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 MIL-HDBK-419A
The total shielding effectiveness is the sum of the absorption loss and the reflection loss. Use
c.
Figure 1-86 to determine the reflection loss of various metals to magnetic fields; use the nomograph of
Figure 1-87 to determine the reflection loss of electric fields. The procedures for using these nomographs are
similar to that described previously for determining absorption loss. Note that the right hand scale is based on
the ratio of relative conductivity to relative permeability instead of the product of the two as used on the
absorption loss nomograph.
Determine the reflection loss for plane waves with the use of Figure 1-88. Simply lay a straightedge between
the metal of interest (or the correct
ratio) on the right hand scale and the frequency of interest on the
left hand scale; read the reflection loss of the scale in between.
Thin shields with low values of absorption loss can experience re-reflections which may cause the estimates of
shielding effectiveness to be in error. If the absorption loss is less than 10 dB, see Volume I, Section 8.3.3 for
ways to account for the effect of re-reflections.
Consider the use of thin metal foils for shielding high frequency (broadcast frequencies and above)
d.
plane and electric fields. Use Figures 1-89 and 1-90 to estimate the amount of shielding that can be achieved
with copper and aluminum.
1.8.4 Construction Guidelines.
Securely ground all metal shields.
a.
All seams and joints must be well bonded. Welded seams are highly desirable in enclosures which
b.
must provide a high degree (≥ 80 dB) of RF shielding or are intended for EMP protection. Where welding is
impractical, solder or knitted wire gaskets should be used to supplement the mechanical fasteners (see
Volume I, Section 8.5.2). Figures 1-91 and 1-92 show two recommended techniques for constructing seams in
shields.
c.
Limit openings (windows, doors, ventilation ports) and penetrations (signal lines, power lines,
utilities) to the lowest possible number and restrict their dimensions to a minimum.
(1) If holes through the shield are necessary, see Volume I, Section 8.4.3 to determine the
optimum size and spacing.
(2) Use honeycomb (see Volume I, Section 8.5.3.1) for the shielding of ventilation ports wherever
possible. Where forced ventilation is used through ports shielded with either honeycomb or wire mesh, predict
the pressure drop with the aid of Figure 1-93. (A larger blower will generally be necessary to provide the same
volume of air through a shielded port than would be required through an unshielded port.)
Peripherally bond metallic utility lines to the shield at the point of entrance. Nonmetallic lines
d.
entering through waveguide-below-cutoff (see Volume I, Section 8.5.3.1) ducts or tubes may also be used for
water, gas, compressed air, etc.
1-162
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