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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