I found another lens in my optical parts box, slightly bigger but very bendy, not ideal…
I 3D printed a frame to hold it straight and fitted it to my Receiver, i found maybe 3dB more gain than with the small lens… Well at least it’s something.
I fitted the small lens in front of the modulated source ‘the Tx’, and used the larger lens on the Rx.
Outdoor tests eventually worked at 55m distance, alignment was not easy at all, the beamwidth when using the lenses is very small.
I ended up using an LED at the focal point of the Rx and looking for the red dot to illuminate the heater element on the Tx.
This is very similar to optical work, a sight for alignment is going to be essential.
Signal was not strong but easily seen in a few hundred mHz bin.
