Hi Paul -
I buildt a small, 100 watt, "balanced " tuner, using a "T" circuit, with
"floating chassis", insulated shafts and torriod at the 50 ohm end. Judging
from rf ameter readings in the balanced line and operation of the xmt amp
and results, it looks good. I use it for portable operation with a 45 foot
centerfed dipole as near vertical as I can get it and some 450 ohm line. It
works great on 10-20 and somewhat on 40. Adding horizontal loading wire to
the top end gets me on 40 and 80. With a carribean prefix (and being next
to saltwater) worked lots of EU and JA, including EU on 80 SSB.
Gene / W2LU
----- Original Message -----
From: "Paul Christensen" <w9ac@arrl.net>
To: <towertalk@contesting.com>
Sent: Monday, November 30, 2009 1:50 PM
Subject: [TowerTalk] Open wire lines
>> "Another point that it's critical to understand. Open wire lines can
>> carry
> significant unbalanced (common mode) currents if what is connected on
> either
> end is unbalanced." One method to help with a portion of the unbalance is
> to
> remote-feed the open wire line with a remote-controlled balanced tuner,
> with
> an attempt to push the tuner as close as reasonably possible to the feed
> point, again taking open wire portion of the feed line length into
> consideration. I am using a remote-controlled balanced tuner, based on
> AG6K's design. A couple photos of the indoor and outdoor units here:
>
> http://www.qrz.com/db/w9ac
>
> My dipole antenna and line is optimized for the harmonically-related
> bands,
> starting with 80m. To extract the best efficiency on the WARC bands
> really
> demands a change in open wire line length (even with the use of the
> tuner),
> but for me, that requires the addition of DPDT relays with selectable line
> lengths chosen on the open wire line.
>
> Although I don't have any measured data myself, W8JI and W7EL have claimed
> that moving the output balun to the input on a common "T" design
> accomplishes no useful benefit and may under some circumstances degrade
> performance. There again, unless great care is taken, it's easy to
> unbalance currents at the tuner if the chassis is part of the balanced RF
> return. It takes floating the modified circuit above a metal cabinet to
> achieve some semblance of balance and a safe operating environment. But
> with all that work, a better alternative for open/ladder line use calls
> for
> a symmetrical balanced tuner or link-coupled tuner.
>
> Paul, W9AC
>
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