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.
A 481THz transceiver and separate receiver
at the test site for a >10km speech contact
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. So far I have used 100mm optics (lenses available from Poundland) mounted in 110mm drain pipe. Although I have managed one 10km line-of-sight 481THz AM speech contact, this aspect is less interesting to me.
On 3 attempts copying G4HJW's optical signal it has been (1) 30dB S/N, (2) no copy and (3) 20dB S/N in 0.67Hz bandwidth over an 8.63km NLOS path. I am 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 is the furthermost NLOS signal I have copied and it verified my RX kit is 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.
See the next page in this section for more details of tests using my own QRSS3 optical beacon over non line-of sight paths/
This beacon uses 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.