Induction loop

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1 History
2 Implementation
3 Other definitions
- 3.1 Modern day applications
- 3.2 Historical applications:
4 External links
5 References

[edit] History

“Induction loop” is a term historically used to describe an electromagnetic communication system, which exploits a phenomenon that was (according to anecdote) first noticed accidentally, on a battlefield during WWI. 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 by way of 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 Licence" 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 Rediffusion/ 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 (120kHz) frequency modulated system.

[edit] Implementation

The "aerial" system of an induction loop installation normally consists 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. The 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.

[edit] 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.

[edit] Modern day applications

vehicle detection at traffic lights
car park Parking Guidance and Information systems
metal detectors

Another use of induction loops is the provision of hearing aid compatible "assistive listening" telecoil.

[edit] Historical applications:

An Anti-submarine indicator loop was a device used to detect submarines and surface vessels using specially designed submerged cables connected to a galvanometer.^[1]