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.

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

Optical mirages - NOT amateur radio

These have fascinated me for years. When I first created this page many years ago there was very little on optical mirages on the internet, but now there are lots of resources.

There are some indications that some extreme amateur optical DX may be influenced by optical refraction. The most extreme "over the horizon" (NLOS) amateur optical DX is by cloud bounce when signals are reflected off the base of clouds.

Under very favourable conditions places way beyond the normal horizon become visible. This is most common in conditions when warm(er) air passes over very cold water. The common mirages seen over roads, that look like puddles are just the opposite (hot roads and cooler air above).

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

Optical communications

Optical RX

Until my stroke in 2013, one of my joys was building and testing optical communications gear. Although I had a successful line-of site speech contact, my "thing" was weak signal communication over the horizon.

Even without any visible signals in the sky I was able to detect my beacon from home on 481THz (red optical) way over the horizon, even though it was firing through double glazing. Had I been able to carry on, there were so many things I wanted to try including higher powerLEDs, further distances and a better tripod.

Sadly, these days I am too wobbly to do this portable work. As all the gear can be built and tested in the "back of a garage" with simple test gear, this could appeal to those interested in serious, but low cost, experimentation. Over the horizon optical work could do with attention.

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

Optical RX improvements

G8CYW Optical head (April 2011 RadCom)

I've now bought some NE5534 low noise op-amps so can implement the full G8CYW optical head circuit which many users have successfully adopted. Stuart describes this as "exquisitely sensitive". I have BPW34 PIN diodes but think the sensitivity should be similar to that with the SFH2030 diodes used in Stuart's design.

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.

Nanowave over the horizon tests - nearly ready

Original optical beacon

Just about ready now to restart my optical non line-of-sight tests again using my 481THz CW/QRSS beacon and a choice of 2 sensitive receivers. I'd intended to go out this evening but (a) the XYL needed the car to go to her choir and (b) the fog has descended!

Optical beacon capable of continuous subcarrier (choice of freqs) or QRSS/CW

SpectrumView in action - it's brilliant

The beacon design needs to change somewhat: currently I can TX either QRSS3, QRSS30 or CW at around 830Hz subcarrier, using the IRF640 fed directly from my K1EL keyer IC. However, I cannot actually send just a 830Hz tone continuously. This would actually be very useful when aligning TX and RX over a non-optical path e.g over a hill. So, when the grandchildren go home on Sunday I'll make a modified version of the beacon keyer so that I can choose a number of subcarrier frequencies, including some lower than 830Hz (detector sensitivity is greater at lower audio frequencies) and also add the option of a continuous subcarrier signal. This will involve adding a 4060 based oscillator and divider to be keyed by the K1EL keyer IC. So, it may be next Tuesday or Wednesday before I actually start testing again.

I also want to optimise the use of the iPod Touch 4g as a handheld audio spectrum analyser. I have used it for this sort of test before using an excellent package available free called SpectrumView (see screenshot) available from Oxford Wave Research. With a laptop PC, running Spectran, the problem is the brightness of the screen which emits an interfering optical signal. With a tiny iPod Touch it is much less bright and can be held in the hand. Unfortunately the bandwidth can't be screwed down as narrow as with Spectran on the PC, so there will be S/N limitations. One test will be to see how far away I can detect my beacon (about 0.5W into the LED) over the horizon using just the optical receiver and the iPod Touch.

The beauty of 481THz (red light) work is the kit is simple: everything that matters is at audio frequencies and can be built and tested with the simplest of test gear.

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

Optical kit

One of my Facebook memories today is of testing my 481THz optical kit in the garden of the old QTH. In those days I was fit and well. These days, field work has proved impossible although I still hope to restart one day. The photo is here as a reminder (to me!).  And, no, I was not looking at that attractive girl up the street!

All the optical kit was homemade and easy to assemble and test in the back of the garage with very basic test gear. Even the software used was free. Over the horizon testing with optical kit was challenging and great fun. Results amazed me.

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

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

481 THz tests

I have resurrected my optical gear. It had stood largely unused for 10 years.

As yet, I have not got Spectran working with the optical RX so I can only do optical tests with continuous carrier or CW and not QRSS. Tests so far are a reflected (off a wall) signal on CW. Good copy over a reflected path of just 100m. At least the optical gear works again!

UPDATE 2020z:  Expections were well exceeded. I set up my 481 THz beacon firing out of our lounge double-glazed window at a wall of a house at the very far end of our close. I walked with my RX to the other end. So the path distance was about 0.25km with one reflection. Signals were S9+ even by ear. The gear really works well. I must get Spectran working and try some NLOS tests. with G4HJW. I suspect we will have greater success with cloudbounce. My main issues are exhaustion and giddiness.

