Paul,
Great info, and awesome workmanship!
Having said that, what are the benefits of doing all of that work, over 450
ohm ladderline, then using a well made 4:1 current balun outside the
shack, and a short run of rg-213 into the shack, and using a Dentron MT-3000A
tuner? I've used this setup for awhile here, at 1kw, and an 80 meter dipole
at 50', and have been thrilled with the results. No RF in the shack, no
heating of the balun, or the tuner, and, everything works great. Am I missing
out on something?
73- Chuck KI9A
In a message dated 11/30/2009 12:51:16 P.M. Central Standard Time,
w9ac@arrl.net writes:
> "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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