Electromagnetic shielding
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.
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.
Common sheet metals for shielding include copper, brass, nickel, silver, steel, and tin.
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.
Properly designed and constructed RF shielding enclosures satisfy most RF shielding needs, from computer and electrical switching rooms to hospital CAT-scan and MRI facilities.
[3][4] 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).
[5] EMI shielding is often achieved by electroless plating of copper as most popular plastics are non-conductive or by special conductive paint.
From the perspective of microwaves (with wavelengths of 12 cm) this screen finishes a Faraday cage formed by the oven's metal housing.
RF shielding is also used to prevent access to data stored on RFID chips embedded in various devices, such as biometric passports.
[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.
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.
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.
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.
[8] 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 alloys can be used, such as sheets of permalloy and mu-metal[9][10] or with nanocrystalline grain structure ferromagnetic metal coatings.
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 saturated.
Entry holes within shielding surfaces may degrade their performance significantly.
Additionally, superconducting materials can expel magnetic fields via the Meissner effect.
Suppose that we have a spherical shell of a (linear and isotropic) diamagnetic material with relative permeability
In this particular problem there is azimuthal symmetry so we can write down that the solution to Laplace's equation in spherical coordinates is:
This coefficient describes the effectiveness of this material in shielding the external magnetic field from the cavity that it surrounds.
