Understanding the wave nature of EMI
It is important to understand that waves are two dimensional objects. Picture an EM wave as a piece of paper flying lengthwise through the air. The width of the paper is the amplitude of the wave, and the length of the paper is the wavelength. For purposes of understanding this example, assume the paper is actually infinitely thin.
Now try to fit this paper through your window. If you open your window just the tiniest amount possible, you will see that you can slip this piece of paper through that little crack. However, if you open the window wide, but you have a screen in it to prevent bugs from flying through; you will see that you cannot fit the paper through the holes in the screen. Despite the fact that the screen is not solid, you can’t get the paper through, yet in the case of the tiny wide crack, the crack in the window might be less than the distance between the wires in the screen, but you can still get the paper through it.
The point of this example is to help you understand that EM waves can easily escape through seems in electronic enclosures, but they cannot pass through holes, as long as the hole is significantly smaller than the wavelength of the wave. So one can have a metal box with an EMI generating device in it, and you can have rather large holes in your box, and the device will still be perfectly shielded as long as the box does not have a seem.
If the box is hinged, so that it opens and closes, likely the two sides of the box are not in perfect contact at all points, and EMI will easily pass through the seam, just like the paper passed through the tiniest crack in the window.
This leads to the concept of a Faraday Cage, where a mesh box is used to shield a device from EMI/RFI.
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