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 Specifically, when electronic equipment is being used to process, display, or
store plain language (decoded) text of classified information, the shielding
utilized to prevent its compromise is called TEMPEST shielding. If the
equipment within the shielded facility is not used to process classified
information, but it is necessary to prevent the EMI generated by the equipment
from being transmitted outside where it can affect the operation of sensitive
equipment the purpose of the shield again is for EMC. An example of the
latter situation is a medical laboratory facility with large magnitude EMI
generating equipment such as the CAT scanner, Nuclear Magnetic Resonators, or
Linear Accelerators operating adjacent to signal processing computers. This
manual will address design, specification, construction and testing problems
associated with shielding for both TEMPEST and EMC. Refer to Naval Shore
Electronics Criteria Handbook, NSWSC 0101, 106, Electromagnetic Radiation
Hazards for guidance on Hazards of Electromagnetic Radiation to Personnel
(HERP), Ordnance (HERO), or Fuel (HERF) shielding. Refer also to NAVFAC
DM 12.02, for shielding criteria for electromagnetic pulse (EMP) protection
from high altitude nuclear detonations. Whether the purpose for the EMI
shielded enclosure is to prevent EMI from entering or leaving the enclosure,
the problems and solutions used in the design and construction are basically
the same. TEMPEST shielding requirements however, are slightly more complex
than EMC shielding requirements and will be described later in the following
sections.
2.3.1
Shielding Effectiveness Level. The shielding effectiveness (SE)
level in decibels (dB) that must be provided and the frequency range over
which it is necessary is the first shielding requirement to be determined
during the initial planning phase of a project. The available shielding
effectiveness is dependent on a number of parameters including frequency, the
intrinsic electrical properties of the chosen shielding material, and the
number and configuration of discontinuities in the shielding material, which
will be necessary to accommodate personnel, (access doors and hatches),
equipment installation (piping, conduits, HVAC duct penetrations) and the
interface with the parent building construction (columns, beam penetrations,
and grounding). Propagation of the EMI may be radiation and conduction, and
the required SE must be provided for the total shielding systems including all
discontinuities and attachments.
2.4
Characteristics of EMI Waves. An electromagnetic (EM) wave is an
energy field which radiates from a source and propagates through a surrounding
medium such as air. An EM wave may also be conducted from one point to
another by means of conductors arranged to form a transmission line. The EM
wave is composed of an electric field component (E) in volts/meter and a
magnetic field component (H) in ampere-turns/meter. The ratio of the (E)
field to the (H) field is called the wave impedance (Z = E/H) in ohms. The
(E) and (H) fields are time-varying due to a continuous reversal of the
polarity of the field components propagating in the medium, or a reversal of
currents and voltages propagating along a transmission line. The rate at
which the field, voltage, or currents alternate with time is called the
frequency of the wave and is measured in Hertz (cycles per second). The
relationship between frequency (f), velocity of propagation (v), and
wavelength ([lambda]) (the distance the wave travels during one cycle of
oscillation) is:
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