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 Section 2:
BASIC ELECTROMAGNETIC INTERFERENCE
SHIELDING THEORY
2.1
Introduction. The use of sophisticated communication and electronic
equipment has increased drastically in recent years to meet the operational
requirements of the Navy. The solid state electronic components of the modern
equipment are susceptible to upset or damage from external electromagnetic
source caused by induction of unwanted electrical currents and voltages in the
connecting circuitry. The high speed switching (baud rate) of recently
utilized equipment is capable of emitting electromagnetic signals in a broad
frequency range. The acquisition of these signals by unauthorized signal
detection equipment may result in the compromise of sensitive information if
the equipment emitting the signals are processing decoded classified
information.
2.1.1
Increased Need for Shielding. NAVFAC has been tasked to design and
construct facilities with shielded enclosures to provide proper protection for
communications and electronics equipment. As a result of a greatly increased
size and complexity of communications and information processing systems, the
size of necessary shielded enclosures has increased dramatically, with many
system floor space requirements exceeding 20,000 ft.2- (1,858 m.2-). In
addition to increased size, the frequency range and shielding effectiveness
requirements have also been expanded resulting in a greater complexity in the
shielding construction design. This results from necessary accommodation of
numerous penetrations, structural interconnections, thermal expansion and
contraction, and electric isolation requirements, on a larger scale than
previously experienced with free standing shielded enclosures. As a result,
the problems associated with design, construction, and testing of the shielded
enclosures have substantially increased.
2.2
Sources of EMI. When the problems associated with Electromagnetic
Interference (EMI) were first encountered and investigated, the phenomenon was
identified as Radio Frequency Interference (RFI). Earlier, the bulk of the
problems were with radio communications systems transmission and reception.
With modern expansion of the use of signal processing, military and industrial
electronics systems, the broader Electromagnetic Compatibility (EMC)
terminology is used. In addition to deliberate radio communications sources
of interference, we must consider digital switching and solid state power
supplies, high intensity discharge (HID) lighting systems, automotive
ignition, electric motor switching, arc welding, electrical power system
faulting and switching, corona and static discharge, intermittent contact
between metal objects in an electromagnetic field, lightning discharges,
fluorescent lighting, and intermodulation products from nonlinear junction
mixing, are among the many sources of this problem.
2.3
Purposes for EMI Shielding. There are basically two purposes for
providing EMI shielding in military construction projects. The first is to
prevent external EMI sources from penetrating a sensitive environment. This
sensitive environment may contain electronic equipment, personnel, ordnance,
or fuel supplies which are susceptible to the presence of EMI. This type of
shielding is for Electromagnetic Compatibility (EMC). The second purpose for
shielding is to prevent electromagnetic signals generated from certain
electronics equipment within the facility from being transmitted or conducted
outside the controlled area in sufficient magnitude to be received and
recorded by the most sensitive receiving and signal recovery systems.
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