Optical communications

The last time I did serious field work on 481THz over the horizon optical communication was before my 2013 stroke. Since then I have been too wobbly. 

This photo shows the gear used in these tests.

Optical Communications

 Before my 2013 stroke I enjoyed doing experiments at 481THz (red light). Although my main interest was "over the horizon" digital communication, I had one line of sight AM QSO over about 11km. 

On YouTube is a video of a German optical communicator that was used in WW2. As the beam was very narrow it was ideal for secret communication up to 5km. Mechanically it was a work of art.

481THz beacon

This was the beacon I used on optical frequencies many years ago before my stroke. It was received well over the horizon at many kilometres and was capable of being copied far further. 

I still have it and would love to use it again .

Optical DX - NOT amateur radio

For years I have been fascinated by mirages and increases in refractive index at optical frequencies. What is often not realised is that how far we can see changes depending on the refractive index .This photo was a Facebook memory from some years ago.

Over the horizon optical tests

 It’s many years since I last did these tests and I remember being amazed that on a cloudless night I could clearly receive my 481THz beacon way over the horizon without any signal visible. Had my stroke not got in the way, I am sure much much further would have been possible even with my simple QRP system. The receiver I used is attached. It was also in RadCom. It is also on my main webpages.

Over the horizon on 481THz (optical)

Before my 2013 stroke I enjoyed experimenting at optical frequencies and was amazed that I could decode my QRSS CW well over the horizon with no clouds in the sky. Sadly, my stroke put an end to this. The circuit diagram shows the beacon I used. As you can see it is simple.

See https://sites.google.com/view/g3xbm4/home/optical-nanowaves/over-the-horizon .

Optical DXing over the horizon

Some years ago, when I was fitter, I really used to enjoy going out in the car to experiment with non line-of-sight optical beaconing. I would set up my beacon at 481THz on my windowsill indoors and see if I could copy it over the horizon. Sadly my stroke ended these tests.  

The photo shows my beacon, that was connected to 100mm drain pipe and a Poundland lens.  The LED was bought off eBay. Latterly I bought some much brighter Phlatlights, but I never got around to using them before fate intervened.

I was always amazed how well my tiny signal was copied over the horizon. I presume this was clear air scattering as there were no clouds.

481THz beacon

 Another Google photo from many years ago. This was an optical beacon used for over the horizon tests.

Optical illusion - NOT amateur radio

This optical illusion (actually 2 circles) was seen recently. 

It certainly messed up my head! 

Optical over the horizon

This was 2012.

New experiments to try?

As many know, I enjoy experimenting. In recent years, my health has not been too great, so I have been looking at things I can do from home rather than going out and about. In the past I have tried VLF and over the horizon ((NLOS) optical tests, both with great success.

My problem is I am running out of new things to try!  

I have tried 500uW WSPR on 10m (best DX USA) , got to the Caribbean, USA, Canada and South Africa with 2.5W and a dipole on 8m FT8, 630m QRP WSPR (got to Finland) and have tried the 22m and 8m ISM bands with QRPP WSPR.

If you have any ideas, I'd like to hear from you. These are the criteria:

• Must be able to do it from home.
• No complicated antenna erections.
• Not expensive.
• Does not involve fine motor skills as I am no longer good at these sadly.
• Interesting and fun.
• Contributes to knowledge.

I am sure there must be things to have a go with.  What I would really like is something "meaty" that needs doing. With plenty of time on my hands, things can spread over many months.

Some of the things I might try:

• More 137kHz CW and QRSS3 tests.
• 1296MHz  QRP
• FT8 with real QRPP on 10, 12, 15, 17 and 20m.
• 8m this Es season with ISM power and WSPR.

2012 optical experiments

Way back in 2012 I was doing optical work at 481 THz. 

The photo shows some initial tests along the road at the old QTH. 

Later, some tests were done "over the horizon" non line-of-sight. I was astounded by the results.

481 THz testing

If this photo has been on recently, please accept my apologies. 

For some reason, my XYL laughs when this appears. As a geek, I cannot understand why!  😉

It is some time since I did any serious work at optical frequencies. With the dark evenings I really should try.

481 THz RX

 

This is me resurrecting some 481 THz optical gear.

