w3tw_2 wrote:
BTW any recommendations on an external tuner for open wire lines?
This could become an endless debate, but my experiences over the years
with the typical unbalanced transmatch with associated 4:1 balun on the
output have not been good. Under widely varying, highly reactive loads
and depending on the frequency, baluns do not provide balanced output to
each feeder wire. They weren't designed for this service. For an
explanation of this problem, I refer you to an article by Rich Measures,
AG6K, in the February, 1990 issue of QST (if memory serves me
correctly). The result is that your feedline becomes part of your
radiating system which can possibly pick up any noise, etc. being
generated from your equipment, as well as possibly cause rfi, TVI, and
the like. I found the problem to be worse on the higher bands 20-10
meters and not so evident on 80/40 meters.
I've used homebrew tuners and for the past decade have been using a
Johnson KW Matchbox which provides a true balanced output.
I'veexperienced absolutely no rfi problems even when running my Hercules
II amp. The ladderline feeder to my 80 meter center fed doublet acts as
a feedline, as it should.
Another matching network which shows promise to provide a balanced
output is the Palstar AT4K which uses a 1:1 balun at the INPUT side and
utilizes components floating above ground. The balun sees 50 ohms at
both the input and, when the matching network is properly adjusted, at
the output as well, thus allowing it to make the balance-to-unbalance
transformation. I beta tested a 300 watt version ofthis circuit and it
seemed to do well up through 10 meters. For information on this matching
network, see: www.palstarinc.com (again, if memory serves me correctly).
For what its worth...
Vince/K7NA
--
FAQ on WWW: http://www.contesting.com/tentecfaq.htm
Submissions: tentec@contesting.com
Administrative requests: tentec-REQUEST@contesting.com
Problems: owner-tentec@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|