Quantcast Table 8-14 Comparison of Measured and Calculated Values of Shielding Effectiveness for No. 22, 15 Mil Copper Screens (8-8)

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MIL-HDBK-419A
Table 8-14
Comparison of Measured and Calculated Values
of Shielding Effectiveness for No. 22, 15 Mil Copper Screens (8-8)
Test
Calculated
Measured
Type
Effectiveness
Effectiveness
(dB)
(dB)
(MHZ)
Magnetic
0.085
31
29
field
1.000
43
46
49
10.000
43
118
Plane
0.200
124
Wave
1.000
106
110
5.000
100
95
80
100.000
70
Electric
65
0.014
**65
field
**The value assumes a wave impedance equal to that of a 30-inch square waveguide.
The mesh construction should have individual strands permanently joined at points of intersection by a fusing
process so that a permanent electrical contact is made and oxidation does not reduce shielding effectiveness. A
screen of this construction will be very effective for shielding against electric (high-impedance) fields at low
frequencies because the losses will be primarily caused by reflection. Installation can be made by connecting a
screen around the periphery of an opening.
8.5 SHIELD DISCONTINUITY EFFECTS (APERTURES).
An ideal shielded enclosure would be one of seamless construction with no openings or discontinuities.
However, personnel, powerlines, control cables, and/or ventilation ducts must have access to any practical
enclosure. The design and construction of these discontinuities become very critical in order to incorporate
them without appreciably reducing the shielding effectiveness of the enclosures. Since most mechanically
suitable metal enclosures will give enough shielding above 1 MHz, EMI leakage above 1 MHz is due primarily to
discontinuities. EMI leakage (the amount of EM energy that will leak from a discontinuity) depends mainly on:
a.
maximum length (not area) of the opening,
8-41





 


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