[Laser] One way 3 mile laser NLOS contact using PSK31

[Kerry Banke]

John - I think you're right about better scatter with teh shorter 
wavelengths. I went with the IR as I found some 1W diodes on Ebay cheap & 
took a chance on them. The other issue with higher power visible is 
distracting vehicle drivers/pilots. This is getting real publicity so I'm 
more hesitant to shoot visible beams around at least if they are a a narrow 
spot beam. For scatter purposes I think a larger beam is better and 
certainly provides much more eye safety.  I believe that more dense fog 
will scatter all light wavelengths fairly equally according to an article I 
read.   There's a group in France using 10mw red lasers to do cloud bounce 
and have done 6 km so far using 30mm lenses for Tx & Rx.  I'm anxious to 
try my system some time with very high clouds.  I think that with this 
horsepower we can go a long way. Unfortunately I'm stuck with working with 
a slight haze most of the time as we don't get clouds often here in San 
Diego.  So we'll just continue working to see what is practical with the 
typical haze conditions here.   I have a good quantity of Pin photodiodes 
if you need any. I also have several lenses in the 4" size range and maybe 
8" FL good for the Rx end of things if you need one or two.
  - Kerry N6IZW -


At 11:04 PM 12/13/2004, you wrote:
>Congratulations ... getting close to a two-way QSO ... makes me think about
>trying NLOS with visible light.  Scatter should be better at higher freqs?
>
>BTW, I have been looking into IR propagation at 1400 nm.  Higher power at
>those lower freqs ... I'm told ... is relatively "eye-safe" because the
>cornea/lens is opaque and doesn't focus the IR light to a small spot on the
>retina. If so, I wonder how it is for scatter.  Maybe haze / water droplets
>are sufficiently flat to scatter that freq / wavelength well too?
>
>John Matz KB9II
>
>----- Original Message -----
>From: "Kerry Banke" <kbanke at qualcomm.com>
>To: "Free Space LASER Communications" <laser at mailman.qth.net>
>Sent: Monday, December 13, 2004 9:41 AM
>Subject: [Laser] One way 3 mile laser NLOS contact using PSK31
>
>
> > Last evening Chuck (WB6IGP) & I made our first one way NLOS laser contact
> > using PSK31 between our homes which are about 3 miles apart. The 1W 910 nm
> > laser with 4" beam expander was located at my home and precisely pointed
>in
> > the azimuth  direction of Chucks home and a couple of degrees above the
> > horizon.  There was slight haze which I have found to be great for
> > producing scatter signals.  At Chuck's home the 260 Hz carrier was readily
> > audible using the 4" lens ahead of the K3PGP pin detector/preamp
> > configuration.  Looking at the signal with SpectrumLab showed the carrier
> > to be steady at about 35 dB above the noise with a 1 Hz BW. A test message
> > was sent with perfect copy. The narrow  laser beam could be followed with
> > the detector over maybe 30 degrees elevation  with the signal slowly
> > increasing towards the horizon. In this case we were limited near the
> > horizon by a Sodium vapor street lamp so could not view the bottom maybe 5
> > degrees.  My recent propagation experiments have shown the following with
> > this configuration:
> >
> > On totally clear, cold nights following a rain, there is no useable
>scatter
> > at 3 miles out (at least for levels required for PSK31) . Most
> > evenings  there seems to be a cone of useable signal  for both back
>scatter
> > and forward scatter with the signal dropping off as the receiver
>approaches
> > 90 degrees to the transmit beam. Of course clouds produce an amazingly
> > strong spot or line. Light rain  also produces great scatter.
> >
> > The next step is to finish the equipment to do a two way contact. Chuck
>has
> > assembled a second  working receiver & I'm in the process of building a
> > second  laser transmitter. We'll then be ready to see what kind of two way
> > distances are possible and will also try the LaserScatter software as the
> > hardware has been built with this in mind.
> >   - Kerry N6IZW -
> >
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