>From: John Seboldt <rohrwerk@pcOnline.com>
>However, it is also true that any balun on the output will see
>variable impedances in the typical multi-band "tuned line"
>approach. This means that if the line input impedance is high,
>the balun's reactance may not be adequate to act properly.
>Thus, in many circumstances, a balun on the output of an
>unbalanced tuner is pretty dumb, becuase it just can't do its
>job. So if you're gonna do it, put it on the input for sure.
Hi John, this seems to be turning into a myth. Roy, W7EL, has
a mathematical proof on his ftp site that it doesn't make any
difference whether the choke/balun is on the input or the output
of the tuner. It sees exactly the same common-mode impedance in
either place and the common-mode impedance is exactly what a
choke/balun has to deal with. You are talking about the
differential mode impedance, not the common-mode impedance.
Common-mode current is what stresses a choke/balun. A 1:1
choke/balun completely ignores the differential mode currents
and differential mode impedances. How much flux flows in a
1:1 choke/balun because of a differential mode mismatch? None,
because the fields are completely contained in the coax and
no current is flowing on the outside of the coax.
The choke/balun installed at the antenna feedpoint of a dipole
has to deal with the current trying to flow down the *outside*
of the coax. What impedance that current sees is in parallel
with the antenna impedance. It has everything to do with the
length of the feedline and feedline pickup and is generally
unknown. It can be nearly zero ohms.
73, Cecil, W6RCA, OOTC
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