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 adjusted to repeat the original reference reading on the receiver meter. This
type of representative SE reading can only be obtained on partial shielding
construction when the leakage around the shielding is less than the leakage
through a discontinuity. The SELDS equipment, sometimes called the "sniffer,"
is not a calibrated instrument and cannot be utilized to make final
performance SE measurements to be substituted for magnetic field measurements
required by the performance specifications, MIL-STD-285 or NSA 65-6. It is
beneficial to install test loops under the shielding floor during
construction, so that floor leaks can be immediately identified and corrected
before surface finishes limit accessibility for repair. These loops when left
in place may be utilized at later times to determine degradation of the
shielding system. For small to medium size enclosures the SELDS source can be
used to drive the entire enclosure by attaching source leads to opposite sides
or corners of the shielding. For very large enclosures, driving the entire
enclosure spreads the signal out so that the sensitivity for determining leaks
on the shielding surface is reduced. It is usually desirable to drive
sections of the shield with large loops from the size of the smallest panel
(typically 4 by 8 ft) (1.2 m x 2.4 m) to approximately 8 by 24 ft (2.44 m x
7.32 m). The source loop provides the greatest sensitivity directly behind
the loop conductor which is normally arranged to coincide with a shielding
seam.
4.9.2
Testing After Shielding Completion. The other EMI shielding test is
MIL-STD-285 or NSA 65-6 final performance testing which requires completion of
the shielding system prior to testing. MIL-STD-285 was published in 1956
before the range of performance requirements extended to a high of 10 to 30
GHz and before the low frequency end extended to as low as 30 Hz. The
required frequency range of performance and the required levels of SE must be
clearly identified in the specification. It is useful to provide a SE curve,
and to specify the frequencies that testing is to be performed. TEMPEST
shielding requirements are clearly identified on the graph in NSA 65-6 (Figure
15 of this manual). Shielding performance (SE) requirements for other than
TEMPEST shielding should also be similarly identified in the specification.
Merely stating that MIL-STD-285 testing is required will result in a lower
frequency measurement of magnetic field SE between 150 to 200 kHz with a limit
of 70 dB SE, and a plane wave upper frequency measurement at 400 MHz with a
limit of 100 dB of SE. Test point locations required must also be clearly
identified in the specification. MIL-STD-285 is still useful in defining the
test arrangement for antennas, including distances from source and receiving
antennas from the shielding, and from each other during the reference reading
along with the polarization of the antennas, including required probing with
the receiving antenna during the performance measurements. Specifying NSA
65-6 performance testing will result in test frequencies of 1 kHz, 10 kHz, 100
kHz and 1 MHz magnetic field and electric field, 10 MHz electric field only,
and 100 MHz, 400 MHz, 1 GHz and 10 GHz plane wave fields and with leakage
probing no closer than 2 in. (51 mm) from the shielding around door
peripheries, accessible joints, filter and air duct penetrations. It also
requires measurements at four wall locations, and placement of the source
antennas on the inside of the enclosure. The theoretical shielding
effectiveness of a particular shielding system should be identical whether the
source antennas are located on the inside or the outside. It is possible to
excite the entire enclosure at plane wave frequencies with the source antenna
on the inside, and then to probe the exterior for leakage that is not as
greatly affected by standing waves as it would be with the receiving antenna
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