One of my interests has been over the horizon optical beaconing. Using all homemade equipment, I have received QRSS signals way over the horizon. There was no visible light in the sky yet perfect QRSS signals were visible on the PC. LEDs were used at the TX end and RX end with homemade optics based on a cheap magnifying glasses and drainpipes.
Sadly my 2013 stroke put an end to my field experiments. I am convinced far further would have been possible than I achieved. One of my main problems was the tripod stability of the receiving optics. Aiming was critical, but I was unable to repeatably aim in the right direction.
My interest in communicating at optical frequencies (infra-red and visible around 481THz) goes back to the mid 1960s when a school friend (Francis Wood) and I did some experiments using a small torch bulb and an OC71 transistor (with its paint scratched off). The torch bulb was modulated with a small audio amplifier and the OC71 used as a photo transistor at the receiving end. Despite no real optics we managed to communicate across the street one way and were very pleased.
Since then technology has moved on rapidly. Today there is a growing group of enthusiasts working around 300-500THz with quite advanced equipment and achieving results that are staggering. In the UK the DX record is now well over 100km. In other countries the distances covered have exceeded 250km using cloud bounce techniques and high powered lasers or LED arrays with excellent optics.
My main interest has been weak signal detection over the visible horizon using QRSS and CW beacons and sensitive receivers feeding a PC running Spectran weak signal software. I have managed to copy G4HJW's signal over the horizon at nearly 9km and my own weaker beacon over 4.8km, but much further will be possible. I used 100mm optics (lenses available from Poundland) mounted in PVC drain pipe.
Bernie G4HJW was 20dB S/N in 0.67Hz bandwidth at my local test site 8.63km NLOS from his QTH "over the horizon". He was using his powerful Phlatlight beacon with 1.082kHz tone with Fresnel lens optics. The sky was clear and starlit, so this was clear air scattering not cloud bounce. About 15 minutes after I first copied him it seemed to get more hazy and I was struggling to find his signal at all. Best results appear to be with a clear sky and aiming at the horizon.
On 3 attempts at copying G4HJW's optical signal it was (1) 30dB S/N, (2) no copy and (3) 20dB S/N in 0.67Hz bandwidth over an 8.63km NLOS path. I was using the BPW34 detector in 100mm optics.
There was hardly a cloud in the sky, so this was largely by scatter from the mist/dust in the air. Bernie says signals have been stronger on other nights. Best reception was with the RX aimed just above the horizon, maybe 5-10 degrees, at most, above. This was very gratifying as it was the furthermost NLOS signal I had copied and it verified my RX kit was working at good sensitivity. When I got back home I did try to copy the beacon by ear out of the bedroom window but the tree cover and lights made copy not possible.
A simple CW/QRSS3 optical beacon TX
This beacon used a K1EL beacon IC to drive an IRF640 switching a super bright LED at around 150mA. It was successfully detected over a 3.6km non line of sight path with 10dB S/N in a 0.34Hz bandwidth and later at just over 8km away. Further was possible. If my stroke had not got in the way I was going to try much further. These days I am just too wobbly!