Samir Popaja, 7S7V (SM7VZX) wrote:
>I would like to build OCF dipole, 137 feet long, with feedpoint located 80%
>from one end. It's 27.4 ft from one end.
>(http://www.w8ji.com/windom_off_center_fed.htm). I would like to use it on
>80, 40, 20, 15 och 10m, not interesting of WARC band.
>If we compare this antenna with FAN DIPOLE (for respective bands), is it any
>difference between them?
First, you WILL need a tuner to use an OCF dipole. They have many merits,
and some demerits, which you can read on W8JI's site, or in the white paper
on the Yankee Clipper Contest Club website.
Second, I believe that a 4:1 balun is not correct, based on VE2CV's articles
in QST, some years ago.
Third, the radiation pattern of the OCF wire will be very different on the
higher bands than that of a simple resonant dipole on each band.
The problem of filling nulls on higher frequencies was attacked in an OCF
dipole variant called a "carolina windom". These are presently sold by
Radioworks. (radioworks.com) You can see further information on their
website. The concept is that they force feedline radiation to fill nulls on
the higher frequencies.
Fourth, reported VSWR figures are misleading. The feedline length is a
factor in what you measure at the TX, and where the antenna is never
precisely matched, you'll always be transforming some complex impedance.
My solution? Ignore the vswr, so long as the auto-tuner can handle it.
Use the best coax you can to minimize losses, and recognize that you'll have
to accept inefficiency for broad bandedness.
For the no-tuner situation, a fan dipole may be a better solution.
I've used OCF dipoles for many years, as utility antennas. Always with a
tuner, and always with considerable ferrite common mode chokes.
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