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>Transmitter Failure Alarm

You would have it installed near your high power transmitter. Having worked on high power AM transmitters, the fences surrounding the antenna can pick up the signal and corroded joints act as a detector, the result is the fence becomes a receiver. So I have no doubt a relay could be powered.

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What? Does the fence itself vibrate and reproduce the sound? Or do you need to complete the circuit using the fence as the detector?

I'm not entirely sure what process is converting to sound, but the metal fence makes a great antenna and we are talking about 10's of feet from the antenna with 500 kW AM at 200 kHz going into it.

It could actually have been arcing:

https://www.youtube.com/watch?v=uo9nGzIzSPw

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