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.

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 DXing

Since my 2013 stroke I have not felt fit enough to do field work. I really miss my 481THz over the horizon NLOS (non line of sight) optical work.

Testing the optical kit

The results back in 2013 suggested much further would have been possible NLOS. All the gear was home made and the experiments were fun. Sadly I am just too wobbly these days.

It would be good to try infra-red frequencies. What amazed me was the total lack of a visible signal, yet perfect decode using QRSS3 on the PC on a very non line of sight path over the horizon.

See https://sites.google.com/site/g3xbmqrp3/optical
See also https://sites.google.com/site/g3xbmqrp3/optical/481thz-nlos

481THz NLOS (over the horizon)

Before my stroke in 2013 I enjoyed building and operating optical communications gear. My stroke has rather scuppered this, for now! Although I had one over 10km line-of sight speech contact with G4HJW/P, my main interest was weak signal, over the horizon, communication. I did tests at over 8km non line-of-sight using QRSS3. Although there was no evidence of the optical signal in the sky, the QRSS3 signal was clearly decoded on the PC at the receive end. Overall, I was left with the impression that much further would have been possible.

At some point I really want to continue this work. All my optical gear was home-made using material that was low cost and easily obtained. Test gear for optical work was simple and low cost too. In my view, optical work is more rewarding than microwaves and the test gear simpler.

See https://sites.google.com/site/g3xbmqrp3/optical

481THz over the horizon

Facebook reminded me that 4 years ago I was doing QRSS3 tests with light at 4.8km over the horizon.

"Much better results this evening with 481THz light beam signals over the horizon at 4.8km. Last night I was 2.5 degrees out and signals were much weaker. Tonight QRSS3 could be comfortably copied at 20dB S/N in 0.34Hz bandwidth. At the receiving end the signal is not audible and nothing visible in the sky from the transmitter. Aiming is VERY critical just above the horizon."

Visible Light Communications

There is a nice precis on Wikipedia about this. It is some time since my health has allowed me to do experiments with light. My special interest is NLOS data comms. That is sending data over the horizon using light.

See https://en.wikipedia.org/wiki/Visible_light_communication .

Australian IR tests

VK4EBP reported that he has carried out successful tests over a 10km NLOS (non line of sight) path using infra red.

See Australian Optical DX   https://groups.yahoo.com/neo/groups/Optical_DX/info .

One of my aims for 2015, assuming I am fit enough, is more NLOS tests including IR over greater ranges. To maximise my chances I shall probably use QRSS3 again rather than rely on aural reception. My previous tests, using clear air scatter, was over a 8.5km distance.   Much further should be possible.

Optical communications - over the horizon (NLOS)

The DX record for communications at optical frequencies is phenomenal these days. There is a growing interest in communications over non line of sight paths (NLOS) using cloud-bounce or clear air scattering. To my knowledge, in recent times experiments are (or are about to start) by F1AVY, VK4EBP , VE7SL and G3XBM (when fit again). I am sure there are others too. Weak signal modes certainly help. I used QRSS3 over an 8.5km NLOS path, but much further has been achieved.
 
See http://en.wikipedia.org/wiki/Non-line-of-sight_propagation .

A couple of useful optical Yahoo groups are:

Australian Optical DX   https://groups.yahoo.com/neo/groups/Optical_DX/info

UK Nanowaves              https://groups.yahoo.com/neo/groups/UKNanowaves/info  

Optical article in June RadCom

June's RSGB RadCom carries my article, "Over the Horizon at 481THz" describing my non line-of-sight optical experiments last year. My poor health (stroke) has prevented me doing more with this in 2014. The article was written last year (2013) before my stroke.  Hopefully, a return to good health (by the autumn?) will allow me to continue this fascinating part of our hobby. At the moment I am just too clumsy.

Testing the optical RX

8.5km "over the horizon" NLOS by clear air scattering (in a cloudless night sky) was very straightforward with simple, low-cost optical gear that could be made for next to nothing.  Unlike microwaves, optical kit is very inexpensive and requires only very basic (audio frequency) test gear to set up. Optical alignment is critical and more stable tripods would have helped. 

What amazed me was that nothing was visible or audible at the RX yet the signal decoded and displayed perfectly with Spectran software on the PC.

If you get this magazine, I hope you enjoy the article.  Please let me have any feedback.

481THz RX issue - now fixed

As mentioned in an earlier post, my sensitive non line-of-sight NLOS optical (red LED at about 481THz) RX packed up when I clumsily dropped it yesterday when out in the field at the test site. An investigation this morning revealed a wire had dropped off the battery feed and this was easily fixed. The  RX now works as before.
 
