Early lightbeam tests

Waverley Rd, Kingsbridge, Devon, UK

My interest in light-beam communication goes right back to the 1960s when an old school friend and I used a torch bulb as the TX and an OC71 transistor with the black paint scraped off as the RX detector.

One of our first tests was across the street shown in this photo. I lived in the first house on the RHS and we received the test in the house with bay windows on the left. DX was about 25m! We AM modulated the torch bulb biasing it at half rail if memory serves me correctly. It had a very low frequency response as the torch bulb had a poor HF response, as I recall.

We moved to this house in 1959 but the photo is years and years earlier.

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 .

50MHz, 432MHz and 481THz this evening

Well, this was an interesting evening. Just after teatime I noticed 50MHz was open with some strong Es signals from Europe. A few stations were worked on QRP SSB using the newly erected halo antenna (S52NR, S53OQ and OE5FIN). Heard, but not worked, on 50MHz were CT8/K0RUI in HM68 (Azores Is) and 4X4DK in KM72. Both DX stations were good solid signals on the halo.

Stations worked on 432MHz this evening (5W/4el)

Later I went out portable with my FT817 (5W) and my small "coat hanger" 4 element yagi for the RSGB 432MHz activity contest and worked 14 stations in just over an hour before closing as it was dark and I could not see the logbook very well. Best DX 155km although I did hear, but not work, F8BRK in IN99.

Finally, for the very first time, I have detected GB3CAM on 481THz (red light beacon) at a distance of 32km from the Nine Mile Hill site where I was active in the UHF contest. Signal was weak with lots of scintillation, no doubt caused by the rain that started up just as I began listening with my 100mm optical receiver (KA7OEI/K3PGP hybrid). If I can find another 6dB sensitivity I think the signal will be solid in low haze/clear conditions over this LOS path. GB3CAM is a very low power light source, so copying it is a big challenge.

AM lightbeaming down the street

With my wife manning the TX this afternoon and pointing it out of the landing window we managed a daylight speech test today with 100mm optics at both ends at 0.3km using the v.simple AM lightbeam TX described in the previous post. The LED was run with an 80mA standing current. Biggest problem was aligning the TX beam and the lack of talkback, but I managed to get 59+ copy of my wife saying, "I'm bored with this. Come home and have some tea" when I was aligned on the TX beam.

There is no doubt the simple circuit can be refined to improve audio quality when clipping, but it works. The RX is far more sensitive at night too.  I am now finding a way of replacing the XYL for these tests by using another source of speech such as a radio receiver or MP3 player. She is good at cooking and other things, but not at doing optical tests!

Later this week I'll begin experiments with a simple single LED AM 481THz transceiver.

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.

Further 481THz G4HJW reception "over the horizon"

G4HJW received on 481THz over an 8.63km non line-of-sight path

Bernie G4HJW was 20dB S/N in 0.67Hz bandwidth earlier this evening at my local test site 8.63km NLOS from his QTH "over the horizon". Bernie was using his Phlatlight beacon with 1.082kHz tone. The sky was clear and starlit, so this was clear air scattering not cloudbounce.  About 15 minutes after I first copied him it seemed to get more hazy and I was struggling to find his signal at all.  Best results appear to be with a clear sky and aiming at the horizon. 

On 3 attempts copying G4HJW's optical signal it has been (1) 30dB S/N, (2) no copy and (3) today 20dB S/N in 0.67Hz bandwidth over an 8.63km NLOS path. I am still using the BPW34 detector in 100mm optics.

Getting Going on "Bottom Band"

Well, guess what, I am a year younger (amateur radio wise) than I thought. Having looked up some old RSGB Bulletins this evening I see that I actually joined the RSGB in 1963 and not 1962, so I've only been a member for the last 49 years. Suddenly I feel a year younger!

I was looking for an article about lightbeam communications that was in the very first RSGB Bulletin (later Radcom) I ever received and this was in April 1963 and not 1962. It was called "Getting Going on Bottom Band". The article was an April Fool's one but I didn't realise this and got the reference books from the library. But, the fascination with communicating a long way over a beam of light was kindled in that very article and only now am I giving the subject proper attention, some 49 (and not 50) years later. I feel as if I have a whole year extra in which to enjoy the hobby, HI.

SpectrumView for iPod/iPhone

SpectrumView 0-24kHz displays of waterfall and spectrum

Looking around for useful (free) software to help me with VLF and lightbeam tests I came across SpectrumView yesterday. This is a fabulous application for the iPhone, iPod Touch and iPad which provides a 0-24kHz spectrum analyser and a waterfall display.  The app is produced by Oxford Wave Research.

To make best use of this I need to connect an external audio input into my iPod Touch 4g.  A 4-pin jack plug when inserted, rather than a 3-pin one, allows an external audio input (or a mic) to be connected so I can feed my VLF receivers, used for both 8.7-9.1kHz earth-mode comms and for lightbeam baseband/sub-carrier detection, directly into the iPod.  Resolution will be down to around 2Hz at best, so this will be of somewhat limited use, but certainly easier than a laptop for quick field tests or quantitative measurements.

The mic input (on a 4-pin jack) is on pin 4 (nearest the plug cover) and the ground is the next one down the connector (pin 3). The tip (pin 1) and pin next to the tip (pin 2) are the two audio outputs for a headset.


UPDATE 16.3.12: to tell the iPod Touch 4g that an external mic is connected you seem to need to have a 4k7 resistor to ground across the terminals. See later post for details.

481THz progress update

TX beacon optics

This morning I bought a length of 110mm waste pipe from the local Plumb Centre shop. It came in a 3m length and only just fitted in the car to get it home! After lunch I created my first attempt at some optics inserting my Poundland 100mm lens into the end of a cut section of the pipe. At the focal length I mounted my 10 pence high brightness, prefocussed, LED beacon with Bluetac. With this (crude) set-up on an old tripod and with a taped on gun-sight to help with aim, I set the TX baseband beacon running and pointed it from the stairway through a double glazed window aiming down the road. With my handheld baseband head using just the high brightness 10p LED as the detector I walked down the road to see how far I could get. In the beam in daylight I could detect the signal at the receiver at 120m. With a magnifying glass in front of the RX LED signals at 200m were strong (could not test at greater range), but it was difficult to keep the RX aligned. The red LED was quite strong visibly at the 200m test point even though running just 15mA or so through it. The double glazed window no doubt added a small amount of attenuation. What I'm not sure about is how good (or bad) this result is. Clearly with a really high brightness Golden Dragon LED the range even with this system would be considerably further.

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.

111km optical QSO in Yorkshire

G8AGN/P and G0EWN/P achieved a 111km speech QSO on optical communications using 0.5W red LEDs and Fresnel lenses on April 6th 2011. Brilliant stuff.

Lightbeam communications

Before I became a radio ham, a friend and I had our first phone wireless QSO using a small torch bulb modulated with a couple of germanium transistors. The DX was across the road, about 20m at most. At the far end the receiver was based on an OC71 with its black coating scraped off, which made quite an effective photo transistor. These days there are much better ways. For some examples, see some of the fascinating links at  http://www.carolinaflashers.org/ . One of these days I must revisit optical communications. The picture on the right is from http://www.laud.no/ww2/lispr/index.htm and shows a WW2 lightbeam communications device used by the German Army.