[EMAIL PROTECTED] wrote:
In a message dated 10/13/07 4:58:04 PM Eastern Daylight Time, [EMAIL PROTECTED]
writes:
There is no difference if the balun is on the input or output side of an
unbalanced tuner.
In theory, no.
In practice, there can be a big difference.
See
<http://fermi.la.asu.edu/w9cf/articles/balun/balun.html#SECTION00050000000000000000>
He's not looking at the big picture.
His point is that if you use an unbalanced tuner with the balun on the
input, then there is a common-mode reactance that is equivalent to the
differential-mode reactance that is being canceled by the tuner which is
reflected back to the balun. So the problems that the balun has on the
output do not go away when you move it to the input.
Using a *balanced* tuner followed by a balun or a link-coupled tuner
like the Matchbox solves this problem. But what many of us want to do is
to feed a balanced line from a radio with a built-in automatic (but
unbalanced) tuner, like the KX1-K3, without adding an additional tuner.
If the balun is ideal, or close enough to ideal, or if the shack-end
impedance of the balanced load are within a certain range, the
unbalanced-tuner-with-balun-at-the-antenna-end idea works fine. Thousands of hams use it with no
problems and good results.
But in some cases the shack-end impedance of the balanced line can be very
high, very low, and/or highly reactive. Under those conditions some baluns don't
work well, and all sorts of odd things happen. Sometimes the end result works
well enough that the ham doesn't notice anything wrong, particularly if s/he
has nothing else for comparison.
Absolutely correct, and I recommended exactly what you do: measure the r
and x on the balanced antenna and then either change the line length,
add reactance to compensate for the x, and choose an appropriate balun
ratio to get as close as possible to the r. Of course, you can't always
do this easily for all bands!
You can't just blindly increase the number of turns on a wound-core balun to
increase the impedance because you may set up self-resonances that cause all
kinds of fun.
Although if you are only interested in a few bands it's easier.
The best approach IMHO is to model the antenna-feedline system and see what
the actual shack-end impedances are. Or measure them. Then decide what tuner
setup is needed to do the matching job.
Modeling it is harder than it looks because all kinds of things affect
the real antenna, and sometimes the impedance at a particular distance
along the line from the antenna changes rapidly. My antenna was an 88'
inverted V with the apex at 40 feet, so the ends were not that far from
the ground, causing incorrect modeling results on the lower bands.
I have an Autek VA-1 antenna analyzer which is small and has a
self-contained battery, so I just put it on a large carton and hooked it
directly to my parallel feeders. An MFJ with an AC supply got incorrect
results because of capacity to ground. I was lucky, and on the bands of
interest, 80 and 40 meters, I got r ranging from 15 to 160 ohms and x
was inductive in both cases. So I chose a value of capacitance which
canceled the inductance on 80 (the band on which tuning was the
sharpest) and which reduced the reactance on 40 significantly (and also
incidentally changed the sign).
Once I did this I got good results from a 1:1 balun. I tested it by
attaching a thermocouple to the core and measuring the temperature rise
before and after adding the reactance compensation. Without the
capacitors, the temperature went up several degrees when I transmitted
at 100 watts for one minute. With the capacitors, there was no
measurable temperature rise.
The SWR seen by the radio was below 8 to 1 throughout the CW bands on
both 80 and 40 (as low as 2:1 at one point on 80) so the internal tuner
had no trouble matching it.
I also checked for balance by using an RF sniffer on the feedlines at
the exit of the balun and got good results.
Performance seems to be no worse than with the Johnson Matchbox -- and
now I can operate on 80 and 40 meters with this antenna without tuning
anything, something that makes life easier in contests.
The balun I used was a huge home-made choke with six (!) ft-240-31 cores
wound with 8 turns of RG-58/u. This is only intended for the low bands
and would probably be poor on the high bands, but the Elecraft balun
should work well on all HF bands.
There's an article in the latest QEX that describes a method of using a
combination of a voltage balun and a current balun to get better
performance in situations characterized by wide ranges of impedance. I
would have tried this if I hadn't been able to meet my needs as
described. The Elecraft balun is a current balun, and one could add a
simple voltage balun. Someone should try this!
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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