volume=15 |issue=6 |pages=064006 |doi=10.1103/PhysRevApplied.15.064006|arxiv=2101.01275 |bibcode=2021PhRvP..15f4006P |s2cid=230524109 }}</ref><ref>{{Cite journal |last1=Liu |first1=C. -Y. |last2=Andalib |first2=T. |last3=Ostapchuk |first3=D. C. M. |last4=Bidinosti |first4=C. P. |date=2020-01-01 |title=Analytic models of magnetically enclosed spherical and solenoidal coils |url=https://www.sciencedirect.com/science/article/pii/S0168900219312719 |journal=Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment |language=en |volume=949 |pages=162837 |doi=10.1016/j.nima.2019.162837 |arxiv=1907.03539 |bibcode=2020NIMPA.94962837L |s2cid=195833040 |issn=0168-9002}}</ref><ref>{{Cite journal |last1=Mäkinen |first1=Antti J. |last2=Zetter |first2=Rasmus |last3=Iivanainen |first3=Joonas |last4=Zevenhoven |first4=Koos C. J. |last5=Parkkonen |first5=Lauri |last6=Ilmoniemi |first6=Risto J. |date=2020-08-14 |title=Magnetic-field modeling with surface currents. Part I. Physical and computational principles of bfieldtools |url=http://aip.scitation.org/doi/10.1063/5.0016090 |journal=Journal of Applied Physics |language=en |volume=128 |issue=6 |pages=063906 |doi=10.1063/5.0016090 |arxiv=2005.10060 |bibcode=2020JAP...128f3906M |s2cid=218718690 |issn=0021-8979}}</ref><ref>{{Cite journal |last1=Zetter |first1=Rasmus |last2=J. Mäkinen |first2=Antti |last3=Iivanainen |first3=Joonas |last4=Zevenhoven |first4=Koos C. J. |last5=Ilmoniemi |first5=Risto J. |last6=Parkkonen |first6=Lauri |date=2020-08-14 |title=Magnetic field modeling with surface currents. Part II. Implementation and usage of bfieldtools |url=http://aip.scitation.org/doi/10.1063/5.0016087 |journal=Journal of Applied Physics |language=en |volume=128 |issue=6 |pages=063905 |doi=10.1063/5.0016087 |arxiv=2005.10056 |bibcode=2020JAP...128f3905Z |s2cid=218719330 |issn=0021-8979}}</ref> referred to as ''hybrid'' shielding,<ref>{{Cite journal |last1=Royal |first1=Kevin |last2=Crawford |first2=Christopher |last3=Mullins |first3=Andrew |last4=Porter |first4=Greg |last5=Blanton |first5=Hunter |last6=Johnstone |first6=Connor |last7=Kistler |first7=Ben |last8=Olivera |first8=Daniela |date=2017-09-01 |title=Hybrid Magnetic Shielding |journal=APS Division of Nuclear Physics Meeting Abstracts |url=https://ui.adsabs.harvard.edu/abs/2017APS..DNP.EA034R |volume=2017 |pages=EA.034|bibcode=2017APS..DNP.EA034R }}</ref> so that there is broadband shielding from the passive shield and additional cancellation of specific components using the active system.

