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Scenarios

So, I set up an induction loop, just a simple single wire loop around a house with a little pickup coil for picking up the loop audio.

Lets say I want to run this in REVERSE

I have heard "If an antenna can transmit, it can usually receive as just as well".

I hook up the amplifier to the loop around the house, and all I hear is 60hz HUM, and when I play audio up to the little pickup coil, i don't hear anything through the amplifier, just 60 hertz.

Can I indeed run an induction loop in Reverse, or not?

Second Scenario

When hitting an Amateur Radio repeater, I sometimes can hear it full quieting, but when I transmit, they can't hear me.

So is "If an antenna can transmit, it can usually receive as just as well" True?

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Yes, it's true; it's known as the principle of reciprocity.

The thing you're missing is that all other things must be equal. This includes the transmitter power, receiver sensitivity, noise and interference levels, etc.

In the case of your inductive loop, the large transmitting coil is much better at picking up the ambient 60 Hz field than the small receiving coil is, so when you use the transmitting coil as a receiver, the signal to noise ratio is vastly different.

In the case of your repeater, your transmit power is significantly less than that of the repeater, while the receiver sensitivities are probably very similar. Therefore, you can hear it even if it can't hear you.

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Reciprocity is one of the most useful (and fortunate) property of antennas. Reciprocity states that the receive and transmit properties of an antenna are identical. Hence, antennas do not have distinct transmit and receive radiation patterns - if you know the radiation pattern in the transmit mode then you also know the pattern in the receive mode. This makes things much simpler, as you can imagine.

Taken from here and note that the send or receive patterns for a big loop and a small loop are massively different. You are citing reciprocity but you are not applying it correctly - these are two different scenarios.

Your big induction loop will pick-up 60Hz whether it is transmitting or receiving. When transmitting, the 60Hz in the loop is of no consequence because your amplifier is pumping out amps and defeating this current on all fronts. Your little receiver coil is also picking up 60Hz but the magnitude of the magnetic signal from the big loop is overcoming the 60Hz it picks up.

On the other hand, when you are transmitting on the small loop, you are generating only a fraction of the flux you generate with the big loop. This means the voltage induced into the big loop has to be amplified significantly more than the small loop. Also, because most of that big loop isn't magnetically coupled to the small loop, the background noise it picks up can be massive. This is not true when the big loop transmits - all of the small loop is "bathed" in flux from the big loop.

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You have two issues there:

  • You are picking up a wide frequency range, including 60 Hz, where you can hear the electromagnetic waves emitted by the power lines in your house.
  • Your induction loop runs in parallel to the horizontal distribution to your outlets, so it is bound to pick that up easily, and at a power level greater than other transmissions.

If you amplify this and output to a speaker, you will hear the dominant frequency.

To extract the other frequency components, you need appropriate filters. These can be either analog or digital if you use an ADC to sample the data. As you have a dominant interferer which would easily saturate your ADCs before it could pick up the other signals with good quality, analog is the way to go here, for example a high pass filter.

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"Usually" is an unnecessary hedge. Reciprocity implies that an antenna works equally well in either direction. I'm not going to summarize the proofs here, Wiki has it all.

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