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 Reference is made to the following tables and graphs from chapter 8 of
MIL-HDBK-419, Grounding, Bonding, and Shielding for Electronic Equipments and
Facilities, Volume I:
TABLE
SUBJECT
8-1
Table of conductivity, permeability and aborsoption
loss of 21 different shielding materials at 150 kHz.
8-2
Table of permeability and absorption loss of iron,
copper, and aluminum shields, 1 mm thick versus
frequency (60 Hz to 10 GHz).
8-4
Table of permeability and H-field, E-field, (in the
near field), and plane wave reflection losses for
iron, copper, and aluminum shields versus frequency
(60 Hz to 10 GHz) with a source to shield separation
of one foot.
2.6
Penetrations. The theoretical shielding effectiveness of a large
variety of conductive metal materials would appear to provide sufficient dB of
SE to be available for construction of 100 dB enclosures if it were possible
to use the infinite sheet values utilized in the theoretical equations for
reflection and absorption. Unfortunately, the ideal enclosure formed from a
seamless envelope of metallic material with no openings or penetrations is not
a reality. The obtainable overall SE for both welded and bolted seam steel
enclosures, soldered and bolted copper enclosures, and bolted and welded
aluminum enclosures is severely limited by first, the door closures, second,
bolted or gasketed seams, third, penetrations by metal conductors, and fourth,
the wave guide below cutoff air duct, vent and drain penetrations. Beyond
this, cracks in welds, corrosion of seam metal-to-metal mating surfaces, and
uncontrolled penetration by fasteners such as screws, nails, or bolts add to
the degradation from the theoretical or ideal SE of a shielding material.
2.6.1
Waveguide Below Cutoff Penetration Theory. It is possible to
provide penetrations which will pass light, air, or liquids through the
shielding surface without passing electromagnetic waves lower in frequency
than a certain fixed value by means of an effect called waveguide below
cutoff. Waveguides are formed by metal tubing or ducting and are used to
deliberately propagate electromagnetic waves in transverse-electric (TE) modes
and transverse magnetic (TM) modes for wave frequencies above a cutoff
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