Yes, moving a current balun to a position before the tuner is a
"useless" exercise but it doesn't render the balun "useless" -- it's
equally effective at the input as at the output (provided that the tuner
case is floated if the balun is at the input).

To answer your question about why not use one at the input and output, I
recommend against using one at the input because the case of the tuner
has to be floated for one at the tuner input to be effective. This has
some potential safety implications. If you want to use two baluns in
order to increase the common mode impedance, I suggest putting them both
at the output. And for maximum effectiveness, space them about a quarter
wavelength apart at the most troublesome band.

Roy Lewallen, W7EL

Cecil Moore wrote:
Jack Painter wrote:

6. Conclusions
As noted by Roy Lewallen, W7EL,[2] putting a choke balun on the input
of an
unbalanced tuner to drive a balanced line is useless.


The choke on the input is *still* in series with the common-mode
current and is still performing the choking function. Moving the
balun to the input doesn't relieve the stress on the choke but it
also does NOT render the choke non-functional as the word "useless"
implies.
--
73, Cecil http://www.qsl.net/w5dxp


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