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

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.

"Over the horizon" optical experiments

Since 2013, my health has precluded proper field experiments at 481THz or thereabouts (optical). I am just too wobbly. I tried once, but I just could not manage it sadly.

One of the things I enjoyed was experimenting with weak signal optical signalling over the horizon. My own tests used QRSS (slow CW) received on a PC . There was absolutely no sign of the optical signal in the sky, yet there was a clear trace visible on the PC. 

Had I been well, these experiments would have continued and far greater ranges would have been possible.

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

Optical communications

G4HJW's over the horizon 

(9km disatan) optical signal

Before my 2013 stroke, I really enjoyed my experiments out in the field with homebrew optical kit. Everything was home made: the electronics, the optics and even some of the test gear. This really is an area that is ideal for simple homebrewing. Sadly, I am too wobbly to do field work these days.  Truthfully, all the gear can be made in the "back of the garage" (mine was!) and it is easier than microwaves as all you need is a scope, multimeter and a weak LED source.

My "specialisation" was over the horizon weak optical signals. I managed to reach 8.1km non line-of-sight using a 481THz red LED with QRSS3. Nothing at all could be seen in the sky, yet there was perfect decoding on the PC. Had I been fit, I am sure much further would have been possible. As there were no clouds, I think this was clear air scattering.

What was needed was a much better tripod which was more stable and could be repeatably set in the right direction. My cheap tripod was wobbly and far from suitable. Accurate alignment is important. Once aligned there was the signal.

G4HJW's signal was way above the noise at 9km non line-of-sight (see photo above).

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

More optical tests looking for elusive GB3CAM

This evening, thinking it would be a good opportunity to look for the GB3CAM optical beacon (yet again) I took the trip to nearby Nine Mile Hill (32km from the beacon) with the kit recently used to detect G4HJW's Phatlight beacon over the horizon at 8.6km. My RX is now pretty sensitive. However, I failed yet again to detect anything of GB3CAM. Actually it was a little hazier than my first tests (when I had sensitivity issues) but I was disappointed not to detect a thing. All I managed was a chat with the farmer in whose field gate I'd set up my optical kit and PC: he seemed happy with my explanation.

I've asked the beacon keeper if the beacon was actually on-air - it could have been off I guess. 

My other question of Bernie G4HJW, the beacon keeper, is what the ERP of the GB3CAM beacon is compared with his phlatlight beacon at home that I'm consistently able to copy at 20-30dB S/N in 0.17 - 0.67Hz bandwidths non line-of-sight, whereas I've so far been unsuccessful receiving the Wyton optical beacon line-of-sight. At 32km haze will matter more and it may just be that on the occasions I've tried I've either had insensitive kit or poor optical conditions. I did try very carefully scanning the horizon this evening after sundown for a good 15 minutes without detecting a thing and I had a very good idea where to aim.

241GHz or optical?

Although the world record for DX on the highest microwave band (241GHz) allocated to radio amateurs is measured in several tens of km, the UK record is quite modest.

My own view is 481THz optical is much easier: test gear is trivial and all the gear can be made inexpensively by nearly anyone. Microwave gear is much harder. I know people say you can "do microwaves" with minimum test gear, but I think you will find most who are successful are, or were, in the business professionally and have access to good microwave test gear. With optical gear this is definitely not the case. Before my stroke I built all my optical gear in the back of a garage and I achieved very useful results. Others have achieved far more than me. All you need are a scope, a multimeter and an audio generator. Much of the test gear needed for optical work is freely available software on the internet.

See http://www.microwavers.org/?241ghz.htm .

481THz (light) - first test results

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.

For the receiver I built the first stage of the optical head described in the RadCom articles (March-May 2011) using an identical  red LED (reverse biased) as the detector. I also tried a BPW34 detector, but it was not that different. This was followed by a couple common emitter transistor stages using my ubiquitous 2N3904s feeding a crystal earpiece. With the TX "beacon" running from a 9V battery and aiming out of my garage I walked across the road with the receiver and a 4 inch magnifying glass. Across the road, at about 25m range, this produced a quite respectable signal as long as the magnifying glass was focussed onto the RX LED. This was all a bit "Heath-Robinson" as I had to hold everything in my hand and move things around to get it spot on. There was quite a bit of interference from the street lights nearby.

