ng joint. Loss (dB/km) The individual tail cables have to be air tight and to retain the electrical characteristics of the main cable. For convenience of equipment installation and maintenance, the housings are best impedance of the cable is 50ohm. The pressure coefficient of the mounted to one side of the main cable in a dedicated shallow cable is remarkably low at about 0.5% per 2km of sea depth. manhole or footway box. Water tightness of the repeater housings is This cable has been used for 1840 circuit systems operating up to assured by air pressurisation which may be integrated into the main 13.7MHz and for 5520 circuit systems operating up to 44.3MHz. cable system. The cable loss is given in Table 15.8. (The standard unit of length in submarine system work is the nautical mile (n.m.) of 6087 feet (1.855km).) Laboratory studies demonstrated that systems of much 15.5 Optical fibres higher frequencies were feasible, but this work has been dropped in favour of the development of optical fibre systems. Telecommunication transmission systems operating over light guid- Many thousands of nautical miles of STC's submarine coaxial ing optical fibre waveguides were first proposed in 1966 by Hock- cable systems give reliable service throughout the world. ham and Kao working at the STC Technology Ltd. laboratories in Harlow, Essex. One of the earliest and most ambitious field trials of an optical fibre system was the 140Mbit/s system designed, manu- Line plant for copper cables factured and installed by STC plc between Hitchin and Stevenage in 1977. Since then the technology has progressed to the extent that optical fibres have virtually ousted metallic cables for new telecom- Cables can be installed underground or overhead. For overhead munication work except for the local area network and short dis- installation the cable can be lashed tightly against a previously tance applications. installed messenger wire. Alternatively, the messenger wire, or Optical fibres, or light guides, operate by total internal reflection ca