Cloudbounce and scatter optical and IR tests

This morning I read a most interesting article about French cloudbounce and scatter optical tests using lasers. The article is some years old but makes a fascinating read. My own experience with clear air forward scatter at optical frequencies using relatively low powered red LEDs makes me believe that a LOT more is possible in this area. It is a bit like people saying, "UHF is only line-of-sight". Rubbish! With decent, easily built kit it is possible to copy QRP amateur signals over the horizon even at IR and visible optical frequencies.

See http://sd-1.archive-host.com/membres/up/22679775843705539/CBVUK.pdf

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.

Optical cloudbounce propagation theory

Bernie G4HJW and I are interested in carrying out some "cloudbounce" tests using our 481THz optical kit and we both are none to clear about the physics of scattering of signals from water droplets and dust in clouds or the sky. However, today a new tool to help our understanding was shown to us on the UKNanowaves Yahoo Group in a posting by Barry Chambers.

The free software is available at  http://www.philiplaven.com/mieplot.htm and allows us to work out the intensity of a scattered optical signal as a function of scattering angle, droplet size and wavelength. Scattering is best when the angle of incidence is at grazing incidence and the droplet size is small. If I've understood the results correctly, aiming at the underside of a cloud at 45 degrees would result in a scattered signal some 50dB weaker than if at grazing incidence. So, depending on how far apart the 2 stations are and the angle at which the optical signal hits the underside of a cloud then signals can be quite strong or extremely weak. This is why weak signal modes like QRSS60 may be needed to work a given path by cloudbounce.

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