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Electromagnetic shielding

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Electromagnetic shielding cages inside a disassembled mobile phone.

Electromagnetic shielding is the process of reducing the electromagnetic field in a space by blocking the field with barriers made of conductive and/or magnetic materials. Shielding is typically applied (1) to enclosures to isolate electrical devices from the 'outside world' and (2) 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.

The shielding can reduce the coupling of radio waves, electromagnetic fields and electrostatic fields, though not static or low-frequency magnetic fields[citation needed] (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.

Materials used

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.

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.

Example applications

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 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 between the wires.

RF shielding is also used to prevent access to data stored on RFID chips embedded in various devices, such as biometric passports.

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.[1]

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.

How electromagnetic shielding works

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

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 electrical resistance of the conductor, the excited field does not completely cancel the incident field. Also, most conductors exhibit a 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.

In the case of high-frequency electromagnetic radiation, the above-mentioned adjustments take a non-negligible amount of time. But then the 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 called the skin effect. A measure for 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. For static or slowly varying magnetic fields (below about 100 kHz) the Faraday shielding described above is ineffective. There exists a limited possibility of passively isolating a volume magnetically by using shields made of high magnetic permeability metal alloys such as large crystalline grain structure foils or sheet metals of Permalloy and Mu-metal,[2] or with nanocrystalline grain structure ferromagnetic metal coatings.[3] 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. The effectiveness of this type of shielding decreases with the material's permeability, which generally drops off at both very low magnetic field strengths, and also at high field strengths where the material becomes 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.

Because of the above limitations of passive shielding, an alternative used with static fields is active shielding; using a field created by another magnet to cancel out the ambient field within a volume. Solenoids designed to do this are called Helmholtz coils.[4]

Additionally, superconducting materials can expel magnetic fields via the Meissner effect.

See also

References

  1. ^ Martin Vuagnoux and Sylvain Pasini (2009-06-01). "Compromising Electromagnetic Emanations of Wired and Wireless Keyboards". Lausanne: Security and Cryptography Laboratory (LASEC). {{cite journal}}: Cite journal requires |journal= (help)
  2. ^ [1] Magnetic shielding. Lecture notes from the University of Texas.
  3. ^ [2] Interference Technology Magazine Whitepaper on Ferromagnetic Nanocrystalline Metal Magnetic Shield Coatings
  4. ^ [3] NMR Magnet Shielding: The seat of the pants guide to understanding the problems of shielding NMR magnets.
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