It is possible to create a tuner specifically to be fed with coax
and output to balanced line. The transformer that couples the
signal has its secondary floating and the capacitor on that side
is split, or ganged so that half is on each end of the secondary.
The primary is a standard grounded circuit for input with the
coax. It can be looked at as a floating secondary with a pair
of ganged tuning capacitors to which the input signal is coupled
with a singled ended tuned loop. By adjusting the turns ratio
you can convert any reasonable impedance range up or down.
The simple approach of including a balun is widely used in commercial
tuners because it allows one to add a balanced input easily to
an unbalanced input/output design. If you really want a tuner
designed specifically to hook only to balanced wire, simple
designs exist, though the split capacitors the secondary wants can
be hard to find. Often capacitors can be found that have a bit of
shaft protruding at the rear and a coupler can be used on this
to couple to a second identical capacitor.
Check out OLD ARRL Handbooks.
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