C

Abstract

An electrical cable is one or more conductors surrounded by an insulating medium and a protective sheath. Such cables are used for the transmission of electric power and for transmission of communication signals. The power cables have relatively few conductors of heavy gauge and are insulated for high voltages. Such cables are frequently filled with oil to increase the insulation strength. The outer sheath is commonly of lead, although for submarine work this in turn is often further strengthened by a second sheath of steel strands. Communications cables usually contain many pairs of small-gauge copper conductors, paper-insulated, surrounded by a lead sheath. Sometimes the entire cable is nitrogen-filled under pressure. The various pairs of conductors are arranged by twisting and placing to minimize pick-up between them (see Cross-Talk). Common practice is to include two extra pairs for spares for each hundred active pairs. Cables used for submarine circuits have fewer pairs and are heavily insulated and armored to withstand the severe strains to which they may be subjected in laying and by ocean currents. The coaxial cable (see Coaxial Line) is a special type in which the pair of conductors is formed by a center wire and the outer sheath. In this case the sheath is copper and the insulation is often a gas with solid dielectric spacers at intervals to hold the inner conductor centered. This coaxial cable may in turn be enclosed with others in a lead sheath for protection. Coaxial cables have a wide usable frequency range and hence are used for transmission of television programs. They are also often used for radio-frequency transmission lines as the electromagnetic fields necessary for the transmission of signals penetrate the space exterior to the sheath of the cable to a negligible extent. See Fig. 1.Construction of a network coaxial cable. The center conductor may be bare copper. copper-clad aluminum. copper-covered steel, silvered copper, or tinned copper. The shield may be aluminum, bare copper, foil plus aluminum braid, foil plus tinned copper braid, silvered copper, or tinned copper. The dielectric may be an air dielectric polyethylene, solid Teflon® foam Teflon® foam polyethylene, or solid polyethylene. The jacket may be Teflon® fluorinated ethylenedipropylene, noncontaminating polyvinylchloride, polyethylene, or poly-vinychloride. (Illustration provided by M/A-Com, Inc., Hickory, North Carolina.)