Shielding

Potentially effective shields may be compromised or destroyed by improper termination to ground. A-low impedance path to ground is critical in order to gain maximum shielding benefits. "Pigtail" connections from a shield to ground have inductance, resulting in an impedance which increases with frequency. This type of connection will work at frequencies below 10-kilohertz -- but it will create problems at higher frequencies. Opting for short connections with large cross-sectional area minimizes the inductance caused by pigtails; however, the best connection is a 360-degree contact between the shield and connector or chassis.

Solid shields are, theoretically, best for noise reduction; however, they are generally harder to manufacture and apply. Instead, most cables shielded braided, which also improves ease of termination, flexibility and strength. Braided shields not as effective as solid shields since they only cover 60% to 98% of the cable. Decreased shielding effectiveness is more prevalent at high frequencies where the holes in the braid are large compared to a wavelength. For maximum (optimum) shielding, reliability, and ease-of-use, cables with combined shields are available which utilize both solid and braided layers.

Shields can also be configured with a low permeability shield on the inside and out with high permeability shields in between. In any case, the low-permeability shield should always be closest to the source of interference.

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