Optical communication in daylight

Earlier, I noticed the the reflection of the sun from my watch face on a distant wall. This got me thinking of heliographs. These were used to send Morse code messages in sunny countries. I think ranges were remarkable.  See https://en.wikipedia.org/wiki/Heliograph. I seem to recall ranges were up to 80km. The best DX (line of site) was 295km.  Basically, the sun's reflection was obstructed to send CW.

My mind turned to QRSS beaconing. For this to be possible it would be necessary to divert the beam when the QRSS was not on. One possibility would be to have a loudspeaker with a mirrored sheet (kitchen foil?) over it with a tone keyed.  It would also be necessary to have a receiver that was optimised for daylight.  I have no idea how good this would be. 

I guess the same idea could be used for speech. The mirror would be deformed as speech was said. I seem to recall there was a 19th century patent for this called the Photophone. For speech I guess one could simply speak against the metal foil mirror.

I would imagine an optimised system could be used for over-the-horizon (NLOS) tests. Sadly, my health these days means I can only dream about this. It may already have been tried.

Optical Experiments

One of the fallacies going around is that optical communication is just "line-of-site" and that both the TX and RX have to be able to see each other. This is just not true! 

With very low power and simple, homemade, gear I could detect my beacon on a PC way over the horizon. In Australia, the "over-the-horizon" (NLOS) distance record is way over 100km. With my own very modest gear, I can well believe this. 

My tests were done on cloudless nights, suggesting the propagation was scattering off atmospheric dust. There was no sign of the light from the TX in the sky at all.  This is definitely where everyone, licenced or not, may experiment. It would be good to find out how far we could achieve non line of site (NLOS) in the UK. Incidentally LEDs are generally considered better than lasers due to scintillation (look it up!).

My own beacon and optical gear were made in the back of the garage. If I was fit, I would be experimenting now.

My optical gear used LEDs bought off eBay. Far more powerful ones are available. I used 100mm lenses used in magnifying glasses bought at Poundland. Better equipped stations used Fresnel lenses. My tripod was hopeless. With better LEDs, lenses and tripods, ranges could be increased a great deal. Even with my system ranges could be considerably greater than the nearly 9km I managed.

See https://sites.google.com/view/g3xbm4/home/optical-nanowaves/over-the-horizon.

Optical communications - over the horizon

When fitter, I really enjoyed experimenting with this. Actually, I don't think this needs a licence at all, but it is real experimentation. Anyone can do this without costing much. 

All my gear and much of my test gear was homemade and very inexpensive. It is real "back of the garage" stuff. All my gear was homemade in the back of the garage. The only special test gear was a very weakly lit LED. If you have a multi-meter and scope, you really don't need much else. In my view optical communications is easier than microwaves as the test equipment is easy.

I was quite surprised how easy it was to copy an optical signal "over the horizon".

See https://sites.google.com/view/g3xbm4/home/optical-nanowaves/over-the-horizon . 

Plans for Monday

My 481 THz test software and lead are my priority. When I did the optical tests way back more than 10 years ago PCs had separate mic and headphone jacks. All my PCs now have combined jacks! 

So, I have to interface my optical RX and the PC in a new way. With soldering skills very hard these days, this requires some thought. Years ago I would just have bought a new jack socket (with mic input) and just changed the plug! I think years ago I just used a lead with standard 3.5mm plugs at each end.

I may put the 10m WSPR beacon on too.

UPDATE 1038z: 10m WSPR beacon is ON and my audio setup (interface from 481 THz RX to the PC) is working with Spectran. At G1KQH's suggestion I used a USB sound card I already had and it worked first time. Thanks for the suggestion Steve. 

UPDATE 1144z:  No 10m WSPR spots of my 500mW beacon. I suspect things are very disturbed. I checked all was well on another RX.

UPDATE 1515z:  4 stations have spotted me today so far on 10m WSPR. My 481 THz TX beacon and RX work with QRSS and Spectran. 

Plans for next week

My plans for next week are currently as follows:

• Restart experiments on 40.680 MHz QRP FT8 TX now the Es season is almost here. 
• Put Spectran software on the shack PC and carry out local 481 THz optical tests with it.
• Go on 10m QRP WSPR TX whilst on 8m. 
• Perhaps try 80m TX with the FT-710 using the earth-electrode "antenna" in the ground. Maybe I should try 80m FT8?? 
• With Spectran on the PC, I could also perhaps try some 136 kHz QRSS tests.

481 THz testing

The photo shows me testing my optical kit. It was recently tested locally. I want to do some over the horizon tests with G4HJW.