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 MIL-HDBK-419A
where
a = area of each hole in square inches
n = number of holes per square inch
= correction factor for penetration of the conductor at low frequencies
(8-22)
where
p = ratio of the wire diameter to skin depth,
where
(8-23)
K3 = correction factor for coupling between closely spaced shallow holes
As an example, determine the shielding effectiveness of a No. 22, 15 mil copper screen when subjected to a
predominantly magnetic field from a loop source 1.75 inches away and operating at a frequency of 1 MHz. For
such a screen, there are 22 meshes per linear inch; the center-of-wire to center-of-wire distance is 1/22
(0.045) inch and the opening width is smaller by an amount equal to the wire diameter, 0.015 inches. The depth
of the apertures is assumed equal to the wire diameter.
Thus
The impedance ratio for the magnetic wave and rectangular apertures is given by
and the reflection term is
8-35
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