Evening light - NOT amateur radio

In the UK, the days have extended by almost an hour in the evenings since the shortest day. Even in the mornings things are getting better. 

It will be a few months before spring really comes, but the light has changed for the better. We may yet have snow and ice, but it is as if a corner has been turned. 

Optical Communications

It is some years (before my stroke in 2013) that I last tried seriously with optical communications using red LEDs that I bought at low cost via eBay. These days, I expect a google will bring up lots of links. This is a fun branch of our hobby, with very low costs and lots of homebrewing. This is a branch of the hobby I'd like to try again. There is much to learn.

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

Lightwave Comms

Because of my poor health I have not been able to pursue 481THz (lightwave) experiments especially "over the horizon" NLOS tests. Thankfully, others are "carrying the torch" .

See http://qsl.net/ve7sl/westcoast%20lightwave%20project.pdf

481THz experiment - abject failure

Well, I am sorry to report that my 481THz lightbeam experiment was a total failure.

Although I could detect my QRSS3 signal strongly across the house bouncing off the windmill I was unable to detect any sign of the signal at my remote site. I had problems with aiming which did not help, but I was not expecting the total exhaustion this test brought on. In days gone by this would have been easy, but even soldering a headphone jack tired me. Doing the actual test left me totally "done in".  I really should repeat the test with better aiming, but I am not ready in my body.

If one thing this has taught me it is that I am still too feeble for field work. It was a great disappointment, but there will be other opportunities.

In past years I loved doing this sort of experiment. Sadly, my body is telling me, "not yet".

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 .

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.

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.

481THz AM transmitter

This morning I took a step further towards making a complete 481THz lightbeam transceiver by breadboarding and testing a simple AM transmitter for use with a high brightness red LED. The circuit could hardly be simpler but works very well. The bias on TR2 is adjusted to give around mid-rail so the LED has a standing bias before audio is applied. Listening (without optics) on my modified KA7OEI receiver head and looking on a scope, the modulation looks good with around 150mA current into the LED.

Failed cloudbounce 481THz optical test (7.6km)

Possible weak 481THz reception at 7.6km NLOS

This evening I tried a 7.6km non line-of-sight (NLOS) path between Burwell and Newmarket Heath with the 0.7W input red LED beacon running QRSS3 on an 820Hz subcarrier (at home) and modified KA7OEI receiver at the RX end, both with 100mm optics. My PC was running Spectran.

The best I can say is reception was very doubtful:  the screen capture above shows a signal at the correct subcarrier frequency in the right direction, but too weak to identify CW characters. The recent 3.6km NLOS test produced good 10dB S/N signals in 0.67Hz bandwidth - much easier.

With a largely clear sky, few clouds and a slight haze, 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.

481THz update: 1.6km test

Today I did my first test beyond the end of my street and in daylight.  I set up my 1.082kHz subcarrier CW beacon and 100mm optics 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 I have from home.

Then I went up to the Devil's Dyke 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 about 50m along the path. 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.  1.6km is my best distance so far. I'm using a BPW34 detector with some reverse bias with the PIN diode's anode connected directly to the FET gate in a KA7OEI optical head. This feeds into a feedback biased common emitter stage into a crystal earpiece. Recovered audio was a bit low in the wind.

Some progress in the right direction.

Lightbeam receiver progress

Today I did the part of projects I hate most - drilling holes in aluminum boxes. I can never find the right size drill bits, they are blunt and so on. Despite all this, I managed to get a small metal box complete with BNC socket, phone jack and switch mounted on the end of my optics pipe and carried out some first tentative tests. I am using part of a simplified version 3 KA7OEI optical head based around a BPW34 PIN photo diode. In my case there are no op-amps with discrete transistors used for the amplifiers and buffers following the FET cascode stage.

In the next day or so I want to add the sighting scope and find a way of attaching the whole optical unit onto a stable tripod. At this point and can try some more serious tests for example looking for my own 1kHz beacon at greater distance and, hopefully, GB3CAM at 22km if the system works OK.

The output of the head can feed either a crystal earpiece (to listen to the baseband tones or speech received) or into a PC so that it can be used with an SDR to copy various modes up to around 22kHz.

481THz (light communication) progress

This afternoon I did a bit more on the optical front. I repeated my outdoor range tests using a low-cost high brightness LED as both a baseband optical transmitter and receive detector. With around 10mA TX current into the LED I was able to copy a 1kHz tone at 20m with another high brightness LED as the detector, without any additional optics other than the LED's built-in lenses at each end.

A 100mm lens properly focussed has a gain of >24dB (nearer 30dB if correctly adjusted). Assume 24dB "antenna" gain at each end of the link and we will have 48dB system gain over the simple LEDs on their own. Based on these calculations my optical transceiver should have a range of at least 5km.  If the gain of the lens is 30dB then the range could be as great as 20km. This is without using high power LEDs as the TX or using larger Fresnel lenses which would have even higher gain.

I now have the 100mm lenses I intend to use as well as 2 gun sights bought off eBay. The next step is to buy some drain pipe to house the optics. I have still to decide whether to build fully self-contained FM transceivers (simpler) or to build the optical transceive heads with separate transverters to use with the FT817.

More 481THz lightbeam progress

670nm receive head and converter to 80m

Today I did some further light beam experiments, this time using a 25kHz modulation signal on the light beam and receiving the signal on my FT817 with this head/converter unit above. It consists of a BPW34 photo-detector feeding a cascode FET/transistor stage into an emitter follower and SBL1 mixer to convert the signal to around 3.584MHz. With a current into the TX LED of just 10uA (a very dim glow from the red LED) the signal was 20dB S/N in 0.67Hz bandwidth on Spectran at a distance of 25cms without optics. If my calculations are correct, this means a range of around 100m could be obtained even with this miniscule power if 100mm lenses were added at each end to give some 27dB gain at each end. Using the same TX LED at 10mA (1000 times the current) then the range is already in the many km region, and this is without using power LEDs. This is encouraging progress. Tomorrow I want to repeat the test with the same LED as the TX as the detector.