Ian Jackson wrote:
People keep saying that they use 4:1 or 9:1 baluns.
Often I can see no reason for this (such as when simply feeding a dipole
with coax, or on the output of an ATU).

I'm obviously missing the point. Why the impedance transformation?
Ian.

The feedpoint impedance of a dipole varies with frequency from a
low value of about 50 ohms to a high value of about 8000 ohms.
In between, it can look like 1000(+/-)j1000 ohms.
Baluns with a high transforming ratio try to take the impedance
lower so, for instance, with a 4:1 balun, the above values would
ideally go from 50 ohms to 12.5 ohms, 8000 ohms to 2000 ohms,
and from 1000+j1000 ohms to 250+j250 ohms. You can see that the
second set of values have a smaller absolute range than the first
set, thus making them an easier match for the tuner.

That's the simplified version. There are disadvantages to using
baluns with a high transforming ratio.
--
73, Cecil http://www.qsl.net/w5dxp


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