In article aP7wg.626$ok5.203@dukeread01,
"Glenn Ashmore" wrote:

"Wayne.B" wrote
.

Thanks Glenn, looks like I have the LMR400.

Are you saying that path loss at 2400 MHz precludes distances over 2
miles regardless of antenna gain or height?


You can get considerably further with a super high gain antenna but on a
sailboat there is a practical limit. Antennas achieve "gain" by restricting
the radiation to a desired pattern. With a beam you can get a lot of gain
but restricted to a narrow cone which on a boat moving around at anchor it
would be useless. Omni directional get higher gain by flattening the
doughnut. A 5db omni has about a 60 degree vertical spread. an 11db might
have less than 10 degrees of vertical spread. Once you get past about 11db
the pattern is so flat that the slightest rocking will make the signal fade.
8.5 to 9db is about optimal on a sailboat.

If both antennas are at 10' the line of site distance is about 8.5 statute
miles. Assuming the receiving antenna is not laying on the ground the
height over about 10' or 15' is not going to make a lot of difference
because at the low power of wifi the signal is not going to make it that
far.

You add and subtract DB to find the effective radiated power so a 200mw
transmitter is 23db. Subtract 2 db for loss in the LMR400 and add back 8.5
for the antenna and you are at 29.5db radiated in the desired pattern.
The receive sensitivity on the UEB-362 is about -92db which is about as good
as it gets. Add a net of 6.5 for the antenna and cable and you are
at -98.5. You also need 5 or 6 db to separate out the signal from the
noise. The total maximum field loss you can have and still maintain a link
will be 29.5 + 98.5 - 6db margin = 122db. In absolutely perfect line of
site conditions a field loss of 122db is about 7 miles but that is rarely
possible. Just 2 or 3 db difference in performance or field loss will cut
that dramatically. As a practical matter you will be lucky to get a
reliable connection most of the time at much more than half that distance.

very nice articulation of "Radio Path Engineering for Dummies 101"

Bruce in alaska
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