I shall have to try the NLOS test again when my wife can drive me out there. I shall have to do it soon as tree leaves will soon obscure the path at the home QTH end. Just wish I was not so clumsy still following the stroke last year. In the field I feel so much less able than last year. I feel really disabled and hate being so useless and clumsy.

UPDATE 2050z: for now, all my tests will be shack-based because of my poor health, so no immediate plans to repeat NLOS tests until I feel a lot less wobbly.

Aborted 481THz NLOS test

I am beginning to think that ALL tests outside the house should stop until I am a lot better (stroke). The best description is I feel, all the time when on my feet, as if I have drunk 8 pints of beer and am very drunk and clumsy.

This evening, my wife drove me out to an NLOS test site where I managed to drop the RX head hard on the ground. The head immediately stopped working even before I started hunting for my signal! I could hear no hiss at all. There was no sign of the very visible moon noise in the RX.

The test would have been marginal anyway, especially with a bright full moon, but to be unable to carry out the test at all was a great shame.  It is just that everything is SO hard for me at present and setting up optical kit is stretching the limits of my current abilities. Feeling clumsy nearly all the time is tiring too.

Even getting my VHF/UHF beam up at home will require help. Andrew G6ALB has kindly offered to erect it for me. This should be an easy task for me - normally it would be - but not at present. At least, when erected, VHF and UHF activity should be straightforward at home in UKAC sessions on Tuesday evenings. Sitting down my balance is fine. I just hope this soon improves. Walking is getting better but balance/giddiness are still big issues.

Non line-of-sight (NLOS) 481THz paths from home QTH

Before venturing out, I have been trying to plot possible NLOS paths with the optical TX at home (inside) and the RX at a convenient roadside spot at a moderate (local) distance. One possible path is VERY remote down very uneven fenland roads which would have to be navigated in the dark with my wife driving the car. I am thinking about her driving the car. I am not allowed to drive as a result of my stroke.  Many of the NLOS paths are blocked by nearby houses and trees. It is a surprisingly hard task.

 Back to Google Earth to see where might be worth trying. From previous tests it is necessary to aim very carefully.

Successful 481THz garden tests

Today I repeated my optical tests across the garden receiving signals on my RX with both 600 ohm phones and Spectran software with QRSS3. This time the tests were very successful with signals at least 40dB over noise when scattered off distant bushes (0.34Hz BW).   I think we are nearly ready for the NLOS tests again!

Non line of sight (NLOS) 481THz light

My attention is turning back to 481THz (red light) and slightly lower frequency (IR) light communications over the visible horizon (NLOS). From the new QTH there is a good local marker (roof top chimneys on a house in the High Street) for a 6.18km NLOS path to the place I used to go to check VLF earth-mode at limit range. If I can set things on the right beam heading from home,  I then have to persuade my wife to drive me out there after dark where the roads are VERY bumpy and rough - it is way out in the fens - to see if I can pick up QRSS3 signals using Spectran software. This is where I really miss not being allowed to drive (because of my stroke) myself.

Infra-red "over the horizon" tests?

Reading Stuart G8CYW's article on the history of optical communications in the latest RadCom, I am considering repeating my recent NLOS optical test using infra-red (IR) rather than visible light as there may be some advantages with clear air forward scatter propagation by going to lower near optical frequencies. At least IR beams are not visible to the casual viewer, so are less likely to create problems.  However, IR can be more dangerous than visible light because the blink reaction that occurs with visible light does not happen. When using high power IR LEDs it is therefore even more important not to look into the beam and to take care where the beam is aimed, especially at close range. Never ever look into the beam at close range.

A suitable IR PIN diode is the SRH203-FA from Osram, widely available on eBay.  As you can see from the image, the PIN diode detector has IR filtering (black colour) , so may be usable in daylight with less degradation than would be the case with red LEDs.  I already use the SRH213 PIN diode for visible red optical comms.

I am looking for a suitable 1W-3W IR LED and they are available, but I have yet to find one in the 10mm "fat" standard LED package that I use currently on 481THz.

There are 3W IR LEDs available from China that should be suitable. See eBay item 370784927290 for example in the star package. This would be some 10dB more output than I currently use, although I am not sure of the beam divergence which may be greater than with the 10mm package.

Nanowave over-the-horizon experiments are very much like microwaves, except that the test equipment is a GREAT deal simpler!

Clear air forward scatter optical kit

The schematics below are of the RX and TX used in the recent "over the horizon" NLOS 481THz optical tests. As you can see, not too much complexity involved considering the useful science that results. All parts are inexpensive and readily available. Nothing critical apart from the PIN photodiode and the 280000mcd red LED which I got from Hong Kong via eBay. 100mm optics used at the TX and RX ends.

