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
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