[Laser] Ramsey Kit range

[John Matz]

I just thought I'd mention ... if it really is true PWM, not on a subcarrier
... then the bandwidth need only be 3 kHz on the receive end.  Recovery is
simply lowpassing the PWM transmission.
John Matz KB9II
----- Original Message -----
From: "Andrew T. Flowers, K0SM" <flowersa@alltel.net>
To: <laser@mailman.qth.net>
Sent: Sunday, November 09, 2003 7:01 PM
Subject: Re: [Laser] Ramsey Kit range


> This reminds me,
>
> It seems there have been several folks who have put together these
> ramsey kits.  In fact, Dave and I have had the pleasure of using these
> kits in a radio contest or two.  Does someone want to find a way to "hot
> rod" these things to get a little better performance out of them?  It
> seems to me that someone could come up with a "front end" that beats the
> heck out of the phototrasistor that comes with it, keping in mind that
> it needs to have a respose up to over 100KHz or so (it's 18KHz PWM, and
> you need to resolve the waveform fairly accurately I think).  It's
> pretty clear to me that most of the light is getting thrown away due to
> the teeny-tiny active area of those devices.  If someone wants to look
> at this I can show you a schematic what the RX looks like.  It might be
> a worth doing since many folks seem to be getting interested in laser
> comm stuff through these kits...and it's also in the a "ham spirit" to
> figure out how to make a good thing better....
>
> Andy
> K0SM/2, Rochester NY
>
> David D. Rea wrote:
>
> >On Sun, 2003-11-09 at 15:20, TWOSIG@aol.com wrote:
> >
> >>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?
> >>
> >
> >Hi James-
> >
> >You're fairly close to on-target with your theory. The received power
> >will fall off with the square of the distance, but this only occurs in
> >freespace. Keep in mind that you've got atmospheric nasties to deal with
> >as well - after you get above 1 Km or so, you'll start noticing the
> >effects of humidity and purturbations in the air; i.e. you'll lose power
> >due to absorption and dispersion of water mollecules, and you'll see a
> >"shimmering" effect as the beam traverses different thermal planes
> >between the transmitter and the receiver.
> >
> >There has been quite a bit of work done in this subject, as I found out
> >when writing a paper on laser communication during an undergrad
> >independent study. You can get as deep in the math as you want; there's
> >no shortage of folks out there who have written PhD theses on this
> >stuff...
> >
> >To add a layer of complexity, remember that no lens is perfect, either.
> >If you're dealing with a nice AR multi-coated glass lens, maybe you'll
> >throw away 5% of your signal (if you're lucky). But if you're using a
> >less expensive (but MUCH larger area) fresnel lens, plan on tossing
> >about 50% of the inbound light back toward the receiver. This is where
> >you compensate for reflection with sheer lens size...
> >
> >Anyway - hope this helps a bit...
> >
> >73 de Dave K2THZ
> >
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>
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