What Is Coaxial Cable Made of?
Several different coaxial cable designs are in common use. All use some configuration of the four basic components: central core, dielectric sheath, conductive shield and insulating jacket. The actual materials used for these different layers vary. Different types of materials result in different physical strengths, signal loss rates and transmission power.
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Central Core
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In the most common coaxial cable, the inner core is a solid copper wire. Where flexibility is a priority, this may be a copper braid. The core of higher quality cables may be a silver-plated solid copper wire. In a few applications, a core of copper-plated steel wire has been used. For some high-power lines such as those feeding radio and TV antenna towers, the central core may be a hollow tube of copper or copper-plated aluminum.
Dielectric
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The role of a dielectric is to provide a space between the core and the shielding which will confine signals radiated from the central wire. In high-power hard lines, the dielectric may be air or nitrogen, and the shape of the dielectric space is maintained with nylon spacers that support the core. For more common cables such as for cable television, a solid or foam layer of polyethylene is the usual choice. In flexible coax, the dielectric will reflect the signal back to the source if distorted, so a dielectric must be dimensionally stable when bent.
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Shielding
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Common coaxial cable covers the dielectric sheath with a layer of braided copper or aluminum. This shield is grounded and prevents the signal from transmitting beyond that layer. The shielding also prevents external electromagnetic signals from reducing transmission quality. Solid tubes make more efficient shields but are more easily damaged. Sometimes a layer of coated aluminum foil is placed between the dielectric and the braid, or the foil is the only shield used. High power shields in hard-line coax may be copper, silver or even gold tubing.
Insulation
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Most coaxial cable uses a tough outer sheath or jacket of polyvinyl chloride to provide both electrical insulation and protection from environmental damage. The jacket also protects the dielectric layer from distortion when the cable is bent. Buried coax requires a more waterproof material, usually polyethylene. Propylene has a wider range of heat tolerance and chemical resistance. The jacket of a hard line is usually PVC. Critical applications may require additional protection such as gopher tape or steel wire reinforcement.
Multi-Channels
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Long-line coax such as the submarine cables used by Bell Telephone in the 1960s and '70s has now been surpassed by lighter fiber optic cables with much greater bandwidth. Bell's long-line cables were waterproof and surrounded by gopher armor, a protective material impervious to rodent damage. Each contained 20 separate coaxial channels and an array of lines for damage alarms, test procedures and maintenance communications.
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References
Resources
- Photo Credit Photo from Wikipedia Commons at http://en.wikipedia.org/wiki/Coaxial_cable
