13 Mar 2013

Amazing NLOS 481THz reception tonight!

This morning I realised that I was 2.5 degrees out in my aim last night with my optical beacon, so this evening I repeated the over-the-horizon optical test, moving instead to a new location fully in the beam at a distance of 4.8km. The difference was astounding! Last night I was struggling to copy the signal even with a long carrier. Tonight it would probably have been audible in 10wpm CW!
Excellent NLOS 481THz copy this evening
I ran QRSS3 tonight and was rewarded with signals 20dB over the noise in 0.34Hz bandwidth when beaming just above the horizon. At higher elevations the signal was weaker. With this strength I am very confident that with QRSS3 the signal should be copyable non line-of-sight (NLOS) out to at least 10km by forward scatter and possibly much further, even with my 1W LED running at 250mA in 100mm optics.  This was an excellent result and I am now confident the RX is working very well indeed.  Cloudbounce is another mode I have yet to properly exploit. The reception tonight was as a result of clear air scattering presumably from water droplets or dust particles.

This time I made recordings which are linked from my website. These can be played back through Spectran or Spectrum Lab software and further analysed. See https://sites.google.com/site/g3xbmqrp3/vuhf/optical/481thz-nlos .

Rapidly I am learning the absolute importance of accurate aim: 2.5 degrees out last night probably reduced the signal level by around 20dB. For serious tests over longer ranges a means of very accurately aligning the TX and RX is essential. Unlike with line-of-sight where the red dot can be seen by eye usually, NLOS requires careful alignment from map features.


Virgil said...

Very interesting!

Anonymous said...

Yes, very interesting. I will try similar experiments when my aiming system is more reliable. Keep up the good work. Dave G4FEV.

Todd Dugdale said...

Why such a narrow bandwidth (0.34Hz)?

Roger G3XBM said...

Todd, 0.34Hz is the optimum bandwidth for QRSS3 giving best S/N ratio. Any wider and the signal will start to be buried in noise. Any narrower and the signal will slur and be less readable. If you go to my main website and play the recording back through Spectran you can try adjusting the bandwidth to see the effect first hand.

Hope to try at around 10-15km next week.

73s Roger G3XBM

Graham said...

Indeed, very interesting work.

Being out by 2.5 degrees at a range of 4.6 Km is about 44 meters one side of center line or your light beam. (based on 6400 mils per 360 degrees and your range being 4600 meters - one mil is the angle subtended by 1 meter at a distance of 1000 meters) and the decrease in power of your light source decreasing at the rate of distance squared. If you know the beam width of your light source and optics you could do some simple math and estimate reasonably well where you were with respect to the center line of your light source the first night.

In any respect, being 2.5 degrees off at 4.6Km may not seem like much but it really is and you where in fact quite successful in your earlier test.

This has me thinking that perhaps I should try something similar. Spice of life and all that.

cheers, graham ve3gtc