| MadSci Network: Engineering |
Shielding electromagnetic energy is a common problem in the design of almost any electronic device. There are two objectives: suppress the EM (electromagnetic) energy the device generates to a level where it won't interfere with other users or be detectable, and reduce the susceptibility of the device to undesirable effects created by other sources of EM energy. Electromagnetic waves are shielded effectively by electrically conductive material (typically, metal); shielding of magnetic fields is provided by ferromagnetic material (e.g., steel or other alloys). A Web search on the term "electromagnetic shielding" will reveal hundreds of hits to companies with something to sell. Related concepts that are worth searching include "electromagnetic interference", "electromagnetic compatibility", "EMI", and "EMC". It's not practical to shield electromagnetic waves all the way down to 0 Hz (and why do you really want to?) The issue is that the depth to which an electromagnetic wave can penetrate into conductive material is dependent on several factors. Specifically, the so-called "skin depth" of a material is expressed as d = 1/sqrt(pi*f*mu*sigma) where d = distance at which the amplitude of the EM wave has decreased to 1/e = 36.8% as strong. At distand 4*d, the EM wave is only (0.368)**4 as strong, or about 2% of its original strength. At distance 14*d the wave is only 1 millionth as strong. The other parameters are f = frequency, Hz mu = magnetic permeability of the material sigma = conductivity of the material You can find this equation and the necessary parameters in a good college physics textbook or engineering textbook on electromagnetics. Plugging in the parameters for copper gives d = 0.0661/sqrt(f) meters Thus, at 60 Hz the skin depth is about 8.5 mm, or 1/3 of an inch. It would take a considerable thickness of copper to provide significant shielding. At lower frequencies, the skin depth grows larger. At higher frequencies, things get much better. At 100 MHz, say, the skin depth is 0.06 mm, or about 0.026 inches(about 1/32 of an inch). At these frequencies and above, a fairly thin piece of sheet metal or even metal foil becomes an effective shield. What this means is that electromagnetic energy is conducted more by regions near the surface of a conductor than by the interior; a metal tube can conduct AC current almost as well as a solid rod of same outside dimension. At microwave frequencies, an evaporated metal coating on an insulator will work as well as a much thicker conductor. Shields up, Mr. Worf! Steve Czarnecki
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