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EMC Basics #13: How shield is destroyed

Posted: 30 Jan 2012 ?? ?Print Version ?Bookmark and Share

Keywords:EMC? shielding? seams? penetrations?

Seams and penetrations. These are the two ways by which a high-frequency signal could be destroyed. As discussed in the previous article from this series, even thin conductive materials work well for frequencies above about 10kHz. Thus, the weak points are mechanical rather than materials.

Building a high-frequency shield is like building a wooden water tank. Once the planks are thick enough, the leaks occur at seams, joints, penetrations, and even knotholes. And even a small hole can be a problem: drill a ? inch hole in the bottom of the tank, and eventually all the water leaks out.

Incidentally, the "planks" don't need to be thick for high-frequency EMC shielding, as long as they are conductive. Obviously, wood doesn't work for EMC, but aluminum foil is very effective, along with surface treatments like conductive paint or plating. Remember, however, you still may need the "thick conductive planks" for low-frequency magnetic field shielding (60Hz and harmonics).

Before we go further, we need to look at some simple physics. Although the water-tank analogy is useful, it falls apart in several ways. Here are a couple of key points:

???For seams, the longest dimension is criticalNOT the area. Unlike water, a six-inch seam will leak the same as a six-inch hole under worst-case conditions. The difference is that the seam will be highly polarized, while the hole will not. When designing an EMC enclosure, we need to be pessimistic. After all, Murphy and his law will make sure that the worst case will occur.
???For penetrations, the depth of penetration is critical, NOT the hole size. If a wire, cable, or even a pipe extends beyond the EMC shield and is NOT shorted to the shield, the extensions act like antennas connected by a coaxial cable. For example, carrying wires or cables through a hole to a connector on the circuit board can completely destroy a shield at high frequencies.
A good rule of thumb for seams and penetrations is the "1/20 wavelength rule." Antenna designers often use this guideline as a practical limit when making small antennas. You don't need a half wavelength (or even a quarter wavelength) to support electromagnetic radiation C 1/20 of a wavelength will still do a credible job.

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