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Measuring Energy in an Induction Loop

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Newbie level 5
Apr 5, 2009
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Hey everyone,

I have a question about induction loops, for example those used for hearing assistance or other short-range communication applications. I assume that they are magnetically coupled systems whereby the transmitter generates current in a parallel resonant circuit tuned to a specific frequency. I suspect the receiver has a coil that is "induced" upon transmission.

My question is, what can be used to independently measure the inductive energy external from the circuit? For example, I can take a regular car radio and receive the RF emissions of an AM station. An inductive loop ideally generates only a magnetic field (no E field). So I expect I wouldn't be able to use a traditional RF receiver to capture the data, would I? Is it the difference between a "near-field" communication and "far-field"? Are there specific typical circuit arrangements to receive signals through an inductive loop?

As a first remark, induction loops for hearing assistance are in fact transmitting unmodulated audio signals, so they are wide band rather than tuned circuits. There are however other induction loop systems using modulation.

To characterize the transmission system, you would want to measure magnetic field strength. A loop antenna, possibly an amplifier and either a spectrum analyzer for modulated signals or a wideband monitor, e.g. an oscilloscope, for base band audio, can be used. A RF receiver would need to support the frequency range and modulation scheme of the system.
Thanks for the response and that correction in the first remark.

So if I u derstood correctly, even though an induction loop is missing a significant E-Field, and only has a magnetic component, I can use a standard RF spectrum analyzer or simple tuner circuit to capture a modulated inductors loop transmission? I wasn't sure if by only having a magnetic component that a different device would be needed.

Yes. The output of an antenna, whatever it is and which field component it's sensitive to will be a voltage that can be feed to a receiver, spectrum analyzer, wideband detector or what instrument you prefer. In case of the hearing assistance induction loop, you would simply connect an audio amplifier and a speaker.

I once designed a receiver for a multi-channel AM induction loop system. It used a ferrite rod antenna and a single chip receiver.

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