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 MIL-HDBK-419A
b.
the wave impedance, and
c.
the wavelength of the EM energy.
Maximum length rather than width of an opening is important because the voltage will be highest wherever the
"detour" for the currents is longest. This is at the center of the slot and the voltage increases as the length of
the slot increases. The width has almost no effect on "detour" length and as a consequence has little effect on
the voltage.
Wavelength controls how much the "slot antenna" radiates. If the slot happens to be 1/4 wavelength or longer,
it will be a very efficient radiator; if it is less than 1/100 wavelength, it will be a rather inefficient radiator.
Therefore, slots only .001" to .005" wide but 1/100 wavelength or more long can be responsible for large leaks.
Figure 8-21 shows wavelength and 1/100 wavelength vs frequency for 0"-6" slot lengths typical in normal metal
enclosures. Combinations of frequency and slot lengths to the right of the 1/100 wavelength line would tend to
be leaky. This figure shows why discontinuities in shields, even if very narrow but a few inches long, will
severely reduce the shielding capacity of an enclosure above 100 MHz.
Some types of discontinuities commonly encountered include:
Seams between two metal surfaces, with the surfaces in intimate contact (such as two sheets of
a.
material that are riveted or screwed together),
Seams or openings between two metal surfaces that may be joined using a metallic gasket, and
b.
Holes for ventilation or for exit or entry of wire, cable, light, film, water, meter faces, etc.
c.
8.5.1 Seams Without Gaskets.
Seams or openings in enclosure or compartment walls that are properly bonded will provide a low impedance to
rf currents flowing across the seam. When good shielding characteristics are to be maintained, permanent
mating surfaces of metallic members within an enclosure should be bonded together by welding, brazing,
sweating, swagging, or other metal flow processes. To insure adequate and properly implemented bonding
techniques, the following recommendations should be observed:
All mating surfaces must be cleaned before bonding.
a.
b.
All protective coatings having a conductivity less than that of the metals being bonded must be
removed from the contact areas of the two mating surfaces before the bond connection is made.
c.
When protective coatings are necessary, they should be so designed that they can be easily removed
from mating surfaces prior to bonding. Since the mating of bare metal to bare metal is essential for a
satisfactory bond, a conflict may arise between the bonding and finish specifications. From the viewpoint of
shielding effectiveness, it is preferable to remove the finish where a compromise of the bonding effectiveness
would occur.
8-42
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