EMC Question of the Week: April 18, 2022

desktop computer enclosure

In terms of shielding effectiveness, the most important dimension of a slot or seam in a shielding enclosure is generally its

  1. length
  2. thickness
  3. depth
  4. area


The best answer is “a.” Length is by far the most important dimension of a slot or seam. A shielding enclosure with a resonant-length seam (e.g., approximately a half-wavelength) could radiate more efficiently than the same product with no enclosure at all. Thickness, depth and area are far less important. 

For cooling vents, it is desirable to get the greatest amount of open area using openings that have the smallest possible maximum dimension. Arrays of small round holes are preferable to elongated slots. For cooling apertures with significant cross-sectional area, providing significant depth can create a waveguide-below-cutoff that provides additional attenuation at frequencies where the wavelength is small relative to the longest dimension of the aperture opening. 

In a seam with significant length, additional depth (i.e., more overlap) can help to reduce the impedance observed by currents crossing the seam. Enclosure seams must generally be sealed using finger stock, gaskets or other methods to ensure a good high-frequency connection.   

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