The 481THz receiver

The 481THz beacon transmitter

8.5km "over the horizon" on 481THz tonight

This evening I ventured further afield with the optical kit to see if I could copy my QRSS3 beacon signal in the village of Stow-cum-Quy which is 8.5km from home . This is non line-of-sight and nearly twice as far as my previous tests last week.

Path covered this evening NLOS

Success! After quite careful searching both horizontally and vertically, the signal was copied and recorded at around 14dB S/N (at best) pointing just above the horizon. After searching for a stronger signal, the sky became increasingly "murky" and I was unable to find the signal again, so I came back home.

1W LED TX in 100mm optics, SFH213 detector in 100mm optics

This range is about the practical limit: aiming is very difficult and I find it is hard to get back to the same direction/elevation if I move anything. I need a tripod that is far more stable with some sort of degrees marking (both horizontally and vertically) so that I can go back to the best settings with confidence. At the moment it is a bit "suck it and see" to find the best aim and nearly impossible to get back to those settings easily if the tripod gets knocked.

This was clear air scattering as there was hardly a cloud in the sky. I have still to try cloud-bounce with real low level clouds.

To see the signal play this recording through Spectran with it set to 572Hz in 0.34Hz bandwidth. QRSS3 signal will be clearly visible.

There is a chance that I may try a much longer 27km NLOS path before too long. Looking at the map, this should be a possibility but I may need another 6-10dB from a Phlatlight LED and a MUCH more stable tripod!

Amazing NLOS 481THz reception tonight!

This morning I realised that I was 2.5 degrees out in my aim last night with my optical beacon, so this evening I repeated the over-the-horizon optical test, moving instead to a new location fully in the beam at a distance of 4.8km. The difference was astounding! Last night I was struggling to copy the signal even with a long carrier. Tonight it would probably have been audible in 10wpm CW!

Excellent NLOS 481THz copy this evening

I ran QRSS3 tonight and was rewarded with signals 20dB over the noise in 0.34Hz bandwidth when beaming just above the horizon. At higher elevations the signal was weaker. With this strength I am very confident that with QRSS3 the signal should be copyable non line-of-sight (NLOS) out to at least 10km by forward scatter and possibly much further, even with my 1W LED running at 250mA in 100mm optics.  This was an excellent result and I am now confident the RX is working very well indeed.  Cloudbounce is another mode I have yet to properly exploit. The reception tonight was as a result of clear air scattering presumably from water droplets or dust particles.

This time I made recordings which are linked from my website. These can be played back through Spectran or Spectrum Lab software and further analysed. See https://sites.google.com/site/g3xbmqrp3/vuhf/optical/481thz-nlos .

Rapidly I am learning the absolute importance of accurate aim: 2.5 degrees out last night probably reduced the signal level by around 20dB. For serious tests over longer ranges a means of very accurately aligning the TX and RX is essential. Unlike with line-of-sight where the red dot can be seen by eye usually, NLOS requires careful alignment from map features.

Successful over the horizon 481THz test at 4.6km

This evening, in the freezing cold, I attempted a non line-of-sight (NLOS) optical test using my beacon TX (1W 10mm LED at 250mA in 100mm optics) at home in the bedroom firing out through the double glazed bedroom window and my new improved receiver (SFH213 detector in 100mm optics) at a new test site 4.6km away towards Cambridge. The path is obstructed by rising ground some 10m above the TX and RX height around mid path.

481THz signal (top line) at 4.6km over the horizon tonight

This time, I used a lower TX subcarrier tone frequency of 572.3Hz and a continuous carrier transmission so that aiming would be easier (!) than with CW or QRSS3. I am pleased to report successful reception of the signal in 84mHz bandwidth using Spectran, but aim was extremely critical and the signal was not solid, probably because of  aiming issues and possibly changing sky conditions.

When I got home I checked the frequency to make absolutely sure that this was my carrier, which it was beyond any doubt. The signal was totally inaudible in the headphones and there was no sign whatsoever of the red beam in the sky. The most probable propagation was by scatter off dust particles in the atmosphere.

This was MUCH harder than I expected: I honestly expected to hear quite reasonable signals by ear, but as the plot shows, the signal was only just copyable in 84mHz. When I received my signal NLOS last year at 3.5km on a different path signals were 10dB over noise in a wider bandwidth on QRSS3.

At the moment I am still aiming the TX and RX as low as possible on the horizon, but I need to try greater elevations to see if this gives better to worse results over this sort of distance.

Another 6-10dB ERP from the Phlatlight LEDs (still to be fired up) would certainly help.