Quantcast Table 8-17 Comparison of Cable Shields

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MIL-HDBK-419A
Table 8-17
Comparison of Cable Shields
Multiple +
Single +
Flexible
Layered
Layer
Braid
Foil ++
Conduit
Braid
Conduit ++
Shield
Good
Good
Effectiveness*
Exc.
Exc.
Good
(Audio Frequency)
Shield
Poor
Effectiveness*
Good
Exc.
Exc.
Exc.
(Radio Frequency)
90-97%
95-97%
100%
100%
Normal Coverage
60-95%
Fair
Fair
Poor
Fatigue Life
Good
Good
Fair
Poor
Tensile Strength
Exc.
Exc.
Exc.
*Poor < 20 dB; Fair, 20-40 dB; Good, 40-60 dB; Exc. > 60 dB.
+Effectiveness against magnetic fields is poor.
++For effective magnetic shield, high permeability material must be used
Conduit either solid or flexible, or zippered tubing may also be used to shield system cables and wiring from the
rf environment. The shielding effectiveness of solid conduit is the same, for rf purposes, as that of a solid sheet
of the same thickness and material. Linked armor or flexible conduit may provide effective shielding at lower
frequencies, but at higher frequencies the openings between individual finks can take on slot-antenna
characteristics, seriously degrading the shielding effectiveness. If linked armor conduit is required, all internal
wiring should be individually shielded. Degradation of conduit shielding is usually not because of insufficient
shielding properties of the conduit material but rather the result of discontinuities if the cable.  These
discontinuities usually result from poor splicing or from improper termination of the shield. Zippered tubing
may provide greater than 60 dB of shielding to frequencies below 1 GHz.
For protection against primarily magnetic fields, shielding materials with high permeability are necessary. For
example, iron or steel conduit offers better protection against magnetic fields than does aluminum conduit. In
lieu of ferrous conduit, annealed high permeability metal strips wrapped around the cable are sometimes used.
Multiple layers of counterspiral-wound nickel-iron or silicon-iron alloys, or low carbon steel frequently prove
effective. High permeability tape is also available with or without adhesive backing. Also, combination high
permeability, high conductivity tape is available which provides both electric and magnetic shielding.
8-60





 


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