The problem John, with that experiment is it does not tell you what is
happening just above the very lowest angle. By modelling you can see that
the last lobe to reduce is that contained down near the horizon. We are also
interested in the content between 2 degrees up to 20+ degrees. By moving the
antenna away from the sea the energy contained in that sector reduces. For
example on HF the content between 3-10 degrees is all important. 73 Clive
GM3POI
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of
john@kk9a.com
Sent: 02 April 2015 15:00
To: topband@contesting.com
Subject: Re: Topband: Salt-Water Qth!
I did not recall seeing tests for verticals a wavelength or more way from
the sea so I checked the team vertical website and found the following:
John KK9A
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.
To: <topband@contesting.com>
Subject: Re: Topband: Salt-Water Qth!
From: "Ed Sawyer" <sawyered@earthlink.net>
Reply-to: sawyered@earthlink.net
Date: Wed, 1 Apr 2015 13:13:17 -0400
The best write up and data I have seen on this subject was the "team
vertical" report on test done in Jamaica back about 10 or more years ago.
As I recall, the vertical signal strength to low angle DX went up
"dramatically" within 2 or less wavelengths of the edge of the high water
mark and maybe leveled off as "fantastic" from within 0.5 wavelength. But
further and further away past 2 wavelengths, the signal strengths dropped
away and had very diminishing effects. I don't recall how far back before
the benefits were disappointing but that article has the answers you need.
Just scale it for 160 or 80M vs their 40 - 10M data.
By the way, I used a vertical as 9M6/N1UR at Layang Layang island in the
Spratlys in 1998. 40 and 30M performance was "amazing" but 20 - 10 was good
but not great. The vertical was placed about 100 feet from the edge of the
water. So it would have been just under a wavelength on 40, just over on
30, and 2 - 3 wavelengths on 20 - 10.
Ed N1UR
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
|