This works, increasing the distance between transmitter and receiver increases the delay of the received signal. I used a Raspberry Pi to make the on/off pulse, well of course some 555 timers could do it too. A small 2 transistor switch keys the transmitter oscillator driven from a GPIO pin.
Youtube resolution is a bit low to see the scope, top trace is transmitted 44kHz signal, bottom trace is received signal, trigger is external from the Raspberry key pulse.
10 ms on, 10 ms off.So far so good.
BUT doing it in the 'wind tunnel' with varying air speed, does not give any measurable differences in time of flight. Setup:
So, as I now have the windtunnel I will next test some thermal circuits, simpler, lighter (coils are heavy), and maybe even better. Will see.