RF shielding
|
|
RF shielding is the protection of sensitive electrical equipment from external radiofrequency (RF) electromagnetic radiation by enclosing it in a conducting material. RF shielding is a refinement of the principle of the Faraday cage, which protects equipment from electric fields such as those from electrostatic discharges.
The enclosure may be made of an unbroken conducting sheet, like the metal box surrounding a sensitive radio receiver, or a wire mesh, like that in the door of a microwave oven. Any holes in the box 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.
How RF 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 generates a current that causes displacement of charge inside the conductor that cancels the applied field inside.
Similarly, varying magnetic fields generate current vortices that act to cancel the applied magnetic field. (The conductor does not respond to static magnetic fields, so static magnetic fields can penetrate the conductor freely.) 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 case of high-frequency electromagnetic radiation the time the above-mentioned adjustments take is not negligible, but then the radiation energy, as far as it is not reflected, is absorbed by the skin (unless it is extremely thin), so in that 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.