COAXIAL CABLE SHIELDING GENERAL CONSIDERATIONS
- electrical screening refers to the cable characteristic to either cause interference by signal radiation or to be interfered by the ingress of external signals.
it usually consists of a combination of metallic laminated tape and braid.
it is measured in dbs, and it is known also as:
路screening (referring to the en 50117 standard)
- the metallic laminated tape (foil) is taped lengthways and surrounds the cable
this foil can be made of copper/aluminum + polyester/polypropylene.
- the braid consists of different groups of wires (strands), twisted over the foil, which can be made of different materials as copper, aluminum, copper-coated aluminum, copper-coated steel...
the braid improves:
the cable’s electrical screening.
the cable’s shield; it is one of the main protective elements of the foil.
- braid coverage. it contributes to the efficiency of the screening.
the better the quality of the screening inner layers, the less important the braid coverage.
in most cases the higher the number of strands, the greater the braid surface coverage.
the following is always the case:
the higher the coverage 庐 the higher the number of strands 庐 the heavier the cable 庐 the greater the cost.
- greater coverage means:
- the larger the angle 芒 between the group of strands and the axis of the cable:
- the greater the number of groups of strands (carriers)
- the greater the number of strands (ends).
- the greater the number of carrier crossovers.
- there are two different categories as regards the braid coverage (although some manufacturers make three distinctions):
30% to 60% coverage
60% to 90% coverage
- regarding the number of shielding layers, we can differentiate between:
standard: tape and braid.
tri-shield: uses the standard shielding design, plus an additional outer tape.
quad-shield: adds a second outer layer of braid to the tri-shield design.
- there are even more possible combinations when various other parameters are accounted:
sheath materials, type of dielectric, type of inner conductor, type of shielding foil, cable different diameters, types of protection, messenger wire, siamese, ...
- manufacturing. - some details in certain cables provide an indication of how they will perform.
for example, it is important that the overlapping of the shielding foil (the part that overlaps lengthways onto itself) represent a high percentage of the surface. values between 15% and 30% would be worrying.
in order to guarantee the proper screening of the shielding foil (when it is laminated on a single side) all along the cable, there is a shorting fold that provides a good metal-to-metal contact thus avoiding the isolation that the polyester film would produce.
this can be seen in the ollowing drawing:
- anticorrosive protection: this is normally known as "flooding" and it is highly recommended for burial cables.
it is a low viscosity compound that is easily inserted into the external braid and sheath.
it prevents moisture from getting in through any small rupture in the sheath, and acts as an automatic repairing agent.