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Types of Cable Shields Cheat Sheet by

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Introd­uction: Types of Cable Shields

A shield acts in two ways: It can reflect energy and keep it from reaching the conduc­tors; or the interf­ering signals can be captured “over the air” by the shield and passed to ground. Regard­less, to work properly, the shielding and its termin­ation must provide a low-re­sis­tance path to ground.

As any discon­tin­uities can increase resistance and thus lower shielding effect­ive­ness, it is one of the reasons why foil-s­hielded cable is the least effective and most likely to be damaged. That said, various degrees of armoring around the cable can reduce the potential for damage, but they also signif­icantly increase cost, weight and reduce flexib­ility.

There are two basic types of shielding – foil and braid – can be used either alone or together. Various cable manufa­cturers (including L-com) have developed propri­etary solutions based on these two types, along with unique cable constr­uction techniques to optimize perfor­mance.

Foil shielding

Foil shielding utilizes a thin aluminum sheet (like aluminum foil) laminated to a polypr­opylene or polyet­hylene carrier that increases its overall strength and reliab­ility. Because of their small size, foil shields are commonly used to protect individual pairs of multi-pair cables to reduce crosstalk. Foil provides 360-degree coverage around the conductors and is the least expensive and lightest altern­ative. However, it also provides the least shielding effect­ive­ness, has high resistance that offers a relatively poor path to ground, can break in cables exposed to frequent or severe flexing, and is difficult to mate with connec­tors. The latter is annoying enough that rather than attempting to connect the entire foil to a connector, a wire is often connected to the end of the foil and then to ground.
 

Shielded Cable Types

Braid

Braid is a woven mesh of bare or tinned copper wires and typically covers 60 to 85 percent of the conductor area, depending on how tightly the braid is woven. In contrast to foil, braid provides a highly­-de­sir­able, low-re­sis­tance path to ground and is far easier to terminate to a connector through soldering or crimping. As the braid is copper, it has much higher conduc­tivity than aluminum and also more “subst­ance” – which together provide greater shielding effect­ive­ness.

Braid shields have greater structural integrity while also remaining flexible, and can withstand a fairly rigorous flexing enviro­nment. They attenuate both electrical and RF interf­erence and are well suited for interf­ere­nce­-se­nsitive audio applic­ations.

Foil with Braid over the top

The third category combines foil over the conductors with braid over the top of that, which provides 100-pe­rcent coverage and the highest level of shielding effect­iveness over the broadest range of freque­ncies.

Still other combin­ations for use in the most proble­matic enviro­nments include two levels of braid – a layer of foil, layer of braid, and layer of foil, and even foil covered by braid, covered by foil and another level of braid. Not surpri­singly, the cost and weight of these solutions increase with their shielding effect­ive­ness, so it is necessary to decide what level of shielding is actually needed, and in some cases whether the increased weight can be accomm­odated.

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