My interest in communicating at optical frequencies (infra-red and visible) 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.
On this page I intend to record my own experiments nearly 50 years later than those first tests. Like my experiments on VLF, LF and 500kHz this will grow as I build more gear, refine it and improve it. As a keen QRP man I expect my approach will be "keep it simple" if at all possible. So, it is unlikely my kit will be the biggest and the best. Rather, I expect to share my modest attempts with relatively simple equipment and optics.
On 3 attempts copying G4HJW's optical signal it has been (1) 30dB S/N, (2) no copy and (3) today 20dB S/N in 0.67Hz bandwidth over an 8.63km NLOS path. I am still using the BPW34 detector in 100mm optics.
Successful NLOS test over 8.63km!
Both on Bernie's RX kit and my own we got solid copy by ear of the beacon at a distance of around 9.8km NLOS using 100mm optics. Once found, the signal was solid. I then set up the optics on the tripod and fed the signal into Spectran where the signal was 30dB over noise in a 0.17Hz bandwidth.
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. However I will try again later in the week with Spectran running and narrow bandwidth.
With a largely clear sky, few clouds and a slight haze, it was not ideal conditions for cloudbounce testing. This test will be repeated when cloud cover is better and visibility is clearer. QRSS30 would give me another 10dB so this may be worth a go if QRSS3 doesn't work on a second try. There are several variables that make weak signal forward scatter detection difficult: (a) what vertical angle to use at both ends, especially with few clouds (b) amount and height of cloud cover, (c) horizontal direction of aim. With QRSS3/30 modes it is a case of small aiming increments and wait to see if a signal appears on the screen. It is quite hard work.
The result is disappointing, but there will be plenty of other occasions to repeat this test and do others. All good fun.
Success with non line-of-sight "cloudbounce" test (3.6km) today
With the beacon aiming out through the double glazed shack window at nearby Burwell windmill (as an aiming point) I set off for a road at Landwade which was 3.6km away "over the hill" and on a NLOS path from here. At Landwade I set up the 100mm optics and my variation of the KA7OEI head feeding into my laptop running Spectran. Immediately I got a good signal from the beacon 3.6km away. Signal was around 10dB S/N in 0.67Hz bandwidth. The signal was neither visible as a red glow nor audible in the earpiece despite listening quite hard and panning around for best signal.
This was my first proper NLOS test and it is extremely encouraging. I did try to elevate the RX to higher points in the sky but best reception was with the optics aiming at the lights of Burwell village in the distance i.e. as low as was possible in elevation. At the TX end I was aiming to just clear the slight rise in ground to the east of me near Burwell windmill.
I'm really lucky finding this test path as I can put the TX beacon on the bedroom shack windowsill and fire towards the windmill. In daytime I would be able to align the RX better as I was having to guess the best direction with only Burwell church visible. I had to tweek the alignment to what I thought was the right direction. I did not spend a lot of time trying to peak the signal and better copy is possible. In all honestly I did not expect this test to be successful.
Kit: TX (250mA LED, 100mm optics in one room of house), RX in another room KA7OEI based head (doors closed, no light leakage) with 100mm
optics. PC running Spectran positioned to minimise light pollution to RX head.
First test was 10wpm CW off the tree (0.3km path length total) with strong signals received 35dB S/N in 5.4Hz bandwidth. Signal v.clear in the earpiece too. I could just make out the red light glow in the distant tree branches. Aiming critical.
Second test: TX and RX elevated to aim at roughly same patch of clear sky. QRSS60 signal sent from TX. Signals detectable in Spectran in 0.17Hz bandwidth, weakly, but definitely there. It is less clear on the capture than on the real screen. For this second test I made no great attempt to optimise the RX aim, just aimed at what I thought was roughly the same patch of sky. Now I can't be sure whether the signal is purely from scattering off mist/dust particles or what, but I think it is unlikely signals are coming off other objects as I am aiming quite high into the sky (about 45 degrees up) clearing nearby stuff.
These tests suggest that with very slow QRSS I may be able to get a NLOS signal to G6ALB in the next village 3km away. I wasn't expecting the QRSS skyscatter test to work at all, so was surprised. When the weather improves I'll go out /P with the PC and RX and see if I can detect the "forward" scattered signal at much greater range (1-3km). With proper cloudbounce it should be better I would think.
My 1.082kHz beacon and 100mm optics were set up pointing out through my double glazed bedroom window and aimed it at a local feature called the Devil's Dyke which is 1.6km (1mile) exactly from home. This is the furtherest line-of-sight (LOS) path from home. Then walked along the dyke to roughly the point where I was aiming and started looking with my handheld 100mm optics receiver. Much to my joy and surprise, I heard the beacon before I spotted it by eye.
The beacon could be copied over a stretch of about 50m along the dyke. S/N I'd guess at around 20dB (by ear) in speech bandwidth in daylight. Next time I'll take the laptop and measure S/N with Spectran. It's a bit of a trog up there at night in the dark though as it is some way from the roadside carpark.
