Next field test - optical?

After my very disappointing results on VLF earth-mode yesterday - I must get to the bottom of why results were quite so bad - I think my next test will be at the other end of the spectrum. We have a windmill very close so I may try bouncing optical signals off this to test my optical gear is working still.   It is a long time since I have done any optical experiments.This will not be too far initially, more a test of the gear before venturing any distance.

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.

Phlatlights

When I have repeated my over-the-horizon NLOS optical (red LED) tests at 481THz using LEDs bought from Hong Kong, I must remember to try my Phlatlights which are about 10dB stronger.  I bought these nearly 2 years ago and have not yet fired these up! The ones I have already have heatsinks so it should be quite easy to use these for QRSS3 tests. In theory it should mean aiming a little less critical or greater range when "on-beam".

At 8.7km over the horizon with a clear sky, QRSS3 signals were about 10dB S/N with the 10mm HK LEDs, so with Phlatights and the same optics (100mm lenses) signals should be much stronger. More tests for when I am fitter.

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!

481THz QRSS3 beaconing across the garden

Last night I fired up my 481THz optical kit with poor results: I think the RX unit may have a flat PP3 battery? On the Spectran display the QRSS3 signal bounced off trees and bushes should be very easy to spot, but for an unknown reason, I was struggling.  I shall recheck all the kit today. The kit has to be well focused and working well to work DX non line of sight (NLOS), which is my plan soon.

My stroke symptoms (poor balance and clumsiness) are really not helping: tasks that were very easy last year are very hard this, well at present they are.  Before my wife and I venture out into the dark fens, I want there to be a good chance of success, which means good and thorough preparation.

481THz QRSS3 beacon

This afternoon I dug out my optical QRSS3 beacon, PSU and optics. In the coming days I intend to test this locally from my shack by scatter off local walls and trees with the RX head, 100mm optics and Spectran software in a different room.

If all is working well, I shall ask my wife to drive me to an NLOS (over the horizon) location at 6.18km to see if the signal can be received. This is the first optical test in many months.

At the moment I am struggling with quite simple stuff because of my stroke. I am still so clumsy.

Optical Comms in Vancouver Canada

This morning I received an email from Steve, VE7SL, reporting on his experiments with VE7CA over a 2-way distance of 54km using red LEDs.  They are doing really well and Steve is writing an article for a Canadian journal. They seem to have been encouraged by my own modest efforts,which I hope to restart before too long. At present my poor fitness and temporary inability to drive because of the stroke are real handicaps.

Image of optical path spanned from VE7SL

G4HJW "Finningley" Optical Transceiver kit

The G4HJW optical transceiver kit

Bernie G4HJW is well known for his innovative work on microwaves and optical comms. Last year he designed a neat, part SMA, optical transceiver kit to build at the Finningley round table meeting. The transceiver is capable of very good results and several (around 70) have been built and used very successfully. I believe the best DX QSOs at 481THz have been over 60km using these in 100mm optics.

Exactly when I'll get my kit built I'm not sure but it will be good to have one of these available for optical line-of-sight tests in the autumn. Once assembled, I shall be looking for some 2-way optical QSOs beyond the 10km speech contact I achieved with my own kit last summer. In East Anglia, the issue is finding some hills to allow long line-of-sight paths.

Bernie is, I understand, considering putting together a further batch of these kits, which make a good introduction to nanowave communications. In addition to these electrical kits, all that is needed is a microphone, headset and some simple optics than can be built for a few pounds.

See http://www.earf.co.uk/nanotrx.htm for more details

7 mile optical daytime QSO by Bernie G4HJW and Jenny G0VQH

G4HJW and G0VQH enjoyed a successful 7 mile optical frequency QSO during daylight when in Scotland recently. Bernie also worked GS3PYE/P. There is a video of the QSO recorded by the good folks at Cam Hams. The QSO used the now famous Finningley optical transceiver designed by Bernie last year in 100mm (drain pipe!) optics. They were going to try an infra-red QSO too but as rain set in they abandoned the attempt.

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.

Optimised 481THz receiver

As the weather was miserable here today, I decided to work on the K3PGP derived 481THz optical receiver this morning. I wanted to ensure it was working at maximum sensitivity with subcarrier tones around 100-1500Hz. With some adjustment of the coupling capacitors and the addition of an HF roll-off capacitor on the collector of T2, the sensitivity now appears to be excellent. My test is an AF modulated red LED on the ceiling of my building shack, in almost total darkness, with the test receiver (less any lenses) on the bench about 1.5m away. If the receiver is working credibly then the tone can be heard in the headphones when the LED is barely lit. In the case of this latest receiver I cannot even see the LED lit at all in a darkened room, yet the tone is audible in the receiver. When comparing this with my previous best optical receiver there is around (guess) 6dB more sensitivity. This design uses "blog standard" components: nothing selected for low noise, and not an IC in sight. I am now waiting for some decent weather - and no snow please - to test this on my NLOS test path.

K3PGP receiver for 481THz

One of the simplest, yet highly sensitive, receivers for optical communications is one designed by John K3PGP. The G8CYW design in Practical Wireless in the March and April 2013 editions is based on this. This works really well in darkness, but is easily overloaded in any light. Many circuits are optimised for speech communications but this one works really well with digital modulation at very low frequencies where the detector sensitivity is highest.

http://k3pgp.org/Construction/Frontend/preamp.gif

Today I've been building a version and I am in the process of optimising it for subcarrier frequencies below 500Hz. I have added an extra transistor gain stage after the basic K3PGP design but want to see if lower noise FETs and transistors make a worthwhile improvement. When satisfied that it is working optimally I shall be trying this at the RX end of my over-the-horizon tests. I am quite excited about trying much lower frequencies in the next test using QRSS3 and continuous carrier.

New 481THz QRSS3 beacon ready for action

Completed 481THz beacon electronics

This afternoon I finished off the electronics build of the new dual frequency optical beacon TX for 481THz (red light) over-the-horizon (NLOS) tests. In the end I opted for QRSS3 on a choice of 2 sub-carrier frequencies selected by toggle switch together with the option of a continuous sub-carrier transmission on either frequency to help beam alignment. I have not incorporated an FSK facility at present. All that remains is to align my sighting scope with the optics so that I can use this to help with aiming. Currently the beam appears off-centre in the spotting scope cross-hairs.

As I now have the option of a lower sub-carrier frequency, I need to revisit the improved RX to see if I can better optimise sensitivity at the lower frequency. Theoretically the detector should be several dB more sensitive at a lower sub-carrier frequency which should help with NLOS tests where signals are weak.

The beacon is powered by a 19.5V 4.5A ex-Dell laptop SMPSU, although I only need around 300-400mA with the current LED, which is exceedingly bright.  In the picture above you can see the BACK of the LED and that is pretty bright. At least with the PSU the whole beacon is self-contained.