[Laser] LaserScatter

[aflowers at frontiernet.net]

Kerry,

You'll find that the rolloff on that PGP front end really giveS you MUCH
more S/N in the LF range.  It will depend on the capacitance of the PD
and other factors, but F1AVY managed to measure the 3dB rolloff point
at around 12 Hz (not a typo).  If you bring that modulation frequency
down you should be able to increase the S/N tremendously, unless of
course you are shooting for audible detection.  In that case, the human
ear isn't going to do too much better lower than about 300Hz or so.  I
wrote the program to use 20-31 Hz because I figured that natural things
might add noise below 20 Hz, for example, twinkling starts or branches
swaying in front of distant streetlights.

BTW, I've observed that the .jar file does not want to execute under
W98/W95.  Has anyone gotten this to work?  I have the same JRE as on my
XP machine but it is unable to find the main class under W98.  I'm
guessing there is some arcane path problem, but I make no claims to be
the java expert!

Andy K0SM


Quoting Kerry Banke <kbanke at qualcomm.com>:

> Andy - Thanks for putting together the LaserScatter program.  I'm
> just
> starting to look at NLOS optical communications and hope to give your
> program a try before too long. Last night I was happy to observe  a
> faint
> return from a 1 watt  Luxeon red LED collimated with a 4" lens
> pointed
> vertically in to a clear sky.  I'm modulating the LED at 355 Hz and
> receiving it with a K3PGP MPF102 front end without any  PIN detector
> bias
> resistor. I need to change the frequency a bit as it's fairly close
> to the
> six harmonic of 60 Hz which never goes away unless I cover the Rx
> lens. The
> Rx lens is only 1" a the moment and  I'll go ahead & move up to 4" on
> that
> as well.   For this first test the Tx & Rx units were only separated
> by
> about 100' on  opposite sides of my house.   I'm using the Spectran
> software to observe the returned signal.   The photodiode is a type
> SFH203
> (1mm and 10pF).  I'm hoping to work towards being able to communicate
>  over
> a hill between myself and another Ham a few miles away.  Keep up the
> good work.
>   - Kerry N6IZW -
>
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