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 placed inside the enclosure. For TEMPEST shielding effectiveness testing, NSA
65-6 should be specified with additional test frequencies determined to be of
concern to the shielding occupants. Continuous sweeping of seams at one or
more plane wave frequencies should also be specified, in addition to testing
around all door panels, filters, air duct penetrations and all other
penetrations of the shielding at all test frequencies.
4.9.3
Testing Before Finish is Installed. It is imperative that final
performance MIL-STD-285 or NSA 65-6 testing take place before finish materials
are constructed which make the shielding inaccessible for testing and repair.
All penetrations must be complete and installed, all filters and waveguides
installed, and all doors hung and balanced with sealing fingerstrips in place
before final performance testing is accomplished. Testing should be performed
by a government approved contractor provided independent testing agency (not a
subsidiary of the shielding subcontractor) and must be monitored during
testing by an expert government witness experienced in EMI testing
procedures. Case leakage measurements on the calibrated receivers must be
made during the reference level testing by disconnecting antenna cables to
assure that the reference levels are a result of antenna induced signals
only. Preferably, the receiver should be shielded from the source during
reference measurements, or so located to minimize coupling from the source
antennas. The receiving antennas must also be rotated and moved to find the
worst case readings while probing test locations. Test points must be
properly coordinated between source and receiving antennas to assure that both
are located for measurement at the same test point. Sources often drift in
frequency, and receivers are easily detuned while transporting and positioning
them for measurement, requiring careful retuning before test readings are
taken and recorded. Where specification limit readings are not obtained,
necessary modifications, repairs and retesting must be accomplished as often
as required to provide the specified SE. Then when finish materials are in
place (floor topping slabs, gypsumboard wall finishes, roof insulation and
coverings, etc.) it is recommended that limited testing be repeated. One
plane wave frequency sweep, backed up by magnetic field probing of any new
leakage areas discovered, with repair and retest as necessary to bring the
enclosure back into proper SE performance, is recommended.
4.10
Cost Considerations. The design cost estimate must include a
detailed breakdown of all specific shielding costs including SELDS and MIL-
STD-285/NSA 65-6 testing. Specific projects may also require a cost analysis
of alternative shielding systems (demountable modular construction versus
welded steel construction for example) to determine the most suitable method
of providing the facility shielding effectiveness requirements.
4.11
Design Review. It is important that the planning and design
documents are reviewed to make sure that the shielded enclosure satisfies the
users requirement and that adequate funding has been budgeted. The review
submittal may include facility study, concept studies, basis of design or
design drawings and specifications (i.e. 35%. 100% or final design). Each of
the submittals should provide basic information on the shielded enclosure
including reason for the shielding, required shielding effectiveness, type of
construction such as welded steel or modular (demountable) bolt together, cost
of construction and area to be shielded. The designer is responsible for the
quality, technical accuracy and the coordination of EMI shielded enclosure
design including drawing and specification. The designer shall check and
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