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| {{refimprove|date=March 2010}}
| | == I can not let up == |
| [[Image:Electromagnetic shielding inside mobile phone.jpg|thumb|Electromagnetic shielding cages inside a disassembled [[mobile phone]].]]
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| '''Electromagnetic shielding''' is the practice of reducing the [[electromagnetic field]] in a space by blocking the field with barriers made of [[electrical conductor|conductive]] or magnetic materials. Shielding is typically applied to enclosures to isolate electrical devices from the 'outside world', and to cables to isolate wires from the environment through which the cable runs. Electromagnetic shielding that blocks [[radio frequency]] [[electromagnetic radiation]] is also known as '''RF shielding'''. | | But to say this. Senior students in martial arts that we,[http://www.alleganycountyfair.org/_vti_cnf/rakuten_oakley_16.htm オークリー 激安 サングラス], you and Yang lunatic brother duo, who have been quasi-martial assessment through. 'This young man in white touch his head helplessly,[http://www.alleganycountyfair.org/_vti_cnf/rakuten_oakley_20.htm オークリー サングラス 野球],' see you both pass,[http://www.alleganycountyfair.org/_vti_cnf/rakuten_oakley_4.htm 登山 サングラス オークリー], I can not let up, so we came early this morning Lianqiang it. '<br><br>Luo Feng nodded,[http://www.alleganycountyfair.org/_vti_cnf/rakuten_oakley_43.htm オークリー サングラス ランニング].<br><br>turned out to be a teacher say this, though more than 30,[http://www.alleganycountyfair.org/_vti_cnf/rakuten_oakley_40.htm サングラス オークリー 人気],000 limit number of martial arts students, but teachers a total of only six people! Instructor is just only one! When teachers teach,[http://www.alleganycountyfair.org/_vti_cnf/rakuten_oakley_76.htm オークリー野球サングラス], teaching mostly in the kind of large hall teaching, allowing thousands of students while lectures.<br><br>course to teach senior students do not have a senior student will then point the whole martial arts person only.<br><br>'madman, you pass the assessment,[http://www.alleganycountyfair.org/_vti_cnf/rakuten_oakley_24.htm ゴルフ オークリー サングラス], early in the morning to doing.' young man in white smiles.<br><br>'8 1st, fighter combat assessment I have to prepare myself.' Luo Feng smiled on towards martial arts hall and walked beside the weapons room,[http://www.alleganycountyfair.org/_vti_cnf/rakuten_oakley_24.htm ゴルフ オークリー サングラス], weapons room door is not closed. |
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| The shielding can reduce the [[Coupling (electronics)|coupling]] of [[Radio frequency|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 apertures in a shield to an incident electromagnetic field.
| | == then eventually getting stronger ...... One day == |
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| ==Materials used==
| | Fixed! 'Hung looked Luo Feng,[http://www.alleganycountyfair.org/_vti_cnf/rakuten_oakley_76.htm オークリー野球サングラス],' The most important thing is not to practice than others,[http://www.alleganycountyfair.org/_vti_cnf/rakuten_oakley_63.htm オークリー サングラス 新作], but and their ratio! As long as you have been in progress, has been beyond yourself, then eventually getting stronger ...... One day,[http://www.alleganycountyfair.org/_vti_cnf/rakuten_oakley_76.htm オークリー野球サングラス], you will find ...... the whole world, you could not find opponents. You have in the world! '<br><br>Luofeng Jing Jing listen,[http://www.alleganycountyfair.org/_vti_cnf/rakuten_oakley_33.htm サングラス オークリー 偏光].<br><br>on big brother 'Hung', Luo Feng is a very respectable admirable.<br><br>simply force themselves on strong,[http://www.alleganycountyfair.org/_vti_cnf/rakuten_oakley_43.htm オークリー サングラス イチロー], but in the state of mind that he has been able to rival some of the big brother called saints in the history of the Chinese presence.<br><br>'Haha,[http://www.alleganycountyfair.org/_vti_cnf/rakuten_oakley_14.htm オークリー 激安 サングラス], boss,[http://www.alleganycountyfair.org/_vti_cnf/rakuten_oakley_38.htm ゴルフ サングラス オークリー], you did is so become the world's first?' Thor laugh watching Feng Luo.<br><br>'Yes!' Hung calm eyes,[http://www.alleganycountyfair.org/_vti_cnf/rakuten_oakley_76.htm オークリー野球サングラス], nodded and smiled, 'when I was loyal to a spear in my hands, in the hands of a spear to elaborate world of reason! one step beyond ...... I did not think my big Nirvana period It even became the first person on Earth. Luo Feng, understand my meaning |
| Typical materials used for electromagnetic shielding include [[sheet metal]], metal screen, and [[metal foam]]. 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.
| | 相关的主题文章： |
| | | <ul> |
| 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.
