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Induction loop

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Induction loop is a term used to describe an electromagnetic communication- and detection system, relying on the fact that a moving magnet will induce an electrical current in a nearby conducting wire. Induction loops are used for transmission and reception of communication signals, or for detection of metal objects in metal detectors or vehicle presence indicators. A common modern use for induction loops is to provide hearing assistance to hearing aid users.



The phenomenon that was (according to anecdote) first noticed accidentally, on a battlefield during World War I. A telegraph linesman was running tests on a field telephone cable loop, when a radio operator, within the loop, noticed sounds in a headphone that had a faulty, short-circuited, jack-plug. The original explanation of the phenomenon was a conventional transformer theory. Mr Barry Pyatt, Senior Equipment Engineer and Project Leader at Rediffusion-Reditune (Rediffusion House, Cray Avenue, Orpington, Kent) in 1973 mathematically demonstrated that the phenomenon would vary according to an installation's global geographical location.

Complex computer simulations were run at a London university, taking account the magnitude and dip angle of the earth’s magnetic field, and these gave weight to Pyatt’s assertion that the phenomenon was rather more due to a modulation of the earths magnetic field, rather than simple transformer coupling theory. Rediffusion was, at that time, a multi-national organisation, and tests conducted by the company's South African and American operations also indicated that Pyatt's predictions were measurably correct. Rediffusion-Reditune were granted a "Ministry of Posts & Telecommunications License" and developed one of the first commercial induction loop amplifiers (product code LAA 100) for use in the industrial world for communication in noisy environments. Hearing protection hardware and telecoil assemblies for the system came from A & M Hearing Aids in Crawley, West Sussex.

The required adherence to the maximum average ampere/turn criteria laid down by MINPOSTEL was achieved by indirectly heated thermocouple technology. The whole gamut of the history, technology and Rediffusion's development of induction loop systems was internally published in an aide-mémoire for the (then) global British Electric Traction empire, and distributed amongst an international contingent of engineers and company directors attending an engineering conference at the Bromley Court Hotel, 1974. Mr Pyatt became Group Leader of engineering development at Rediffusion-Reditune, and his team went on to develop a four band VNB VLF (120 kHz) frequency modulated system.


thumb|alt=A An example of the Inductance loop installed in the road for cars and bikes.|An example of the Inductance loop installed in the road for cars and bikes. The "aerial" system of an induction loop installation can consist of one or more loops of a conductive element.

In industrial applications this might be a large single, or multi-turn, loop, or a complex multi-lobed, phase coincident sub-loop design, most effectively mounted above the required reception area in industrial applications.

An audio induction loop might have one or more loops sometimes with a phase shift between them, and either near to or around the area in which a hearing aid user would be present. Many different configurations can be used depending on the application[1].

An induction loop receiver is classically a very small iron-cored inductor (telecoil), although Rediffusion demonstrated a prototype Hall-Effect system in its PLL FM system.

The system commonly uses an analogue power amplifier matched to the low impedance of the transmission loop. The transmission is normally direct rather than superimposed or modulated upon a carrier, though multi-channel system have been implemented using modulation.

Vehicle detection (inductive) loops are used to count vehicles passing or arriving at a certain point, for instance coming up to a traffic light, and in motorway traffic management. An insulated, electrically conducting loop is installed under the road. An electrical voltage is generated when a mostly ferrous (containing iron or steel) body passes close to the wire/loop

Other definitions

A popular alternative definition of "induction loop" is that applied to metal detectors, where a large coil, which forms part of a resonant circuit, is effectively "detuned" by the coil's proximity to a conductive object. The detected object may be metallic, (metal and cable detection) or conductive/capacitive (stud/cavity detection) Other configurations of this equipment uses two or more receiving coils, and the detected object modifies the inductive coupling or alters the phase angle of the voltage induced in the receiving coils relative to the oscillator coil.

An increasingly common application is for providing hearing aid compatible "assistive listening" telecoil. In this application a loop or series of loops are used to provide an audio frequency oscillating magnetic field in an area where a hearing aid user may be present. Many hearing aids contain a telecoil which allows the user to receive and hear the magnetic field and remove the normal audio signal provided from the hearing aid microphone[2] site. These loops are often referred to as a hearing loop or Audio induction loop (more information at this page).

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