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Antennas or aerials
Published in Geoff Lewis, Communications Technology Handbook, 2013
Leaky feeder/leaky wave antenna. A length of waveguide with a longitudinal slot or row of holes, or co-axial cable with purposely poor shielding, can lose energy continuously along its length. This provides an effective way of transmitting signals within a tunnel or underground system. These are often described as fast or guided wave structures.
Low-cost leaky feeder communication for mines rescue
Published in Mining Technology, 2020
Michael D. Bedford, Angel J. A. Rodríguez López, Patrick J. Foster
By way of contrast, a leaky feeder cable (Martin 1975) is a type of coaxial cable that has a deliberately reduced coverage shield so that, although it still acts as a moderately efficient feeder, some of the signals will leak out along its length, while external signals can leak in. This way, users in the vicinity of the cable are able to communicate, but the requirement to be in very close proximity to the cable, which applies to the LF guidewire approach, is relaxed. It is intended to provide communication in confined spaces such as mine galleries or transport tunnels, and is unaffected by bends. For example, users in trains in a subway tunnel equipped with a leaky feeder are able to use mobile phones. Although there are several different designs, one common type of leaky feeder is shown in the bottom part of Figure 1. It can be seen that the shield has periodic apertures cut in it to allow some signal to leak both in and out. However, to compensate for this loss, and therefore allow an acceptable amount of signal to progress along the cable, the cable often has a significantly greater diameter than ordinary coaxial cables, and is therefore much more expensive. Typical leaky feeders are 16–50 mm in diameter, they weight 220–1120 kg/km, and they cost thousands of pounds per kilometre.