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Cables and Wave Guides

From the electrical standpoint alone, open-wire transmission lines compete favorably with cables, including the coaxial type. From the mechanical standpoint, the open-wire line, consisting as it does of exposed uninsulated wires supported on glass insulators, cross-arms, and poles, has serious shortcomings. The open-wire line is subject to mechanical contact with trees, brush, and other wires. The insulators become damaged by natural causes and by man. Furthermore, open-wire lines are subjected to severe mechanical stresses due to sleet, snow, and high winds. The wires must be spaced at least approximately 8 inches apart or they will swing together. The maximum number of pairs of wires that can exist on a modern pole line is approximately 40; that is, 8 crossarms of 10 wires each.

For these and other reasons, cables must be used extensively in modern communication systems. Cables used in communication are of two general types: first, those in which the transmission circuits are twisted pairs of insulated wires, and second, those in which the transmission circuit consists of an outer tubular conductor and a conductor held at the center (called a coaxial cable).

Cables are sometimes supported by steel strands attached to poles and are called aerial cables.1 There is a growing tendency, however, to bury cables so that they will be more nearly damage and storm proof. These are called underground cables.1

Cables of the twisted-pair type are extensively used in telephony for transmission at audio frequencies. They are also used for transmission at telephone carrier frequencies (page 429). Coaxial cables are used in telephony for transmitting many telephone conversations simultaneously (page 432) and for television and radio purposes,


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Last Update: 2011-05-30