[Laser] Ramsey Kit range

[laser@mailman.qth.net]

Howdy

I am James, N5GUI.  I joined this list after finding information about VE7DXG 
sucess with the Ramsey laser kit in the recent archives.  I really appreciate 
that this exists to share experiences and ideas  -- I guess it it my kind of 
techie entertainment.  I hope that I can add something too.

I am sure that I will have bunches of questions, but I would like to start 
with two:

What sort of "barefoot" range has been obtained with the Ramsey laser 
communications kits?  --  Without using lenses to increase the receive area or reduce 
the beam divergence.  The company claims 0.25 mile, and testing with mine at 
about 800 feet has lots of signal strength.


The second question is part theory, part real world practice, and part may be 
pointing out flaws in my thinking.  What is a way to estimate the range that 
a signal can be received when lenses are added to a system.

I was able to analyze the question two ways.  One says that the transmitted 
beam expands to the square of the distance.  If a 5mm diameter detector can 
receive a signal at 1Km, and you then move the detector to 3 Km, the received 
signal should be on nineth.  That suggests that a lens with 9 times the area, or 
3 times the diameter (ignoring losses) should be needed.  Said another way, 
with a 5mm detector, the range should be ten times with a 50mm lens and twenty 
times with a 100mm lens, which is a linear relationship to the diameter ratio, 
or to the square root of the area ratios.  I think this will be true for long 
ranges.

On the other hand, the area of beam, or the power in it, may not be descibed 
by a mathematical relation to the square of the distance.  For "short 
disatances" the intensity of the received beam may be linearly proportional to these 
"short" distance numbers.  If that is the case then the power on the detector 
would be proportional to the area of the lens, so that the useful range is 
factored by the square of the diameter ratio of the lens to the detector.

Both ways of thinking about range seem to be oversimplification of the "real" 
world, but is either a practical estimating tool?




I have been tinkering with laser pointer communication for more than two 
years.  The "spark" that started it was when a florescent desk lamp with a 
magnifying lens quit working (lightning strike or maybe one of the other wiring 
problems with my house).  I took it apart and enjoyed playing with the 4 inch 
diameter lens.  About the same time I was playing with CdS photoresistors.  I 
mounted an LED at one end of short piece of plastic pipe and a photo resistor at 
the other.  For a tone source, I breadboarded a 555 timer circuit to drive the 
LED  and then connected the photoresistor to a battery and headphones...  Well, 
you get the idea.

At some point, I mounted the lens at one end of a tube assembly and a CdS 
photoresistor on the other.  (The tube was made by joining a metal vegitable can 
to the bottom of a large cardboard container for chocolate milk drink powder  
-- My grandaughter drinks a lot of chocolate milk.)  The LED worked a few feet 
in my darkened basement.  With some ideas from the internet, I was ready for 
experiments with a laser pointer.

The first 555 timer laser pointer driver circuit that I found on the internet 
was OK (and very similar to my breadboard) but I did not like the key 
connected to the B+ line.  I put an NPN transistor to pull down the reset line of the 
555 and used the key to ground the base of the transistor.  The other mod I 
made to the circuit was to drive the laser pointer with a (nearly) constant 
current source using an LED (yup, the same one from the breadboard) as the 
voltage reference instead of a zener diode.

This got enough interest a the local clubs, that I gave some demonstrations.  
When I found out about the Ramsey kits, I got two.  They are together(with a 
few mods), mounted in 2" plasitc pipe, as are 50 and 100 mm lens attachments 
for the receivers, and really heavy tripod mounts.  All got set aside for 
several months.  I gotta get back to them.

Wish you all well.
James



 


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