Thank you for explanations Estaban,
There is still one part I don't understand -- The circuitry to measure the phase shift: Do I use only an audio circuit to listen to the 400 Hz transmit signal? And I listen for change in frequency or in amplitude or both? Is there some kind of receiver circuit to build?
What kind of circuit should I use, and what should I measure with this circuit?
Best wishes,
J_P
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