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| | | <li>[http://dept2.hbeu.cn/xiaoyou/bbs/home.php?mod=space&uid=983117 http://dept2.hbeu.cn/xiaoyou/bbs/home.php?mod=space&uid=983117]</li> |
| RF shielding enclosures filter a range of frequencies for specific conditions. [[Copper in architecture#Radio frequency shielding|Copper is used for radio frequency (RF) shielding]] because it absorbs [[radio wave|radio]] and [[magnetic wave]]s. Properly designed and constructed [[Copper in architecture#Radio frequency shielding|copper 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>Radio frequency shielding, Copper in Architecture Design Handbook, Copper Development Association Inc., http://www.copper.org/applications/architecture/arch_dhb/fundamentals/radio_shielding.html</ref>
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| ==Example applications==
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| [[Image:Coaxial cable cutaway.svg|thumb|right|Cross-section through a coaxial cable showing shielding and other layers]]
| | <li>[http://www.ryowa.net/cgi/bkn_v1.cgi http://www.ryowa.net/cgi/bkn_v1.cgi]</li> |
| 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.
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| 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.
| | </ul> |
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| 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.
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| 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=http://www.newscientist.com/article/dn8227-metal-shields-and-encryption-for-us-passports.html |title=Metal shields and encryption for US passports |publisher=Newscientist.com |accessdate=18 November 2012}}</ref>
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| 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.<ref>{{Cite journal|url=http://lasecwww.epfl.ch/keyboard/|title=Compromising Electromagnetic Emanations of Wired and Wireless Keyboards|date=2009-06-01|author=Martin Vuagnoux and Sylvain Pasini|publisher=Security and Cryptography Laboratory (LASEC)|location=[[Lausanne]]}}</ref>
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| 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.
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| == How electromagnetic shielding works ==
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| 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.
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| 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.
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| 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, 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.
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| 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]].
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| == Magnetic shielding ==
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| Equipment sometimes requires isolation from external magnetic fields. For static or slowly varying magnetic fields (below about 100 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>[http://www.magnetic-shield.com/literature/pdf/mu-2.pdf] MuMETAL brochure MU-2 | The most recognized brand of magnetic shielding alloy worldwide.</ref> or with nanocrystalline grain structure ferromagnetic metal coatings.<ref>[http://www.nxtbook.com/nxtbooks/item/emcdirectory-design_2009/index.php#/134] Interference Technology Magazine Whitepaper on Ferromagnetic Nanocrystalline Metal Magnetic Shield Coatings</ref> These materials don't 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 at high field strengths where the material becomes [[magnetic saturation|saturated]]. So 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.
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| Because of the above limitations of passive shielding, an alternative used with static or low-frequency fields is active shielding; using a field created by [[electromagnet]]s to cancel the ambient field within a volume.<ref>[http://www.acornnmr.com/appnotes/shielding.htm] NMR Magnet Shielding: The seat of the pants guide to understanding the problems of shielding NMR magnets.</ref> [[Solenoid]]s and [[Helmholtz coil]]s are types of coils that can be used for this purpose.
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| Additionally, [[superconductivity|superconducting]] materials can expel magnetic fields via the [[Meissner effect]].
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| ==Mathematical model==
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| Suppose that we have a spherical shell of a (linear and isotropic) diamagnetic material with permeability <math>\mu</math>, with inner radius <math>a</math> and outer radius <math>b</math>. We then put this object in a constant magnetic field:
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| :<math>\vec{H}_{0}=H_{0}\hat{z}=H_{0}\cos\theta \hat{r}</math>
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| 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:
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| :<math>\vec{H}=-\nabla \Phi_{M}</math>
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| :<math>\nabla^{2}\Phi_{M}=0</math>
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| where
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| :<math>\vec{B}=\mu \vec{H} </math>
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| In this particular problem there is azimuthal symmetry so we can write down that the solution to Laplace's equation in spherical coordinates is:
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| :<math>\Phi_{M}=\sum_{l=0}^\infty \left( A_{l}r^{l}+\frac{B_{l}}{r^{l+1}}\right)P_{l}(\cos\theta)</math>
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| After matching the boundary conditions
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| :<math>(\vec{H_{2}}-\vec{H_{1}})\times \hat{n}=0</math> | |
