[TowerTalk] Open wire lines

[Paul Christensen]

> "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