So you're after feeding a continuous wave into the TX, with something that disrupts the wave briefly, and on the RX side out the same wave with disruption - maybe followed by a peak level detector that drops down when a peak is missing.
So you're after feeding a continuous wave into the TX, with something that disrupts the wave briefly, and on the RX side out the same wave with disruption - maybe followed by a peak level detector that drops down when a peak is missing.