On Fri, 05 Sep 2008 13:29:36 +1000, Peter Schrader wrote:
>Has anybody on this list had experience with using open wire type
>feeders with verticals? All of the literature on verticals refer
almost
>exclusively to coax feeds.
That's because twinlead is a BAD way to feed a vertical, which has a
rather low impedance at resonance. What you DO need is some means of
resonating it on 160 -- that is, making it electrically longer. The
most efficient method is top-loading, either with a single or with
multiple wire running horizontally or angling down from the top. Study
the antenna books for more details about this. One form of this the
inverted L (one wire), another is a Tee (two wires), another is an
"umbrella" with multiple wires. If you can't do that, a loading coil
midway up the wire is second best. Putting the coil at the base is
least efficient.
Once you get the antenna reasonably close to resonance, it will look
like about 30-40 ohms (including losses in the wire and radials), which
is a pretty good match to coax. Use RG8, and the loss will be very
small.
To cover 80 meters with the same antenna, use a second vertical wire in
parallel with the first one, but don't add top loading (or inductive
loading) to it. It will be close enough to 80M for your tuner to match
it. You can either switch between the two vertical elements, or connect
them in the manner of a fan dipole. I've got a "fan vertical" here -- a
top-loaded 160M Tee that's 86 ft tall with horizontal wires, and a 67
ft wire next to it pulled up with rope from the horizontal wires. I've
made both elements resonant, and have them connected as a fan (that is,
both in parallel).
I'm in California. If I'm on the air before sunrise, I can usually work
the stronger VK, ZL, and JA stations.
73,
Jim K9YC
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