On Thu, 29 Sep 2011 21:06:40 +0000 (UTC), Owen Duffy
wrote:

My own view is that achievement of highest choking impedance in a
current balun is assisted by minimising stray capacitance to 'ground',
so I would prefer to put a current balun in a non-conductive box,
outside the ATU, on a foot of coax to the ATU. (This is one reason why I
haven't tried converting my '949E to current balun connection, the
windings are quite close to grounded metal and I expect stray
capacitance to ground is higher than desirable.)

Hi Owen,

The internal modification would also demand a balanced tuner topology,
and not the garden variety pi configuration. Your stating stray
capacitance, coupling to ground, etc. is a preface to this.

Put simply, and as described, the 949E and its ilk are lumped
transmission line UnUn transformers. You might salvage the box in a
re-design, but it would require gutting the entire interior circuitry.

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.

73's
Richard Clark, KB7QHC