Dave Platt wrote:
... 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.
--
73, Cecil http://www.qsl.net/w5dxp

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Dave Platt wrote:
The other approach is to put a choke balun at the input of the tuner,
"float" the tuner chassis clear of ground, and connect the balanced
line directly to the "unbalanced" output of the tuner. This
pseudo-balanced arrangment can provide good balance once the tuner is
adjusted, because the balun "sees" 50 ohms on both sides... but it
adds the complication of having to float and insulate the tuner.

There's considerable debate on that configuration. If we assume
the common-mode current doesn't change when we move the choke
from output to input, i.e. it is a systematic problem, then the
choke is exposed to exactly the same common-mode current on output
and input. Common mode current travels on the tuner chassis and the
coax shield input and coax shield output may be one inch from each
other with a dead short between them.

And here's something that no one, to the best of my knowledge,
has mentioned. If one succeeds in balancing the currents at
the input of an unbalanced antenna tuner, the currents at the
output will automatically be unbalanced because the current on
the coax center wire will suffer a greater phase delay than
the current on the coax inner braid thus forcing some current
to the outside of the coax. This effect should be easy to measure.

Link coupling solves the common-mode problem. Unfortunately, the
MFJ balanced tuners do not relieve the stress on the choke. Take
a look at the schematic to see why.
--
73, Cecil http://www.qsl.net/w5dxp

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... 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
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

You'll still need a balun even with a balanced tuner.

Save yourself the expense of a pair of ganged roller coasters.

Reg Edwards wrote:
You'll still need a balun even with a balanced tuner.

If it's link-coupled, like the Johnson Matchboxes, the
link performs the cancellation of the common-mode current.
--
73, Cecil http://www.qsl.net/w5dxp


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