Richard Clark wrote in
:

....
The 974 attempts to do this:
http://www.mfjenterprises.com/pdffiles/MFJ-974HB.pdf
and by their stated intentions, they are aware of the design issues.

I am sceptical of the "true balanced tuner" approach.

For one thing, I do not recall having ever seen measurement of the
common mode impedance reported.

If the objective is current balance, high common mode impedance is
essential.

Some folk seem to think that symmetric design is the sure path to
success, but it isn't. If you take a 1:1 Guanella with extremely high
choking impedance, the currents in its output wires will be almost
perfectly balanced, irrespective of the voltage from each terminal to
ground. If you placed 10pF of capacitance from each terminal to ground,
you appear to have preserved symmetry, but the currents in those
capacitors will not be equal unless the load is symmetric. In cases
where the currents in the balun wires are almost equal and the currents
in the capacitances I mentioned are not equal, then those capacitances
have probably compromised common mode impedance.

Most implementations of a "true balanced tuner" have large stray
capacitance from each side to chassis.

But, savvy sellers will offer them to the people who are attracted by
the concept of a "true balanced tuner".

The joke of balun offerings is those designs purported to work well on
isolated loads. If the concept of the load is the overly simplistic two
terminal network, and it is isolated from ground, clearly current into
one terminal MUST equal current out of the other terminal, no balun is
required.

Owen