I wouldn't think about a transformer at the feed-point. The transformer would
be some type of monster with a high impedance AND a high current capacity which
spells "heavy". As the feed-point frequently is as a high voltage point there
will be common mode current on the feed line. It is "just" a matter of
accepting it and, maybe place the floating tuner and the choke close to the
antenna.
I can see to put an outdoor tuner on top of the tower and let the OCF dipole
start from there. The, now remote controlled" tuner will need some RF isolated
control lines as well. I think I will let the radiating feed line be included
in the picture.
Hans N2JFS
-----Original Message-----
From: Jim Brown <jim@audiosystemsgroup.com>
To: towertalk <towertalk@contesting.com>
Sent: Mon, Dec 12, 2016 7:50 am
Subject: Re: [TowerTalk] Fwd: OCF Dipole Questions
Only to the extent that less RF current flows into equipment with Pin
One Problems. But that feedline still radiates -- the only thing that
changes is the tuner chassis is now hot with RF, and the tuner modifies
the current distribution on the line depending on how it's tuned. And
none of this changes the fact that off-center-fed antennas are
inherently noisy BECAUSE they are imbalanced, nor does it change the
fact that it is simply not practical to choke open wire line at the
feedpoint.
What COULD block feedline current is a real transformer on a low loss
ferrite core, IF the transformer had very low capacitance between
windings (because common mode current would couple through interwinding
capacitance). This means that windings could NOT lay next to each other
as is often done (bifilar, but connected as a transformer), but would
have to be on opposite sides of the core. And, of course, to handle
power, a beefy, low loss core would be needed. Something like Fair-Rite
#67, which N6RK has used for high power transformers.
73, Jim K9YC
On Fri,12/9/2016 3:46 PM, Hans Hammarquist via TowerTalk wrote:
> One way to get control over the common mode current is to place a good common
> mode choke between the tuner and the transceiver and let the tuner "float"
> electrically. Don't ground the tuner and leave it far from any grounded
> equipment. You don't stop the contribution from the feeder to the radiated
> feild but you reduce the RF in the shack generated by the common mode current.
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