[Antennas] OCF Long-wire with movable feed point

2002年4月19日 21:46:51 -0400

I was reading an old 73 Magazine, (May, 69') when I saw an article
discussing Off Center Fed longwires. The article examines directivity
pattern of a 3.5 wavelength longwire. Article shows how the radtiation
pattern changes depending on where the feed line is placed along the
length of the antenna to achieve a steerable pattern. This article was
written before antenna modeling programs were invented and I assume the
author's (W2EEY) results were experimental, although he doesn't say.
Per the ARRL Antenna book, a 3.5 wl longwire has about 3db gain over a
dipole.
I have a long narrow lot with several tall trees where 3.5 wavelengths
on 40 meters (about 475 feet @ 7.2mhz per the Antenna book) would be
feasable.
What if one was to erect a folded doublet, 455 long, fed with open wire
to a tuner ? Further, what if one were to arrange the wire of the
folded doublet in a continuous loop around pulleys, so that you could
move the feedpoint where you wanted it along the length of the doublet
like this:
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x
x
x x
x
x
x x x
x
x x
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 x x
 x x
 x x
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 x x
The result should be a 3 dbd gain antenna (on 40) with steerable
pattern. Further, on 20 meters, 455 feet is approximately 7 wl, which
would theoretically yield about 5.5 dbd. The gain would increase with
frequency to about 8 dbd @ 10 meters.
Per the Antenna Book, take-off angle also decreases with wavelength, so
it should be a good DX antenna.
It would seem to be a cheap alternative to a beam if a person had the
room.
Has anyone built and or modeled this antenna? If so, I would be curious
as to the results.
 Match
 Box
 Rig
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