Ron D'Eau Claire wrote:
Kevin Schmidt, W9CF, has a nice dissertation that includes Roy's findings
and a reference to the original paper and where you can download it
(http://tinyurl.com/ctkwb). Kevin agrees with Roy, but Kevin says that the
reason it's "useless" to put the balun at the input is that it makes both
"sides" of the ATU "hot". Of course it does! You need a purpose-built ATU.
> <http://eznec.com/misc/ibalun.txt> (W7EL)
> <http://tinyurl.com/ctkwb> (W9CF)
That's not his point, Ron. He shows that the balun does not do a better job on
the input than on the output, and then says 'why bother building an isolated
tuner when it's no better than a conventional unbalanced tuner with a balun on
the output?' He shows that a true balanced tuner is one effective solution to
the problem.
As you know, the degree of imbalance in a parallel wire line can be expressed as
the ratio of differential current (flowing in opposite directions on both
conductors) to common-mode current (flowing in the same direction on both
conductors against ground). In a coax line one doesn’t talk about imbalance,
but the common-mode current is what (undesirably) flows on the outside of the
coax shield. The transmission line displays a differential impedance – that’s
what we usually refer to as the impedance ‘seen’ by the transmitter or tuner –
and a common-mode impedance which is based on its undesired function as an
antenna for common-mode currents.
What W9CF shows is that an unbalanced tuner, in transforming the differential
impedance to 50 ohms, inserts an unbalanced reactance which actually increases
the common-mode impedance. Therefore, a balun at the input to the tuner is
stressed exactly as much as one at the output.
This assumes that the effectiveness of the balun doesn’t depend on the
differential impedance of the line, and this is more or less true of true
current baluns such as bead baluns or transmission-line baluns. Voltage type
baluns, such as the 4-1 baluns often found on the output of cheap tuners do not
have this characteristic and so are even worse when working into extreme impedances.
On the other hand, a true balanced tuner like the Matchbox will reduce
common-mode current significantly in itself.
In my experience, there's nothing wrong with putting a balun at the input or
the output of a tuner. There's a lot can go wrong if one subjects that balun
to highly-reactive loads!
W9CF shows that for reducing common mode currents, the best choice is a balanced
tuner with a balun on the input.
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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