Quantcast Cable Shields -Cont.

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MIL-HDBK-419A
The proper installation of cables is essential if interference difficulties are to be avoided. Assuming proper
grounding techniques have been employed, the following are suggested as guidelines for good signal cable
practice.
a .  Choose the cable to be utilized according to the characteristic impedance desired, amount of signal
attenuation permitted, environment within which the cable must exist, and characteristics of the signal to be
transmitted.
b .  Where a high degree of shielding is needed, cables with multiple shields separated by insulation
should be used. Double shielding is not effective unless each shield is insulated from each other.
c.
Overall shields of multipair cables should not be used for signal return paths.
d .  Individually shielded cables, where used, should have insulating sleeves or coverings over the shields.
Balanced signal circuits should use twisted pair or a balanced coaxial line with a common shield. A coaxial line
with a shield is commonly called a triaxial cable. Where multiconductor twisted pair cables that have individual
shields as well as a common shield are used, all shields should be insulated from one another within the cable.
e.
Coaxial cables should, in all cases, be terminated in their characteristic impedances.
f.
Coaxial cables carrying high-level energy signals should not be bundled with unshielded cables or
shielded cables carrying low-level signals.
g .  Grounding a number of conductor shields by means of a single wire to a connector ground pin should
be avoided, particularly if the shield-to-connector, connector-to-ground lead length exceeds one inch, or where
different circuits that may interact are involved. Such a ground lead is a common impedance element across
which interference voltages can be developed and transferred from one circuit to another.
Great care should be taken at connectors if impedance characteristics and shielding integrity are to be
maintained.  A shielding shell should be used to shield the individual pins of a connector; a well-designed
connector has a shielding shell enclosing its connecting points. The shell of multiple connectors should be
connected to the shield. Coaxial lines should terminate in shielded pins. The use of pigtail connections for
coaxial lines is undesirable since it permits rf leakage.
Serious interference problems arise when shielded wires or coaxial cables are not properly terminated at the
connector. It is important that the connector be properly grounded. The direct bonds for this ground can be
achieved by maintaining clean metal-to-metal contact between the connector and equipment housing. In those
cases where a large number of individual shields from shielded wires must be connected to ground, it is
recommended that the halo technique be used. The exposed unshielded leads should be as short as physically
possible to reduce electrical coupling between conductors. Interference is caused when a shielded cable is run
into a completely sealed box, but is grounded internally. The correct way to install a shielded rf cable is to run
the shield well inside the connector and bond it around the connector shell.
8-61





 


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