[Laser] Cheap 8x11 Lens

[Bob Williams]

On Sunday 03 February 2002 16:50, Paul Kelley wrote:
> Can you tell us how rigid it is? Does it hold its shape well enough to
> still focus properly when mounted by the edges and perhaps tilted at an
> angle?
>
Paul,

Yes, it appears to be plenty rigid when mounted by the apx. 0.25" lip and 
pointed straight up, I don't get any obvious deflection.  It's fairly thick 
plastic.  I imagine that in a good wind, it might, indeed, flex a bit, 
though. The packing, an envelope, will allow you to judge that for yourself, 
if you find one at an office supply store.

> Reflections of what, exactly? From what? Could you please explain further?
> I have a system ready to go (though using a smaller lens right now), but
> I'm the only one playing with this stuff here. So I'm curious when anybody
> says "reflections".

Well, I'm waiting for some warmer weather or good cloud cover to test 
cloudbounce and some bounces off of a not-too-distant tower with some dishes 
on it that make it look interesting.  My reflections were off nearby 
buildings and trees.  At this point nothing technically that interesting nor 
very scientific.  I use my 800 Hz mod. laser pointer, attenuate the beam by 
holding the whole pointer in my hand and letting the beam peek out enough to 
make a barely visible, fuzzy spot.  I then swept the view of the receiver 
through where the spot should be to see if I got a peak, and did.  By 
squeezing my fingers till the spot disappears, I produce a fuzzy spot right 
at or just below visibility.  Not too scientific, but it indicates a 
significant improvement over the set-up without the lens, where I was able to 
detect visible reflections off of a surface painted flat-black.

I've been thinking that if you wanted some quantative measure of improvement, 
neutral density filters such as used in photography should produce calibrated 
signal degradation.  I've also toyed with the idea of crossed polarizing 
lenses as an adjustable attenuater; because the attenuation of randomly 
polarized light should be roughly equiv. to Cos of the angle, you could get 
at least a repeatable figure of merit by measuring the angle of crossing.  
I'm not sure that it would work as well with coherent light.  There will 
probably be some polarization effects.

In the end, I'm going to wait for a bit warmer weather and then just take the 
setup out in the field.  I believe that interesting non-LOS paths should be 
workable with common visibility to water towers, microwave dishes and the 
like. And with the sensitivity I'm seeing now with this very simple setup, I 
don't see why not some over the horizon scatter.

73

Bob, AA1ZC
Gales Ferry, CT FN31xk
trillium@snet.net