Lightwave Scatter

Looking down the road at possible future 'clear-air scatter' or 'cloudbounce' lightwave tests with stations (VE7CNF and VA7MM) on the other side of Georgia Strait, I spent a few minutes breadboarding a more stable modulator for my lightwave system.


As it is at present, the modulator consists of a 556 tone generator, capable of either a steady tone for CW keying or a two-tone FSK 'beaconing' signal used to help the other station in aiming alignment.

For the slow QRSS CW narrow-bandwidth modes required for the scatter tests, I've always known that a tone which is much more stable and of precisely known frequency would be needed. The tone from the 556 does well as an aural CW keyed tone but would probably be all over the place when viewed in a very narrow-bandwidth and not nearly as stable as it sounds by ear.

The little modulator uses a 4500 KHz crystal (pulled from a old VCR several years ago) in a 4060 oscillator-divider. In this case, output from the chip is taken from either the 'divide-by-8192' pin 2, which outputs a precise frequency of 549 Hz or from the 'divide-by-4096' pin 1, which outputs a frequency of 1099Hz.

courtesy G3XBM: http://g3xbm-qrp.blogspot.ca/search/label/nlos

This tone is then used to drive an IRF 540 power MOSFET which controls current through the 1W Luxeon Deep Red LED in the transmitter. The 4060 modulator will be keyed via a QRSS software keying program that I have used for many years to key my LF transmitter.

The lightwave receiving station will look for the QRSS audio signal with an audio spectrum viewer such as Argo or Spectran. The ability to make automatic overnight screen captures will allow the receiving operator to get a good night's sleep while the system diligently watches for any traces of a signal.

An example of a strong signal capture showing a repeating "SL" identification is shown below, as it would appear in a perfect world. In this case (QRSS3), the short 'dots' are 3 seconds long while the 'dashes' are 9 seconds.

Huge signal gains (the ability to dig into the noise for signals) can be had by slowing things down and using narrower receiving bandwidths. Just going from a normal 12WPM speed CW (aural copy) to QRSS3 yields a gain of ~15db. At QRSS10 (10 second dots), an additional 5db is gained while slowing to QRSS60 (60 second dots), a whopping 28 db over 12WPM CW is gained!

Of course all of this extra 'hearing power' comes at a cost and in this case, the cost is 'time'. On an overnight of automated computer monitoring, time is not much of an issue ... it only becomes critical in 'QSO mode' when some QRSS QSOs can take several hours to complete. In any case, it will be interesting to see if any traces of lightwave signals will show up while bouncing around in the clouds.

The Georgia Strait scatter tests will not take place for some time but in the meantime, I hope to do some local tests here, from one side of my island to the other but will build a new portable receiver for these tests and leave my main system intact.