Jim,
Correct me if I'm wrong but I would not recommend to used a dipole totally
isolated from ground. My personally experience with that is that you will have
problems with electrostatic discharge.
The feedline from my OCF dipole is part of the radiation system even if I
really didn't intend to do that., Unlike most locations, I have no problems
with local noise coupling into the feed line.
I would be interesting to know if it is possible to design a special balun that
can balance the two unequal parts of the OCF with a "no common mode current" in
the feed line but still with a galvanic connection to avoid static build-up.
Hans - N2JFS
-----Original Message-----
From: Jim Brown <jim@audiosystemsgroup.com>
To: towertalk <towertalk@contesting.com>
Sent: Wed, Feb 25, 2015 7:51 pm
Subject: Re: [TowerTalk] Fwd: Fwd: effect of two wire antennas end to end
On Wed,2/25/2015 4:30 PM, Hans Hammarquist via TowerTalk wrote:
> if you try to put a common mode choke at the antenna I can imagine
difficulties with overheating etc as the voltage is high.
Huh? A proper common mode choke can handle legal limit at the feedpoint
of centerfed dipole fed with coax. Study k9yc.com/RFI-Ham.pdf
> I allow the unbalance current through the feeder and I stop it after the
tuner.
That setup WILL have interaction between feedlines, and the feedlines
will pick up RF noise that they will couple to the antenna.
> A well design (unlike mine) Windom, or Off-Center-Fed antenna has a resistive
200 - 400 ohm impedance at the feeding point allowing for a balun at the
feeding
point and a reasonable SWR in a coax feeder. I will not recommend what type of
balun to use but imagine that the "right' balun reduces any common mode current
outside the coax.
There's FAR too much imagination (as in Alice in Wonderland) in that
paragraph. The word "balun" is far too vague, because it is used as if
it were some magic box that solves all problems, and has no
limitations. I believe in calling a spade a spade. An ordinary
transformer, wound on a low loss ferrite toroid, and with sufficient
insulation between the windings and the core, could isolate the line
from the antenna, thus reducing the common mode current. That
transformer will kill common mode current on the feedline ONLY if it is
at the antenna.
> I have not problem with overheating maybe my 100 W is one reason.
It certainly is. Try running an amp.
73, Jim K9YC
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