... or a link-coupled tuner, which works just fine without a balun

How efficient is link-coupling? I have an old
Sunair PN-099400 auto-tuner that uses motor-
driven variable-link coupling. Too bad it runs
on 28 vdc.

I'm not sure that there's any single answer to that question.

Comments on eHam.net from owners of the old Johnson (or Nye) Matchbox
link-coupled tuner seem to suggest that it's capable of operating with
low losses - significantly lower than are achieved by some of the
unbalanced T-configuration tuners mentioned.

I suspect that the efficiency depends very much on the design and
implementation of the tuner (e.g. air-core coils vs. a toroidal
design, size of core, degree of coupling between the tank and the
link, wire gauge) and on the specific impedance being matched.

Based on what I've read about Z-match tuner design, it seems that
close coupling across the inductive link helps boost the efficiency.
However, the close inductive coupling tends to come with an increased
amount of capacitive coupling, and this can apparently affect the
output balance for the worse in some cases.

I suspect (but don't know for sure) that link-coupled designs which
use a fixed-position, tightly-coupled link (e.g. an air-wound or
toroid-core Z-match) may be more efficient than those which use a
"swinging" link with variable coupling.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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