On Mon, 2 Nov 2009 22:41:06 -0800, Lew Sayre wrote:
>What I have is two fir trees, each about 110' tall and 189' apart. I have
>12GA copper stranded wires going up and over the top of
Hi Lew,
The only thing I would change is to use the trees to support a "Tee"
vertical, with the vertical portion midway between the two trees. This
should minimize loss from the trees. That's pretty much what I'm doing
here. I have a pulley in a tall redwood at about 100 ft and another in a
madrone at about 80 ft. Ropes run from each tree to the top-loading
section, with the vertical section spliced in midway and coming down to
the feedpoint and radials. You have the space to do what I did -- use a
top section that makes the antenna resonant down below 160, then add a
capacitor in series to bring it back up. This gives you a feedpoint Z of
about 50 ohms.
With this configuration, radiation from the horizontal section cancels,
the current peak in the antenna moves up a bit, slightly improving the
vertical pattern. It does NOT improve efficiency of the radial system, but
it doesn't hurt it either.
Be sure to put a weight on one end of the top section so that things don't
break when the trees sway. I use #10 copper THHN (house wire) for
mechanical strength. I use good pulleys from Sherrill Tree Service (in
NC), and the 5/16-inch rope that DXE and others sell. Use insulators that
don't let the rope touch bare copper -- it can arc when it's wet and melt
the rope. As to height -- it's a compromise between maximizing the
vertical section (good for radiation) and too much sway in a big storm if
you go too high. That redwoods holding my antennas are about 170 ft at
their peaks, and my rigging points are typically 30-40 ft below that. Even
with that and very robust antenna construction, I still typically lose one
antenna per year to the wind (I currently have six wires up).
73,
Jim K9YC
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160 meters is a serious band, it should be treated with respect. - TF4M
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