N6AA wrote:
The "Team Vertical" prime movers, K2KW and N6BT, have answered this
question. A good rule of thumb is the antennas should be no more than
a quarter wavelength from the salt water. More information is
contained on K2KW's site, http://www.k2kw.com/verticals/learning.html
Thanks for reminding me of that study Dick. I don't think it's quite
as simple as <1/4 wavelength however. To quote from Lesson 7 in the
article above:
"While field testing the verticals this past summer, we decided to
test the effect of the land-water boundary on the pseudo Brewster
angle. Since our receive site was elevated less than 1 degree across
the bay, we could see any change in the low angle energy. To our
knowledge, there has not been any published tests of this kind. The
goal was to see how far from the water the vertical would loose the
benefit of the salt water on the pseudo Brewster angle. The tests were
done with a 20m ZR vertical, and we moved the antenna away from the
water in 5' steps. The water's edge was considered the reference
point. As the vertical was moved back from the water, there was little
change until we came close to 1/4 wavelength from the water. At that
point there was a 3 dB increase in signal level! Moving farther, the
received signal level dropped, indicating a loss of low angle energy.
This was most significant at 1/2 wavelength from the boundary, being
down about 3dB from the waters edge. Moving farther back to 3/4
wavelength, the signal picked up again, to more than 2dB enhancement
from the water's edge. We could not move the antenna farther due to
obstructions. During the tests, we did not believe the data, and reran
the test. We also observed the same results on the second test. At the
time we only had 20m antennas, so we could not confirm that
enhancement was truly frequency dependent. But based on these results,
more testing is warranted."
Summarizing, they measured:
edge - 0 dB (reference)
1/4 wl - +3 dB
1/2 wl - -3 dB
3/4 wl - +2 dB
It might be logical to assume 1 wl could be another null (-2 dB?) and
5/4 wl could be another peak (+1 dB?) As I said before, I know of
several *very* successful Topband stations "several hundred feet' from
salty water (not necessarily "salt water") with 1 wavelength on 160
being 540 feet.
73, Bill W4ZV
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