"Real World Laser Operation" by Merv McKee, NC1K
For a couple of years now Dave, AA1A and I have been attempting shots across an open water path of about 16 miles. By all accounts this should be an easy shot, I was on a bluff at 50 ft. ASL and Dave was about 6 ft ASL, and we had used this path many times in the past for early 10Ghz contacts. On a clear day it was easy to see each end of the path but being across the salt-water Plymouth Bay we had to contend with a lot of low level fog. At 10Ghz, even on clear days, we had noticed some propogation effects which we had jokingly attributed to the tide being in or out. When we started using lasers we noticed the same effect, only more pronounced, again blaming it on the tide.
We repeated the tests over a shorter 7 mile path, Dave at the same location and me at 6 ft ASL also. We started at dusk and received good signals at first but they soon went down into the noise after the sun went down, and as the tide came in. It was easy to see that the signals were grazing the water at mid point and the moral of the story is to not even bother with an over the water path unless you know for sure that the beam will be well above the water line at any place along the path. Unfortunately in our part of the world we do not have really high cliffs to work from.
The other aspect of this is something that we had read about many times but not really taken heed of, and that is aiming the laser is far more difficult than it seems. When you figure that a well focused beam is only 50 ft. or so in diameter at 16 miles then the slightest vibration will result in a large flicker at the other end; one really cannot overdo the mounting arrangement and you can make the most exact calculations to pinpoint the other end but at night it can be very frustrating, especially if there are no lighted towers, etc. to use as references. We usually end up scanning very slowly across the expected position using 2M for liason; if you are not a Ham or CBer then a cell-phone would do just fine, but some sort of communication is essential. We use a modulated beam both with HeNe and IR lasers and we simply feed the audio of the receiver into the 2M radio so that the transmitting end can hear his own signal; this instant feedback will enable him to zero in on you rapidly. My advice to someone starting out is that for LOS shots devote 80% of your time to the design of the mounting arrangement, a very heavy tripod for instance staked to the ground or strapped to some permanent fixture, and the other 20% on the laser itself. Collect any micrometers, or similar gadgets which have very fine threads, to use for rotating the laser, the receiver will generally be easier to aim. This kind of hardware appears at Ham Flea Markets and yard sales and in many cases merely getting the word out that you are looking for this stuff will get results from people clearing out garages, etc.
Using a visible HeNe laser of 3mW or so, when you finally get aligned, you will be amazed at how bright it is, at 16 miles it is easily the brightest light around. The first time I saw this I had an adrenaline rush that reminded me of my first Ham Radio contact over 35 years ago. With signals that strong you realize that you could go another 100 miles without any problem, if you had an LOS path.
If you are using IR then invest in a CCD camera, a black and white one can be very sensitive and is indispensable
when trying to align and focus the beam.
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