Carlos42 wrote:
Could you elaborate a little on this? I am really
not very knowledgeable on this subject. I thought that by having the
the balun on the input rather than the output you eliminated the
possibility of operating the balun beyond its design parameters.
I was told that by not having the balun look directly into
the antenna you avoided having it see drastic changes in
impedance that it was not capable of handling. Presumably this
condition does not exist as much with the balun looking directly
into the tuner as the impedance is not capable of changing beyond
the balun's design parameters. Again, let me know the validity
of this theory as I am not an expert in this field.

I was a little bit too harsh on the MFJ design. For common-mode
currents, the four capacitors are in a series-parallel configuration
so that reactive impedance is in series with the impedance of the
choke. That impedance can be estimated from the capacitor dial settings.
We normally set the caps as close to maximum capacitance as possible
which is minimum impedance (reactance) so this is (for me) an unknown
in the common-mode equation. If that series reactance is low compared
to the impedance of the choke, it won't have much effect on common-
mode currents.

I gather that by having the four capacitors ganged in groups
two the chances of having the currents in each leg balanced are
eliminated. Also the lack of being able to meter the current in each
leg diminishes the chances of a totally balanced output.

The currents can be balanced by accident or due to a good design.
My balanced tuner design (on the shelf) uses two series roller
inductors and four parallel caps to ground that can be tuned
independently to achieve a better balance. It is actually two
pi-net tuners, one for each line.

So the question is how is the 974H better than my old MFJ
949E? I must say that at this stage,I can see no difference,
but I probably do not know enough to know how much I don't
know!(as we used to say in Nebraska).

The series-parallel capacitive reactance assists the choke by an
unknown amount so the 974H is better than the 949E. How much better
needs to be measured or calculated. How much better could be negligible.

One might take the suggested settings in the 974H manual and calculate
that common-mode series impedance.

Another question is why is a link tuner better in this situation?
(Tuning a balanced feedline antenna?)As I know of no link tuners
that are currently in production,what would you reccomend in this
situation?

A link essentially presents an open-circuit to common-mode currents
except for coil to coil capacitance which should result in a relatively
high common-mode impedance. Back in the good old days before coax was
readily available to hams, a lot of rigs were link coupled to open-wire
feedlines.
--
73, Cecil, W5DXP