In a message dated 8/29/01 6:40:55 PM Pacific Daylight Time, kk9a@arrl.net
writes:
<<
I tried to use a halfwave vertical with a parasitic reflector on the beach
in Grand Cayman (ZF) two years ago. I fed my vertical in the center
as a vertical dipole. I modeled it with AO, but when I put it up the SWR
was terrible. I changed it to 1/4 wave verticals and they worked great
being only 1 meter from the ocean. I never did figure out why the vertical
dipole didn't work. When I took the vertical down and layed it on a
chair the SWR was good. I later suspected the coax running down side
of the vertical may have been the problem and I wish I tried pulling it out
horizontally from the antenna. I'd like to try it again sometime when I'm
on a Caribbean vacation.
73,
John
>>
I would suspect the coax in the antenna field was the problem. Fed with a
balun and run at right angles preferably to the rear would keep it as clean
as possible. Remember also that even with it at right angles there is another
affect that will pull the resonant frequency of the Dipole and its length has
to be adjusted accordingly. The director pulls it one way and the reflector
another way. With a 3 element yagi it about balances out.
Or feed the DE on the bottom with an L network. There will be RF Spill over
down the coax shield however. If the DE is right near the ground (use a
director) a ground rod can cool the coax shield.
in the late 40's W7DND in Bremerton Wa made this very antenna famous. He was
on a salt water inlet facing east and used a 2 element with director vertical
beam. It was fed with a parallel tank circuit about 3' off the beach and I
still have a picture of it in my antenna files. He absolutely dominated the
frequency.
He made other versions of vertical beams that were outstanding. He'd work SA
on 10M in the summer that others couldn't hear and thought he was faking it.
They would stop in to check him out and he was indeed hearing and working
them.
It points out that there is an "unknown world" of signals hiding down at a
very low angle next to the ground or salt water (liquid copper) in this case
that is unheard by horizontal antennas or even a vertical over ground which
both have a null on the ground.
I've run patterns of verticals with radials 10 or more WL long and it shows
some promise. I have a location where I will be able to do that. Has anyone
any experience with this?
Another experiment I wish to do is based on the great results I've had with
LF 1/2 wave verticals on the ground and without ground radial systems. Also
wtih 1/2 wave verticals on mobiles no matter where mounted as previously
described on TT a couple of days ago. It appears to me that a 1/2 wave end
fed with the base 1/4 wave or higher could take on another level of improved
performance. It reduces the local ground effect and loss to a much lower
affect. I plan an experiment using a dirigible balloon to elevate it at
different heights to see what happens. I'll end feed it with a method that
has no RF Spill Over and it should essentially be a vertical dipole at
different heights with virtually no affect of the local ground. I already
have data of this but will have room to compare it with a conventional 1/4
wave at the same time. It's a great contest antenna for the cost of a
dirigible balloon. How about 2 1/2 waves in phase on 160M? You won't be
able to hear all the stations calling due the noise of a vertical antenna.
You will need a Beverage for receive. I'm going lay an insulated Beverage
wire in the mud of a river near my QTH and see how it works. This "Mudder
Beverage" should hear those signals "down under." Insulated wires laying on
the ice of a lake work very well--even a Vee Beam Beverage. Would you
believe it won't cool off a hot receiver either. k7gco
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