01 March 2005
Table 7-2. Shielding Effectiveness vs. Percent Leakage
values do not guarantee high shielding effectiveness.
High resistance values indicate potential problems but
do not provide a measure of shielding effectiveness.
7-4.3. SHIELDING REQUIREMENTS. For practical
purposes, the range of shielding extends up to around
120 dB. Table 7-2 lists the amount of signal passing
through an enclosure for different levels of shielding.
Absorption of the energy by the material attenuates
At 120 dB, 0.0001% of the signal penetrates the shield.
the transmitted wave. When the wave reaches the
(Measurement of shielding levels beyond the 110 to
opposite side of the wall, reflection and transmission
120 dB range is experimentally difficult.) Shielding
again occur at this discontinuity. Figure 7-1 illustrates
requirements cover both susceptibility of equipment to
this process for an electric field. Shielding effectiveness
external signals and the emission from equipment.
(SE) is defined as SE = 20 log (Ei/Eo). SE is shielding
EMI can be radiated through the atmosphere or
effectiveness in decibels (dB), Ei is the incident field
conducted along power and signal lines (see
strength, and Eo is the transmitted field strength. This
ratio of electric/magnetic field strength at a point before
of the signal lines. Test specifications cover individual
and after the placement of a shield for a given external
pieces of equipment or all of the components integrated
source is a measure of the shielding element's ability
into a system. Frequently, individual components may
to control EMI. For example, to shield avionics of older
pass a test but fail when connected together in a
aircraft tested at 20 V/m from an external field of 1000
system. This is due to inadequate shielded cabling,
which acts like an antenna to receive or radiate EMI.
depend on the shield's bulk electrical properties (relative
Operating modes of a system may generate different
levels of EMI depending on the signals transmitted
material, and frequency of the impinging energy.
between the components.
Reflection losses depend on the impedance of the
incident wave and the electrical properties of the
7-4.4. EMI SHIELDING ENCLOSURE MATERIALS.
material. In most applications, magnetic fields present
Conductive gasketing is used to provide electrical
problems only for nonferrous shields, due to the lower
ground and EMI shielding across a joint and often
reflection losses/larger skin depths associated with
employs an environmental seal. They are used on
l o w frequencies. MIL-STD-461 is a triservice
temporary apertures such as access panels. These
specification for testing of electronic systems for
panels have scams that must be electromagnetically
radiated and conducted emissions and susceptibility.
scaled. The form of a gasket is determined by
IEEE299 describes procedures for measuring shielding
attachment methods, force available, joint unevenness,
effectiveness of enclosures. A modified version of this
available space, and applicable EMI shielding criteria.
procedure is used in MIL-G-83528 for characterizing
Table 7-3 lists some of the materials used in EMI
the shielding effectiveness of conductive elastomers.
shielding enclosures. The major material requirements
MIL-STD-464 is an electrical bonding specification,
for EMI gaskets include:
which has been applied to EMI joints. Low resistance