Additionally, [[superconductivity|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 [[relative magnetic permeability|relative permeability]] {{nowrap|<math>\mu_\text{r}</math>,}} with inner radius <math>a</math> and outer radius {{nowrap|<math>b</math>.}} We then put this object in a constant magnetic field:
<math display="block">\mathbf{H}_0 = H_0 \hat\mathbf{z} = H_0 \cos(\theta) \hat\mathbf{r} - H_0 \sin(\theta) \hat\boldsymbol{\theta}</math>
Since there are no currents in this problem except for possible bound currents on the boundaries of the diamagnetic material, then we can define a magnetic scalar potential that satisfies [[Laplace's equation]]:
<math display="block">\begin{align} \mathbf{H} &= -\nabla \Phi_{M} \\ \nabla^{2} \Phi_{M} &= 0 \end{align}</math>
where
<math display="block">\mathbf{B} = \mu_\text{r}\mathbf{H}</math>
In this particular problem there is azimuthal symmetry so we can write down that the solution to Laplace's equation in spherical coordinates is:
<math display="block">\Phi_{M} = \sum_{\ell=0}^\infty \left(A_{\ell}r^{\ell}+\frac{B_{\ell}}{r^{\ell+1}}\right) P_{\ell}(\cos\theta)</math>
After matching the boundary conditions
<math display="block">\begin{align}\left(\mathbf{H}_2 - \mathbf{H}_1\right)\times\hat\mathbf{n}&=0\\\left(\mathbf{B}_2 - \mathbf{B}_1\right) \cdot \hat\mathbf{n} &=0 \end{align}</math>
at the boundaries (where <math>\hat{n}</math> is a [[unit vector]] that is normal to the surface pointing from side 1 to side 2), then we find that the magnetic field inside the cavity in the spherical shell is:
<math display="block">\mathbf{H}_\text{in}=\eta\mathbf{H}_{0}</math>
where <math>\eta</math> is an [[attenuation coefficient]] that depends on the thickness of the diamagnetic material and the magnetic permeability of the material:
<math display="block">\eta = \frac{9\mu_\text{r}}{\left(2\mu_\text{r} + 1\right) \left(\mu_\text{r} + 2\right) - 2\left(\frac{a}{b}\right)^3 \left(\mu_\text{r} - 1\right)^2}</math>
This coefficient describes the effectiveness of this material in shielding the external magnetic field from the cavity that it surrounds. Notice that this coefficient appropriately goes to 1 (no shielding) in the limit that <math>\mu_\text{r} \to 1</math>. In the limit that <math>\mu_\text{r} \to \infty</math> this coefficient goes to 0 (perfect shielding). When <math>\mu_\text{r} \gg 1</math>, then the attenuation coefficient takes on the simpler form:
<math display="block">\eta = \frac{9}{2 \left(1 - \frac{a^3}{b^3}\right) \mu_\text{r}}</math> 
which shows that the magnetic field decreases like {{nowrap|<math>\mu_\text{r}^{-1}</math>.}}<ref>{{cite book |first=John David |last=Jackson |title=Classical Electrodynamics |edition=third |at=Section 5.12 |date=10 August 1998 |isbn=978-0471309321 <!-- hardcover -->}}</ref>

== 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==
{{Reflist}}

== External links ==
* [http://www.mumetal.co.uk All about Mu Metal Permalloy material]
* [https://archive.today/20130213080154/http://www.abschirmungen.eu/en/products/faq/ Mu Metal Shieldings Frequently asked questions (FAQ by MARCHANDISE, Germany) magnetic permeability]
*[http://www.cvel.clemson.edu/emc/calculators/SE3_Calculator/ Clemson Vehicular Electronics Laboratory: Shielding Effectiveness Calculator] {{Webarchive|url=https://web.archive.org/web/20170709124557/http://www.cvel.clemson.edu/emc/calculators/SE3_Calculator/ |date=2017-07-09 }}
* [https://web.archive.org/web/20070927105513/http://www.ets-lindgren.com/pdf/conformity_0705_turner.pdf Shielding Issues for Medical Products] ([[PDF]]) — [[ETS-Lindgren]] Paper
* [http://learnemc.com/practical-em-shielding Practical Electromagnetic Shielding Tutorial]
* [https://web.archive.org/web/20110716141724/http://www.comsol.com/papers/1519/ Simulation of Electromagnetic Shielding in the COMSOL Multiphysics Environment]

{{Authority control}}

{{DEFAULTSORT:Electromagnetic Shielding}}
[[Category:Magnetoencephalography]]
[[Category:Radio electronics]]
[[Category:Electromagnetic radiation]]
[[Category:Electromagnetic compatibility]]