[Laser] Sky illumination experiment

[Kerry Banke]

Tim - The light pollution in town and near San Diego is so high that this 
measly pair of florescent tubes isn't even noticeable. Every building 
commercial & residence has some form of yard night illumination.  I went up 
on a hill a couple miles away with an LOS view to my home  and can find the 
signal with the receiver but had no clue which of the light sources was 
mine visually as there are so many white light sources visible at much 
higher power. Pointing at just above the horizon towards  my source from 
1/2 mile away (NLOS) produces a signal maybe 40-60 dB below the  glow of 
the 120 Hz background of the San Diego City lights some 12 miles West.  I'm 
sure it would be a different story out in the country but I'm surrounded on 
all sides with cities producing heavy light pollution. For clear sky NLOS 
communications the light source beaming straight up is of little value. The 
useful light is that which is headed at a low angle just clearing the 
optical horizon in any given direction.  The ideal source would have a 
reflector that channels all light into maybe a 5-10 degrees vertical beam 
width over a 360 degree azimuth. Cloud bounce is apt to be another story & 
I am waiting for some evening clouds to run that experiment.
  - Kerry N6IZW -

At 03:21 AM 7/5/2005, you wrote:
>Kerry,
>Angry home owners associations, black helicopters and
>astronomers aside, 80 watts is plenty of light. Much
>more and you'd be attracting the men-in-black :D
>
>But yeah, compared to typical football stadiums, light
>pollution is miniscule, especially if you direct most
>of the light straight up and away from the neighbors.
>It might be hard to find a large IR filter for
>something that size but i dont think those tubes put
>out very much light at IR wavelengths anyway, so its
>probably a lost cause trying to stealth them that way.
>A regular filament type light bulb does however put
>out a lot of IR light and would be easier to find IR
>filters big enough cheaply and also to focus a
>narrower beam if you wanted.
>
>I was thinking since these frequencies are all in the
>audio range, and if one was lacking an exotic power
>supply, you could use a regular off-the-shelf audio
>power amp in the 100 watt range. Then match its 8 or 4
>ohm output to the impedance of the tube(s). Matching
>it to such an oddball capacitive load might be a
>problem with the florescent tubes though. A regular
>light bulb would probably be easier to match.
>
>At any rate, PSK31 modulation springs to mind since
>this is all nice and sinusoidal.
>good luck
>-_-
>
>
>Tim Toast
>http://www.aladal.net/toast/exp.html
>
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