[Laser] RE: Sky illumination experiment

[Tim Toast]

I agree, the light going straight up isnt going to
help NLOS much in clear sky. 
One of those circular fluorescent type ring-tubes
might be good for a 360 emitter. Say have it
sandwiched between two wood or plastic disks each a
little bigger in diameter than the ring-tube itself.
These would act as a shade to keep most of the light
in that 5 to 10 degree vertical beam / 360 azimuth. It
could use a reflector of some kind to boost light
output but just a nice decorative water proof shade
for the whole thing to make it look like a bug-zapper
or something ornamental would be good. If it were
mounted up high enough and the bottom disk made bigger
or with some baffles, it might not even be visible to
close neighbors.

>
>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 -
>


On a side note about using incandescent bulbs, I was
thinking in terms of modulating them in frequency
instead of AM (LaserScatter, Jason, etc), so problems
with the filament resistance changing should be
minimal as long as it's fed a constant amplitude
waveform. (and the fluorescents wouldn't be cut off
accidently with FM). 
The modulation percentage may change a little with
incandescents between the low and high frequencies
(15~30 Hz) due to that thermal mass, so it might need
a bit of EQ to even things out if that's a problem.

BPSK is just a two frequency deal, so theres not alot
of amplitude change i wouldn't think??.. but if there
is some then it would get distorted no doubt. I tend
to think of PSK as a form of digital NBFM

>
>Note that the resistance of an incandescent lamp
>changes with the amount of power flowing into it.
>Thus, as the lamp brightens, the filament changes
>resistance.  This might cause a bit of distortion,
>unless you compensated for it with feedback through
>the amplifier.  Then, again, you might just live with
>the distortion.
>
>Dave
>WA4QAL
>

I imagine incandescents as being most efficient as
emitters of modulated light when operated at or near
the frequency where the filament has time to almost
cool off between the cycles. Much lower in frequency
then it would cut off completely and be "over
modulated", and if operated much above the frequency,
then the thermal mass prevents the filament from
cooling off between cycles and there would be a "DC"
component only contributing noise (the amplitude of
the modulating waveform being constant in all cases -
no EQ)

So different types of bulbs may be better for certain
frequencies than others. Regular light bulbs seem to
work great at 120 Hz, but i doubt they are near 100%
modulated, probably more like 50% if that. I bet they
are much better emitters at some lower frequency.

>
>Incandescent bulbs, low power, should work as an
>infrared source for low frequency modulation
>techniques.  The more powerful the bulb's rating, the
>more thermal mass it will have.  Think of the thermal
>mass as a low pass filter. Best way to find out what
>they will do is to wire some up and experiment.
>(Should be worth an article in an experimenter's
>magazine.) I have seen some decorative bulbs with
>very thin, long, filaments.  These should work
>better than an  equivalent power rated bulb with
>thick coiled filament.  Also, the experiment should
>indicate if they are better sources with the
>filament not glowing, glowing dull red, bright red,
>yellow, or white hot. They might  work better as IR
>sources at lower temperature, they might not.  If
>the white light from a hot filament is "noise" then
>the best S/N ratio might be in between.
> 
> 
>James
>N5GUI
>


Tim Toast
http://www.aladal.net/toast/exp.html

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