[Amps] Shielding Effectiveness

[Ward Silver]

Per Ott's "Electromagnetic Compatibility Engineering", section 6.10 - shielding effectiveness in dB is:

S = 20 log (wavelength / 2xL) where L is the maximum linear dimension of the aperture.

If you have multiple apertures, such as an array of holes, the general procedure is to calculate S for one hole and then reduce S as 10 log (n) where n is the maximum number of holes in a straight line in the array.  

For example, if S for one hole is 52 dB (a 2.5 mm hole at 2 meters) and there are at most 20 holes in a line, S would be reduced by 10 log (20) = 13 dB to S = 52 - 13 = 39 dB.

This is an approximation, of course, plus there are seams and other things to worry about.  But it's a good way to at least compare types of apertures in shields.

73, Ward N0AX