> If you don't need the 'auto tune' functionality, I'd
> strongly suggest locating or building a tuner designed to
> drive balanced lines, like my Johnson Matchbox tuners. No
> balun/core heating/arcing to be concerned with.
Let me second this motion concerning ATUs. Most ATUs based on single
ended netowrk design do not have the range of output values necessary to
effect a match at the lowest possible network loss over a wide range of
reactance values. (The X-match commercial design overcomes this to a
large degree on lower bands.) Additonally, most baluns used to change the
single-ended network output to a balanced output have efficiency limits in
the face of large reactive components to the impedance presented by the
line to the terminals. There are techniques of both calculating and
measuring presented line impedances and then selecting line lengths to
place these values within the high-efficiency range of the ATU assembly.
W6RCA uses such a system, which he describes at his web site.
There are alternatives that allow for faster, although manual tuning. One
is the old inductively coupled tuner, which has disappeared from modern
construction due to our love affair with coax. Only one German firm makes
commercial units, to the best of my knowledge.
A second alternative is to construct a balanced network tuner and place a
1:1 balun on the transmitter side. Standard handbook formulas for
converting single ended calculated values to values for a balanced network
(PI, Tee) are available, although the component search and the ingenuity
to couple capacitor shafts, etc. may be initially daunting.
I'd love to see truly balanced tuners back on the market again.
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