I have no idea how much "antenna" gain there is in the built-in lens on the TX LED (a few dB?) or with the magnifying glass on the RX but with this set-up correctly aligned I would think 100m range is certainly possible.  This is just the beginning of a series of tests, but I am already happy that the optical head is sensitive and that good, well aligned optics will be essential to get decent distances. More TX power is easily available by using a power LED.

Next stages are:

• A better beacon TX capable of operating at higher power on both baseband and subcarrier frequencies.
• Putting the optical head into a screened enclosure, even if a temporary one.
• Starting to think about optics. Using the same LED on both RX and TX will save on optics as just one set is needed at each end of the link.

481THz optical treebounce and skyscatter

Optical treebounce (0.3km total path) 10wpm CW 0.501kHz 35dB S/N

This evening I did some further tests with optical treebounce and what I'd loosely call skyscatter. The sky was cloudless, but a little hazy. My test kit was as follows:  TX (250mA high brightness red 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.

Weak QRSS60 signal received by aiming at clear sky

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 non line-of-sight optical signal to G6ALB in the next village 3km away. When the weather improves I'll go out /P with the PC and RX and see if I can detect the "forward" scattered optical baseband signal at much greater range (1-3km). With proper cloudbounce it should be better I think.

This reference looks like it should be interesting (about scattering) http://en.wikipedia.org/wiki/Rayleigh_scattering

Optical setback (of sorts)

RX set-up at Nine Mile Hill on 481THz tonight

Flushed with my local successes (0.4km) last night, I decided to have a go at looking for the GB3CAM optical beacon 32km away from a local high spot where I know others have copied it sending its 1/15kHz FSK sub-carrier data at 481THz (red light). I set up in the daylight so I knew where to aim approximately and had the PC ready with SAQ receiver (0-22kHz SDR) and Spectran all ready. After dark I started to seriously hunt for the elusive signal    .....but with no success.

Failure can be put down to several possible causes:

1. Difficulty in aiming. Although I knew where to aim, I am uncertain that my spotting scope is correctly aligned on the 100mm optical head and that there is no parallax error. Looking for a very weak signal is therefore hard to start with.
2. Inadequate sensitivity in the optical head. Although I could hear the 50Hz buzz from every street light and house light locally, maybe there is another 6-12dB to be had in basic sensitivity in the optical head.
3. Inadequate volume on the recovered baseband audio. I was monitoring the "live" feed with just a crystal earpiece attached to the head and, although this is fine at home and down a quiet street, it was too low a level to hear weak signals over the traffic noise from the main road which adjoined the field gateway at the RX location.
4. Path loss. According to G4HJW, the beacon keeper, the signal varies from 5-30dB over noise at this location, assuming one is aligned correctly. Although the weather was cloudy and good visibility there was drizzle that came in as I was testing.
5. QRM from car headlights. As cars came along the main road near the test site, I got quite bad buzz from their headlights.

So, not to be beaten, I will do things to address all these problems and try again quite soon. I think I need a better local test beacon that I can use to (a) check aim, check RX sensitivity and adequacy of the recovered audio.

Commercial optical communications

Many years ago, before my stroke, I did some work at 481THz, red light. I used LEDs as the TX element, but I see that there are some commercial optical links using LASERS. Light has the advantage of being very secure. I expect these commercial units are expensive. My own optical kit was very inexpensive and great fun.

At some point I hope to get back to this work. Amateur optical work is both fun and inexpensive. It is certainly at the frontier of experimentation.

See http://www.lightpointe.com/laser-radios.html .
See https://sites.google.com/site/g3xbmqrp3/optical .

May Projects

All being well I have a window of a few weeks before visiting grandchildren and the like, so I have a couple of new aims:

(1) Convert the 10m halo to a dual band 10m/6m halo
This should be a fairly easy task as I already have dimensions for the 6m halo and I've air tested it in the loft. All that should be necessary is to take down the 10m halo and change the feed and support arrangements so both the 10m and 6m halos are fed from the same coax. The pair of halos well clear in the air should prove an effective QRP antenna system for my 2 favourite bands.

(2) Complete my first optical speech transceiver
Strange though this is, I have made several quite sensitive optical receivers and a couple of reasonable power QRSS3 and CW optical beacons, but I've still not got around to making a complete optical speech transceiver. Locals hereabouts use baseband audio for TX, so this is what I should do if I want to work people like Bernie G4HJW. It would be good to get a few line-of-sight short range (up to around 10-15km) 481THz QSOs in the log-book soon. Based on my optical work so far this is not a difficult task, just one I need to crack on with.

Target for both projects is within 2 weeks i.e. by May 17th. We'll see.