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April 22nd 05, 04:29 PM

In article ,
says...

On Thu, 21 Apr 2005 10:56:46 -0700, Roy Lewallen
wrote:

Some years ago, John Belrose VE2CV published an article in QST about
off-center fed antennas, based on a combination of modeling and
building. While they look attractive on a simulation program (where the
feedline wasn't also modeled), he found that in practice it was very
hard to keep the feedline common mode current down to a low value. The
consequence of common mode current is that the feedline becomes part of
the antenna, and that makes it very difficult to duplicate or to realize
the idealized performance you might expect if you hadn't taken feedline
radiation into account. Multiple current baluns should be able to reduce
the common mode current to a reasonable value, but they'd have to be
used unless you want a lot of feedline radiation and performance that's
difficult to predict.

Roy Lewallen, W7EL

A few years back I put up an OCFD. Feed it with open line. Seemed to
work pretty well and behaved itself running less than about a
100-watts. However, when I increased my power to above 500-watts I was
able to work every electrical and electronic appliance in the house!
The common mode on the transmission can be a killer!

Danny, K6MHE

If I was going to run open line, I would not go with
an off-center feed. I don't think it buys you much.
On the other hand, off-center feed with coaxial feedline
can buy bandwidth over center feed. Our 80/75 dipole
is off-center fed with a 4:1 voltage balun followed
by a bead balun on the feedline. The SWR is quite low
from 3.5 to at least 3.8 MHz. Solid-state finals and
Alpha 87A finals like it very much. No trouble with
getting into appliances at 1500 watts.

73,
Bob
K3PH

--
+----------------------------------------------+
| Bob Schreibmaier K3PH | E-mail: |
| Kresgeville, PA 18333 | http://www.dxis.org |
+----------------------------------------------+

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