SHIELD TYPES

ALUMINUM/MYLAR DELFOIL: Aluminum strip usually about 1" wide and 0.0015" to 0.0025" thick laminated to the Mylar, using the Mylar as support for the aluminum. This form of shielding requires a tinned copper drain wire, as the aluminum/Mylar is too fragile for a termination point. The drain wire must be tinned so as not to create galvanic action (corrosion between dissimilar metal), applied longitudinally or spirally with 25% overlap, ensuring 100% coverage.

The aluminum (conductive) side can be placed either "in" or "out" against the cable. If the aluminum is "in", care has to be taken during the cable manufacture that the drain wire does not nestle between the convolutions of the conductors and lose continuous contact with the shield. Most effective against ESI (electrostatic interference) inexpensive because it does not require a separate operation during cable manufacturing as the shield is usually applied during the conductor twisting operation.

FOIL FREE EDGE: The Mylar tape is wider than the aluminum, eliminating the possibility of edge to edge contact when multiple individually shielded pairs (or groups) are required. Belden's trade name, Belfoil employ's a process of folding each edge in the opposite direction with the aluminum side "in" and they also refer to it as the Z-Fold.

ALUMINUM/MYLAR/ALUMINUM: The Mylar is sandwiched between aluminum and adding the additional layer of aluminum improves shielding reliability. The shielding is now "in" and "out". Both conductive sides should be drained to give the best shield effect. Usually with the dual faced shield a braid shield is also added overall.

Aluminum/Mylar shields are not recommended for applications where constant cable flexing or extra flexible cables are required.

BRAID SHIELD: A series of copper wires braided around the cable, the effectiveness fo the braided shield depends on the percentage coverage. Leakage is due to the air spaces between the braid weave. Coverage is usually between 65% to 95%.

ALUMINUM BRAID SHIELD: Used mainly for CATV/ MATV applications as a low DC shield resistance is not important. The aluminum braid is also used as a mechanical armour.

SHIELD RESISTANCE: Often an important part of cable equation, two identical cables each having 90% coverage with the same wire size do not necessarily have the same shield resistance! Braid angle determines the length each individual wire has to travel from one end of the cable to the other, i.e. the wires in a cable with a 60% braid angle will be shorter than the wires in a 45° braid angle and thus have a higher resistance (less copper). So lower is better for shield resistance. Shield resistance for camera cable (triax), video and CCTV cable is an important equation.

Tinning the individual wires increases the resistance slightly but the main reason for tinning is the copper is pre-tinned for solder connection (does not require to be fluxed) and tinning also inhibits the oxidation process (greening). Most effective against EMI (electromagnetic interference) and RFI (radio frequency interference).

Very good for flexing applications, using small diameter wire 36 awg or, even better 38 awg with a low braid angle and tight weave (high percentage coverage). The small diameter wire and the low braid angle reduce the fraying (snapping of wires) in a flexing applications, such as microphone cable. The higher braid angle is not as reliable in a flexing application.