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 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.
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.