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 kHz, as seen in Figure 15. This could be provided by two 26 gauge steel
sheets, galvanized and clad on 3/4-in. (19 mm) plywood or particle board with
bolted 1/8-in. (3.2 mm) thick galvanized steel seam plates, available from
numerous manufacturers as modular shielded rooms. At the plane wave frequency
end of the spectrum, 1 to 10 GHz or higher, the methods used for seam closure
construction become very important. In this region of the frequency spectrum,
resonant half-wave lengths are of similar dimension as seam bolt spacings, and
openings and gaps become efficient radiating antennas. Depending on the type
of parent building construction, other than shielding requirements could
dictate the required thickness of shielding material. This is typical in
vault construction, where Defense Intelligence Agency (DIAM) 50-3, Physical
Security Standards for Sensitive Compartmented Information Facilities, require
steel plate with 1/4-in. (6.3 mm) minimum thickness where reinforced concrete
walls are not included.
2.11.1
Bolted, Modular Shielded Enclosures. Bolted, modular shielded
enclosures degrade in seam and penetration closures with time, with the rate
of degradation dependent on presence of moisture, vibration, thermal expansion
and contraction, and on contamination by corrosive materials such as salt
spray or industrial atmospheric pollutants. The bolted seams which compress
plywood or particle board fillers can loosen with cycles of expansion and
contraction which are caused by combinations of moisture and heat. The wood
fibers can be crushed during moisture intrusion, and the seam closure is then
loosened when the fibers dry out resulting in shielding leaks. In addition,
oxidation of the metal to metal surface contact points in the seam closure
results in a non-conductive film which causes an increase in contact
resistance and a resultant loss of shielding effectiveness. The Army Corps of
Engineering Research Laboratory technical report M-296, EMI/RFI Shielding
Effectiveness Evaluation of Bolt-Together Rooms in Long-Term Aging, June 1981,
determined that the SE will degrade by 15 to 20 dB in three years even under
ideal laboratory conditions. With less than ideal environments the
degradation is more severe, and a reduction of 40 dB in 6 months to a year
would not be unusual, especially at door closure surfaces. Welded seams, and
welded penetrations, when properly designed to allow for some flexibility and
motion without cracking of welds, are far superior to the bolted and clamped
closures.
2.11.2
Welded Shielded Enclosures. Welded steel enclosures are preferred
for TEMPEST shielding because of their better seam closures, long term
reliabilty and maintenance of the designed shielded effectiveness. Welded
enclosures for TEMPEST shielding effectiveness requirements, seen in
Figure 15, are typically constructed of steel plate, varying in thickness from
1/4-in. (6.3 mm) to 14 gauge, supported on a system of metal studs such as
seen in Figures 16 and 17, or in Figures 18 through 22, where typical exterior
wall, wall-floor, wall-ceiling, interior wall-floor, wall-ceiling sections are
shown, and including a sliding pneumatic expanding door frame section seen in
Figure 20. The 16 gauge steel studding and 3/16-in. (4.8 mm) tubular channel
seen in these figures serves as both a structural framework, and as a backup
strip for the continuous butt welding of the steel sheets to the framework.
Steel sheets are first tack welded or plug welded to the framework during
assembly by some contractors, and some utilize a fired steel pin which is
later welded. The continuous welding of the steel sheets to the framework is
then carefully sequenced to control warping and buckling of the steel plates.
Sheets are overlapped for continuous welding, seen typically in Figures 23
36
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