I simply had a cascode KA7OEI optical head feeding a feedback biased common emitter stage into a crystal earpiece, although the recovered audio was a bit
low in the wind. I am using a BPW34 detector with some reverse bias but the diode anode is connected directly to the FET gate. Down the street that seemed to give best results, although I have not done extensive tests.
Now 1.6km may not sound like much, but this is my best distance so far. So, some progress in the right direction.
Yesterday my 1W ultra-bright LEDs arrived from Hong Kong. When fitted to the beacon these will be VERY bright and will extend the range possible on 481THz yet further. All good fun!
Today I completed my first keyed optical beacon transmitter (with 100mm optics) which I'll be using for tests at greater range than hitherto. The 481THz (red light) beacon sends my callsign in either 10wpm CW, QRSS3, QRSS30 or QRSS60 as well as a continuous carrier or a 30sec on/off sequence. The TX uses a sub-carrier of 1.082kHz or 8.659kHz, the frequency being derived from an HF crystal divided down in a 4060. The message comes from a K1EL K-ID2 programmed PIC. Initial tests this afternoon at dusk allowed me to copy the 1.082kHz 10wpm CW sub-carrier signal by reflection off a wall across the street at decent strength.
I now need to find some longer (local still) optical paths to test the beacon TX with my 100mm KA7OEI receiver. It would also be fun to try some non line-of-sight tests locally , possibly including cloud-bounce. I think this may be possible with QRSS30 or QRSS60.
In the first instance I need to remeasure the noise floor with the optical head in total darkness to see if the changes indeed make it better than the current discrete component circuit. My first quantitative noise measurements suggest far higher noise floor than I expected.
Assuming this gives improved performance over my 0.4km local "up the road" test range, my next step is to attempt a longer path, so I am looking for a path of around 2-3km to test with my baseband beacon. If this works with decent S/N then I'll retry looking for the GB3CAM optical beacon. At 32km this is a good test of system performance. At the test site at Nine Mile Hill the traffic noise was high (optically and audibly) so I also need to increase the signal level in the earpiece used to align the RX.
There is also soon to be a second optical beacon near Cambridge and this should be a little closer and an easier signal to find. However, I need to do tests before this is installed at Dry Drayton.
Incidentally, I managed to overcome the parallax issue in the optics alignment so now a distant street light is spot on in the cross-hairs of the spotting scope.
Failure can be put down to several possible causes:
The picture shows the test set-up. The dashes in the Spectran trace are a result of me interrupting the beam with my hand.
If you click on the image you can see
the pre-focused red LED pointing out of the house window and the
receiving kit in the bottom foreground.
The image on the right shows the signal as received using Spectran. The signal is well above the noise floor, but the noise floor is well above where it should be! My field test to go searching from GB3CAM will have to wait until the system is improved.
Even with just the stand-alone 20mA TX diode (no additional lenses) and this RX system, the range for voice comms should be be over 10km I think based on 6dB additional loss for doubling distance.
UPDATE AT 1730: For the first time I tried the system in the dark at night. I managed to get around 0.4km from the LED beacon at home (the limit I could find walking along the street and into a field) where the audible signal in the crystal earpiece was STRONG. This is very promising. Interference from sodium street lights was an issue although the RX optics have sufficiently narrow beam width to avoid these.
A 5mm 10 degree red LED using just its own built-in lens was shone out of the stairway window aiming down the street in daylight with 1kHz modulation. It was taking around 20mA. I then walked with the kit to the far end of the street and the signal was solid (S9) in the RX earpiece at a distance of about 0.25km.
The system has been locally tested using SM6LKM's SAQ receiver on the PC (as a 1kHz direct conversion receiver) and using Spectran. All looks very promising in terms of S/N and sensitivity. Next week I hope to go looking for GB3CAM from a site 32km east of the beacon at a location called Nine Mile Hill. See Bernie Wright's page about the optical beacon at http://www.earf.co.uk/light_beacon.htm.
The photo shows the "Heath Robinson" TX optics positioned on the stairs of my home! Note the 4x gun sight which was obtained via eBay. I think this will be an essential part of the alignment procedure, as will good stable tripods at each end.
Next stage of the experiment is to complete one complete transceiver unit. This will probably be a RX/TX head together with a transverter built into the same unit. This will allow FM, CW and SSB operation on subcarrier frequencies up to around 30kHz. Hopefully this will be compatible with other local 481THz operators. It should also allow me to look for the GB3CAM beacon on 481THz at RAF Wyton in Cambridgeshire.
Today I carried out my first optical communications tests since 1966. I
built a small "baseband" (i.e. not on a sub-carrier based) optical
transmitter producing a tone at around 800Hz feeding a standard low cost
high brightness red LED at around 10mA current. The LED has a small
built-in lens which produces a beam of around 20 degrees.