Electrical noise of any type can disrupt proper operation of other equipment and should be shielded to combat the electromagnetic interference effects in cables. Insulation protects a cable mechanically from scraps and abrasion and
environmentally from moisture and spills. But insulation is transparent to electromagnetic energy and offers no protection. Shielding is needed to combat the effects of EMI.
Served shields are spiral-wound groups of small-gauge wire strands surrounding the insulation of the conductor(s). They are easy to unwind and terminate but are prone to be relatively inductive because they are coiled around the
cable. They are the most flexible of shielded cables and are often used in audio applications. Served shields are usually soldered, or crimped to a lug or termination post.
Braided shields are woven over and under one another to form a tight but flexible cylinder of wires. This may result in the need to unbraid or loosen the weave in order to terminate it, though it lends itself to easy coaxial connector termination, and preserves the shield all the way to the connector body. 95% coverage is not unusual on high-quality cables.
Most commonly, braids are formed of groups of small-gauge wires known as “carriers.” These are laid side-by-side, a ribbon-like multi-path conductor. Braids can also be a “strip braid,” using solid ribbons of conducting material, providing a more uniform inner surface to a coaxial conductor. This is an advantage at very high frequencies, and if it is combined with other shield designs, forms a very effective EMI barrier.
Braided coax shields are usually terminated in the field by crimping or clamping, and are occasionally soldered, or are terminated with a heat-shrink shield pigtail. This latter approach is common in aircraft wiring harnesses. Braided
non-coax shields can be soldered if the conductors within are dressed to exit the shield through an opened space in the braid, or can be terminated with a heat-shrink shield pigtail. Foil shields consist of a metalized flexible plastic (Mylar, polyimide, etc.) wrapper, spiraled around the conductor(s). The metalized layer is very thin — on the order of .0003 inch. Foil coverage can be effectively 100%, although its resistance is far greater than the other shield described here, and thus its ability to shunt noise is limited. For this reason, EMI protection is best if foil shields are used in combination with braided (better conductors) shields.