At high frequencies, where the coupling mode is electric field coupling rather than magnetic field, the voltage on the shield will couple to the circuit within the cable based upon the capacitance between the shield and the wire inside. It is important, therefore, to reduce the voltage on the shield. Reducing this voltage can best be accomplished by terminating the shield at both ends and at each point where the cable penetrates a bulkhead.
METHODS OF TERMINATION
The method of shield termination can impact both the high and low frequency shielding effectiveness of the cable shield. The termination must maintain the shielding effectiveness of the cable shield so not to degrade the system shielding. To accomplish this, the termination impedance must be equal to or less than that of the shield.
For low frequency terminations, this means that the terminating device must have, as a maximum, the same DC resistance as the shield. This can be accomplished by a pigtail, an EMI backshell, or a conductive boot.
At high frequencies, the termination is more critical. It must not only
have an equal or lesser impedance as the shield, but it must also have the
same percent of coverage. The 90% coverage will not be obtained for high
frequency terminations with a pigtail. Instead, a form of EMI backshell must
be used. This provides a complete covering of the wires within the cables.
This can be accomplished by using an EMI backshell or a conductive boot. The
conductivity of the termination device must match that of the shield. Figure
4-12 includes examples of typical shield terminations.
Pigtail shield grounds should only be used for low
frequency shield terminations. The upper left example in Figure 4-12
shows a typical pigtail shield termination.
Conductive Heat Shrink Boots
Conductive heat shrink boots can be adequate when used properly. These
boots contain a conductive coating on the inside surface and will provide
between 60 and 80 dB attenuation above 500 MHz. These boots can be added over
conventional braided shields; however, a knurled and grooved surface and tie
wraps should be used to strengthen the contact. Care should also be taken to
assure that electrical contact is maintained between the shield and the inner
conductive coating of the boot. Mechanical strain relief should always be
incorporated into the cable design. Caution should he exercised during
installation to assure that no moisture is trapped within the boot. Trapped
moisture could result in corrosion. Figure 4-13 shows an example of conductive
heat shrinkable boots.
Figure 4-14 compares shielding effectiveness between several styles
of shielding boots. The conductive heat shrink boot provides some
shielding effectiveness below 600 MHz. But then as frequency is increased, the boot
is more effective. The EMI backshell connected to wire braid reaches a
maximum shielding effectiveness in the middle of the frequency range and then
begins to fall off rapidly above 1000 MHz. The boot, composed of a combination of an
EMI backshell and a conductive polymer boot, provides maximum shielding
effectiveness throughout the frequency range.
The most effective shield termination is the EMI
backshell. This consists of cable strain relief, a method of providing
electrical contact around the entire periphery of the shield, and the
termination of the backshell to the connector body. Figure 4-15 shows an
example of an EMI backshell. Figure 4-16
provides an installation example.