I had exactly the same experience as Roel and reached the same
conclusions. I agree that a commercial BALANCED autotuner would be
the best solution for an all-band dipole, if there were such a thing.
Like Roel, I attempted to use a KAT2 and balun arrangement for all-
band feeding of a simple dipole at the center with open wire line. I
put the balun right at the output of the KAT2--no coax at all.
However, both with actual measurements and with simulations using
dipole3.exe, I could find no combination of feedline length and
antenna length for my QTH that would present an acceptable match for
the balun (either 1:1 or 4:1) on all bands. High reactance was
usually the problem, although in some cases the overall impedance was
too high for the tuner. With the best of compromises, I determined
from measurements that on some bands about half of my power was
heating the balun. True, that's only 3 db, about half an S unit.
However, it's like degrading my KPA100 to 50 watts, and not many of
use would accept that. The KAT2 alone, with balanced lines connected
to the SO239 and ground, didn't have sufficient range to match some
bands, and the balance was terrible on others. Maybe I'm
overlooking something, and I'd sure appreciate a response from
someone with difference experience or more knowledge.
Before getting my KAT2 I used a link-coupled tuner I'd built, and it
would match anything with high efficiency and excellent balance. But
band changing with plug-in coils was a pain. Then, like Roel, I
acquired an MFJ-974H balanced tuner, and I am also most pleased with
the results. I built a twin current meter to measure balance in the
feedline, and balance and efficiency is as good with the MFJ tuner as
with my link-couple tuner. I can tune the MFJ tuner for a new band
in a few seconds from a chart of settings I developed. Also, I keep
a noise bridge in the antenna line, with capability to switch it in
when I want it, and that will allow a quick fine tuning (a couple
seconds, usually) without transmitting on the air.
However, it would be so nice to have a wide-range balanced
autotuner. I couldn't find one on the amateur market.
Dick, K0KK
On Sep 6, 2007, at 3:07 , Roelof Bakker wrote:
I purchased a MFJ 974HB and though I did not have very high
expectations of it, I was pleasantly surprised. It matches the
doublet fine, but that is not the point. The balance seems to be
excellent as most of my local noise is now gone, compared to using
a 1:4 balun.
Though this works for me at the moment, it should be nice if
someone (Elecraft?) could come up with a stand alone truely
balanced automatic antenna tuner.
73,
Roelof Bakker, pa0rdt
Middelburg, Netherlands
JO11tm
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