| :<math>(\vec{B_{2}}-\vec{B_{1}})\cdot \hat{n}=0</math>
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| 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:
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| :<math>\vec{H_{in}}=\eta \vec{H_{0}}</math>
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| where <math>\eta</math> is an attenuation coefficient that depends on the thickness of the diamagnetic material and the magnetic permeability of the material:
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| :<math>\eta=\frac{9\mu}{(2\mu+1)(\mu+2)-2\left(\frac{a}{b}\right)^{3}(\mu-1)^2}</math>
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| 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 \rightarrow 1</math>. In the limit that <math>\mu \rightarrow 0, \infty </math> this coefficient goes to 0 (perfect shielding), then the attenuation coefficient takes on the simpler form:
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| :<math> \eta=\frac{9}{2} \frac{1}{(1-\frac{a^{3}}{b^{3}}) \mu} </math>
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| which shows that the magnetic field decreases like <math> \mu^{-1} </math>.<ref>J.D. Jackson. Classical Electrodynamics, Third Ed. Section 5.12</ref>
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| == See also ==
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| *[[Electromagnetic interference]]
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| *[[Electromagnetic radiation and health]]
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| *[[Radiation]]
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| *[[Ionising radiation protection]]
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| *[[Mu-metal]]
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| *[[MRI RF shielding]]
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| *[[Permalloy]]
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| *[[Electric field screening]]
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| *[[Faraday cage]]
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| ==References==
| |
| {{Reflist}}
| |
| | |
| == External links ==
| |
| * [http://www.mumetal.co.uk All about Mu Metal Permalloy material]
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| * [http://www.abschirmungen.eu/en/products/faq/ Mu Metal Shieldings Frequently asked questions (FAQ by MARCHANDISE, Germany) magnetic permeability]
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| *[http://www.cvel.clemson.edu/emc/calculators/SE_Calculator/index.html Clemson Vehicular Electronics Laboratory: Shielding Effectiveness Calculator]
| |
| * [http://www.ets-lindgren.com/pdf/conformity_0705_turner.pdf Shielding Issues for Medical Products] ([[PDF]]) — [[ETS-Lindgren]] Paper
| |
| * [http://www.vitatech.net/pdf/ENG_GUIDE.pdf Guide To Solving AC Power EMF Problems] ([[PDF]]) — [[VitaTech Engineering]] Paper
| |
| * [http://www.learnemc.com/tutorials/Shielding02/Practical_Shielding.html Practical Electromagnetic Shielding Tutorial]
| |
| * [http://www.comsol.com/papers/1519/ Simulation of Electromagnetic Shielding in the COMSOL Multiphysics Environment]
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| {{DEFAULTSORT:Electromagnetic Shielding}}
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| [[Category:Magnetoencephalography]]
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| [[Category:Radio electronics]]
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| [[Category:Electromagnetic radiation]]
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| [[Category:Electromagnetic compatibility]]
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I can not let up
But to say this. Senior students in martial arts that we,オークリー 激安 サングラス, you and Yang lunatic brother duo, who have been quasi-martial assessment through. 'This young man in white touch his head helplessly,オークリー サングラス 野球,' see you both pass,登山 サングラス オークリー, I can not let up, so we came early this morning Lianqiang it. '
Luo Feng nodded,オークリー サングラス ランニング.
turned out to be a teacher say this, though more than 30,サングラス オークリー 人気,000 limit number of martial arts students, but teachers a total of only six people! Instructor is just only one! When teachers teach,オークリー野球サングラス, teaching mostly in the kind of large hall teaching, allowing thousands of students while lectures.
course to teach senior students do not have a senior student will then point the whole martial arts person only.
'madman, you pass the assessment,ゴルフ オークリー サングラス, early in the morning to doing.' young man in white smiles.
'8 1st, fighter combat assessment I have to prepare myself.' Luo Feng smiled on towards martial arts hall and walked beside the weapons room,ゴルフ オークリー サングラス, weapons room door is not closed.
相关的主题文章：
then eventually getting stronger ...... One day
Fixed! 'Hung looked Luo Feng,オークリー野球サングラス,' The most important thing is not to practice than others,オークリー サングラス 新作, but and their ratio! As long as you have been in progress, has been beyond yourself, then eventually getting stronger ...... One day,オークリー野球サングラス, you will find ...... the whole world, you could not find opponents. You have in the world! '
Luofeng Jing Jing listen,サングラス オークリー 偏光.
on big brother 'Hung', Luo Feng is a very respectable admirable.
simply force themselves on strong,オークリー サングラス イチロー, but in the state of mind that he has been able to rival some of the big brother called saints in the history of the Chinese presence.
'Haha,オークリー 激安 サングラス, boss,ゴルフ サングラス オークリー, you did is so become the world's first?' Thor laugh watching Feng Luo.
'Yes!' Hung calm eyes,オークリー野球サングラス, nodded and smiled, 'when I was loyal to a spear in my hands, in the hands of a spear to elaborate world of reason! one step beyond ...... I did not think my big Nirvana period It even became the first person on Earth. Luo Feng, understand my meaning
相关的主题文章：