Richard Clark wrote in
:

....
MFJ is not making a killing on this particular poor winding (OK, call
it historical inertia) practice, and their market for these internal
BalUns is dipoles, not monopoles.

Hi Richard,

I see MFJ getting mention, and whilst they may deserve a bit of a
flogging for some things, I spring to their defence on this occasion.

MFJ make the claim "More hams use MFJ-949s than any other antenna tuner
in the world!" and gauging from questions in online fora, they are
indeed popular, the claim may be correct.

The MFJ949E uses a Ruthroff voltage balun.

I have measured the balun losses in my '949E, and they are as I
discussed in the general case earlier, quite high on high impedance
loads. Anyone with a rudimentary knowledge of balun design would
understand that, but there are those who apparently live in an ideal who
would refute it.

The simple explanation is that most loss in practical ferrite cored
baluns usually results from losses in the core material. The losses in
the core material are related to flux density, which in a Ruthroff
voltage balun is roughly proportion to the differential voltage, and in
a Guanella current balun is roughly proportional to common mode current.
There is no reason to think that these two different types of baluns
would have identical losses.

The MFJ949E could easily be reconfigured as a Guanella 1:1 balun by
changing its end connections, but that does not make it an optimally
designed current balun. I haven't done it, and so cannot comment
further. I cannot see how the cost of manufacturing it wired as a
current balun would be any different.

I think that it is the buyers who determine the market, and savvy
sellers cater to the buyer's wants.

While anecdotal evidence abounds that 4:1 voltage baluns match up
extreme loads better, and users mostly arent't interested in finding the
root cause of the problem and fixing it, voltage baluns will be seen by
most buyers and savvy sellers as the solution.

A good demonstration of the credibility of anecodotal evidence is the
massive online support for the Array Solutions 4:1 Ruthroff voltage
balun / ZeroFive unloaded vertical combination. The configuration drives
high common mode on the coax feed line. (I should note that the sellers
recommendation has recently changed to an unun.)

MFJ is no doubt one of the savvy sellers. They do BTW have some higher
end ATUs with 1:1 current balun, as does the Ameritron label for those
who want a current balun.

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.)

Owen