Aiming high

This evening I caused a few curious looks on our road as I adjusted the optics on my latest 481THz beacon in the dark.  In order to carefully align the powerful red beam, it is important to ensure the cross hairs on the gun sight telescope used for aiming are precisely aligned with the tightly focused red beam.  I beamed onto the rear of a distant car and made some adjustments. As I was doing this, someone walked down the road with a dog and stood, puzzled, at why this car number plate was glowing red very brightly!  The beam was aiming slightly too high, but after adjustment is now precisely in the centre of the cross hairs, so next week, weather permitting, it will be time to try again at the NLOS test.

Trying to see the light (very dimly)

As other nanowave enthusiasts have told me, it is very useful when experimenting with light beams at 481THz to have some sort of lab test range that allows repeatable sensitivity measurements. So, today I rigged one up.

Up on the ceiling of my "lab" I've semi-permanently fixed a standard brightness LED fed via a 1K resistor with a 1kHz tone from my 0-1MHz audio/LF generator. I can now adjust the output until the LED is only just visible to the naked eye, with difficulty,  when the room is totally dark. I then place my optical heads (without lenses) on the same spot on the workbench aiming upwards to the very very dim light. The spacing is about 1.5m. I had to check that the signal being picked up was optical and not inductive coupling from the wiring. With this setup, my most sensitive detector can detect the beam at around S2 in a pair of headphones.

I've been experimenting with the drain current of the detector FET and with optimisation of the post cascode stages of my KA7EOI head and may have made 1-2dB S/N improvement. With this test setup I can make very repeatable sensitivity measurements. However, as the tests require TOTAL darkness I have to judge the S/N by ear. My laptop could be set up outside the room with a long screened audio lead allowing Spectran to be used to measure quantitatively the S/N but I'd have to ensure the darkness of the room is preserved.  For an example of this see http://reast.asn.au/optical/Light_Preamp_Performance_Comparisons_20071119.pdf .

At least now I can do lots of initial tests at home and only venture into the field when I have good confidence in the system performance.

Successful non line-of-sight 481THz test tonight by cloudbounce

QRSS3 signal at 3.6km by non line-of-sight cloudbounce

Armed with my simpler QRSS3/CW beacon (see earlier post) I did a very successful non line-of-sight (NLOS) cloudbounce test this evening using my 1W red LED in 100mm optics (run at 340mA). TX was my "G3XBM" message in QRSS3 (3 second dots CW) at 820Hz subcarrier.

With the beacon aiming out through the double glazed shack window at nearby Burwell windmill (as an aiming point) I set off for a road at Landwade which was 3.6km away "over the hill" and on a NLOS path from here. At Landwade I set up the 100mm optics and my variation of the KA7OEI head feeding into my laptop running Spectran. Immediately I got a good signal from the beacon 3.6km away. Signal was around 10dB S/N in 0.67Hz bandwidth. The signal was neither visible as a red glow nor audible in the earpiece despite listening quite hard and panning around for best signal.

This was my first proper NLOS test and it is extremely encouraging. I did try to elevate the RX to higher points in the sky but best reception was with the optics aiming at the lights of Burwell village in the distance i.e. as low as was possible in elevation. At the TX end I was aiming to just clear the slight rise in ground to the east of me near Burwell windmill.

Weather conditions were light patchy low cloud with pretty decent visibility. I did notice QSB as cloud cover varied.

I'm really lucky finding this test path as I can put the TX beacon on the bedroom shack windowsill and fire towards the windmill. In daytime I would be able to align the RX better as I was having to guess the best direction with only Burwell church visible. I had to tweek the alignment to what I thought was the right direction. I did not spend a lot of time trying to peak the signal and better copy is possible.  In all honestly I did not expect this test to be successful.

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.

Quantitative tests with 481THz kit

Today, in daylight again, I did a repeat of my recent tests with the baseband pre-focused red LED TX (20mA), looking for the signal 0.2km along the road. This time I also took my PC and looked for the signal with Spectran and the SAQ SDR receiver. The audio tone was around 40-50dB over noise on both systems.  At this range I would have expected far better S/N. The noise floor looks high. When I blocked off the lens the noise floor went UP about 15dB. Why I wonder?  The picture shows the test set-up. If you click on the image you can see the pre-focused red LED pointing out of the house window and the receiving kit in the bottom foreground. Based on these results speech communication, even with just the same LED and no external lens on TX, should be possible to at least 5km. With 100mm lenses at the TX end, considerably further should be possible.  Below are the test results.

UPDATE: After dark I repeated the range test, just using the crystal earpiece to listen to the signal detected by the PIN diode head. At 0.4km, the greatest distance I could go line-of-sight from my home QTH, the audible signal was STRONG, which is very encouraging.

See my website https://sites.google.com/site/g3xbmqrp/Home/opticalcomms for more details and links.

Optical receiver head completed

481THz optical receiver with 100mm lens

I now have a 100mm lens set-up in some grey drainpipe with 4x sighting scope attached on a tripod together with a "simplified version 3" KA7OEI head (of sorts) with a BPW34 detector. In my version the cascode stage is just followed by a single common emitter stage driving a crystal earpiece, although I intend to add an emitter follower so I can connect to my PC (to use with Spectran/SDR) or a transverter.

A 5mm 20 degree red LEDusing just its own built-in lens was shone out of the stairway window aiming down the street in daylight with 1kHz modulation. It was taking around 20mA. I then walked with the kit to the far end of the street and the signal was solid (S9) in the RX earpiece at a distance of about 0.25km. I now need to find a more distant test range and see how far this can achieve. Depending on results I hope to go looking for GB3CAM next week.

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.