A leaky feeder is a communications system used in underground mining and other tunnel environments. The layman's term "leaky feeder" implies that the cable is in some way deficient. Manufacturers and cabling professionals use the term "radiating cable"   as this implies that the cable is designed to radiate: something that coaxial cable is not generally supposed to do.
Principle of operation
A leaky feeder communication system consists of a coaxial cable run along tunnels which emits and receives radio waves, functioning as an extended antenna. The cable is "leaky" in that it has gaps or slots in its outer conductor to allow the radio signal to leak into or out of the cable along its entire length. Because of this leakage of signal, line amplifiers are required to be inserted at regular intervals, typically every 350 to 500 metres, to boost the signal back up to acceptable levels. The signal is usually picked up by portable transceivers carried by personnel. Transmissions from the transceivers are picked up by the feeder and carried to other parts of the tunnel, allowing two-way radio communication throughout the tunnel system.
The system has a limited range and because of the frequency it uses (typically VHF or UHF), transmissions cannot pass through solid rock, which limits the system to a line-of-sight application. It does, however, allow two-way mobile communication.
Leaky feeder has been used in the mining industry as a method of wireless communication between miners. The system is used as a primary communication system which has a transceiver small enough to be comfortably worn on a miner throughout an entire shift.
Leaky feeder system is also used for underground mobile communication in mass transit railways. In Hong Kong the leaky feeder aerial was incorporated in the specification of the capital project and installed during construction. This allows emergency services seamless mobile communication from the underground to the surface.
In London, London Underground uses a leaky feeder system for their internal communication network Connect. The emergency services' communications system however was not compatible and did not work underground. The fact that this situation continued to exist after the 1987 King's Cross fire was criticized in the reports from the 7 July 2005 London bombings, where it hampered rescue efforts.
An alternative to using leaky feeder in underground railways is to use Distributed Antenna System (DAS). A DAS system was deployed in some New York City Subway stations by Transit Wireless to provide WiFi and mobile phone and data coverage for customers.
In-flight entertainment systems
Leaky feeder is also being used in warehouses and other industrial buildings where it is difficult to get WiFi coverage using normal access points. Real life installations with 50–75 meters of leaky wire connected to the antenna input of Access Points exist, and are working fine.
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