To N6TJ:
I would have thought TI1C vs 6Y2A in 1998 would have taught you a few things
about verticals on the beach... and you still owe us a beer ;-)
Reflector:
And the original question has seemingly been long forgotten: being on the
salt water, optimum distance before you hit land again is about 100 lambda
if I recall from discussions with N6BV. We had somewhat less than that on
160m from 4M7X and it didn't seem to bother us... The more the better, but
100 lambda should be the target.
Except for N6AA's DXpedition QTH example, how many DXpeditions can place
Yagi's that are 200' above water? Not many. Yes there is a point where a
land-based high yagi or stacked yagis will overtake the OVERALL performance
of verticals on the beach. But the Yagi's have to be very high to have gain
below 10 degrees, even more so below 5 degrees, which are extremely
important take off angles. Very few land-based arrays can beat the gain of
vertical arrays (over salt water) at low take off angles. Yagi's will have
more PEAK gain than most vertical arrays - but you shouldn't worry about
peak gain. You need to optimize gain at take off angles that matter, which
are typically lower angles, especially from DXpedition QTHs that are far
away from the population centers. I'd gladly give up 3-4 dB of peak gain at
25 degrees take off angle (for an average height DXpedition yagi), and have
20 dB more gain at angles below 5-10 degrees.
One year at K4ISV's VP5 QTH, I put up a 3 ele 15m vertical and compared it
to his 50' Skyhawk yagi. The model indicated the Yagi would be stronger
about 1/3 the time, equal 1/3 the time and weaker 1/3 the time. This is all
based on the gain at different take off angles. The predictions were
correct. It was fun to have pileups of JA's calling on the verticals that I
simply could not hear on the Yagi. It's all due to the gain at low take off
angles.
Also, N6AA is again correct about the distance from the water I have
revised my thinking about how far back from the water is OK to go for a
vertical by the beach, but I never updated the website. One could cynically
say that's to throw off the competition, but I have nothing to hide. Life
has taken me in a direction where I no longer have time to update my
websites.
So here's the latest thinking: While placing a vertical 1/4 and 3/4
wavelength back did increase the signal by 3 and 2 dB respectively, the RX
station was directly in front of those antennas about 17 miles across the
bay - not off to one side. To illustrate the situation, let's assume you
have a perfectly straight beach line. If you put the vertical at the water
edge, you have water for 180 degrees and land for 180 degrees.
Now, if you place the vertical 1/4 wavelength back, the distance to the
water is 1/4 wavelength only directly in front of the antenna. At 45
degrees off center, the amount of land increases. Even more land at 60
degrees from center. At 90 degrees off center, the path is entirely over
land. If the antenna is at the water's edge, it's completely over water for
the entire 180 degrees. Which one do you think is better?
I'd gladly give up 2-3 dB gain in one direction to increase the gain in more
directions. (and remember, at 1/2 wavelength back from the water the
signal was down about 2 dB) The farther the vertical is from water, the
smaller the effective aperture over water becomes. Going back much more
than 1/4 wavelength and the paths from 45-75 (??) degrees off center
(direction to the water) could be a few hundred feet over land. At 75-90
degrees off center, the path is entirely over land.
One of the main reasons for the performance of verticals on the beach is the
filling in of the pseudo Brewster angle, which are take off angles below ~12
degrees. If you have a few hundred of feet of land in the direction of the
target, you no longer have the salt water as a reflecting plane to fill in
the pseudo Brewster angle. If this is the case you might as well stay home,
unless you aren't serious about your results.
Moral: keep the vertical as close to or over water as possible. Also,
putting radials in the water, or "coupling the antenna to water" (I hear it
all the time but have no idea what that means) is nonsense. Radials are
there to provide return current for the antenna, and using salt water to
provide the return current is completely ineffective. Salt water is
fantastic as a reflecting plane so the pseudo Brewster angle is filled in
We are talking about two different issues and they should not be confused or
misunderstood. If you don't have efficient current return, you might have
fantastic receive signals, but your TX signal could be down 15-20 dB from
what's possible. Been there, done that.
73, Kenny K2KW
CU from the Bahamas beachside in the 2010 CQWWCW
Message: 8
Date: Mon, 24 May 2010 17:49:40 -0700
From: "Jim Neiger" <n6tj@sbcglobal.net>
Subject: Re: [CQ-Contest] Contest QTH
To: "Bill Tippett" <btippett@alum.mit.edu>,
<cq-contest@contesting.com>
Message-ID: <0559B432925D4167A4D353D84B5DEE49@OwnerPC>
Content-Type: text/plain; format=flowed; charset="iso-8859-1";
reply-type=original
It's a good thing I didn't know all of this before plugging into all of
those horizontally polarized antennas 20 ft from the edge of Ascension
Island. Just think how much louder my puny signal would've been with
verticals............
Jim Neiger N6TJ ZD8Z
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