Controlling Fields Within Test Environments

摘要

Reducing interference from extraneous sources obviously is important when a radio receiver is being developed. Ideally, you would eliminate all signals other than the test signal you deliberately generated. This is the function of a screened room or shielded enclosure. Similarly, if you needed to measure emissions from a piece of equipment, doing so inside a screened room would minimize the errors caused by signals from other sources. How well you can constrain electric, magnetic, or electromagnetic fields depends on many variables including field strength, distance to the source, frequency, and characteristics of the material used to construct the enclosure. Static electric fields have the highest impedance and are completely controlled by a Faraday cage, which approximates a closed conductive shell. According to Gauss's Law, no charge can exist inside a conductor. It all resides on the outer surface. Charge is redistributed on the outer surface of the enclosure to force the internal field to zero—the principle behind screened room performance. The principle works as well in reverse, confining even very strong electric fields inside a test chamber so that the field outside is zero. Faraday cages have almost ideal performance for electrostatic fields, work very well for electromagnetic fields but with some qualifications, and have little effect on low-frequency magnetic fields. For low-frequency electric fields, such as found in a power plant, a Faraday cage can be made from hardware cloth, commonly called chicken wire, and still be nearly as effective as if it were made from solid sheet metal. For higher frequencies, the size of the holes between the conductors and the resistance of the conductors themselves become important, and the enclosure is more correctly considered a shield than a Gaussian surface. Even a small gap is significant at high frequencies, so seams and access doors can be problem areas.