On Sat, 3 Oct 2009 21:08:57 +0200, "Antonio Vernucci"
wrote:

I would like to hear some opinions on the usefulness of using the transformer
and the twin-lead. My arguments a

- above 10 MHz, where impedance is generally fairly high, the transformer could
(??) be helpful to reduce SWR, but below 10 MHz it should be harmful, as its
transformation goes on the wrong side.

Hi Tony,

Good of you to notice that the 1:4 can also be viewed as 4:1 (or did
you notice?) with it reversed. This is not strictly so, as the
classic BalUn consists of wound transmission lines with characteristic
Z at the geometric mean of the load and source Z. However, lacking
this doesn't always stop the experimenter.

- use of the twin-lead is usually justified by its low attenuation, that limits
the extra attenuation caused by the high SWR existing on the line. In my opinion
the 300-ohm twin-lead attenuation is reported to be low mainly because it is
measured in presence of a 300-ohm load, that draws low RF current. But in the
subject application, where the antenna impedance is uncontrolled, what should
count is the ohmic resistance of the twin-lead conductors which is not
particularly low due to their fairly small diameter. Wouldn't an RG-213 do
better than the twin-lead?

If your BalUn has already done the bigger job of turning a High Z to a
modest one, the common logic for the need for twin line has also been
diminished.

Working with, designing, and building BalUn applications demands a
good tool for validation. Do you have something that will measure Z
with accuracy?

73's
Richard Clark, KB7QHC

Good of you to notice that the 1:4 can also be viewed as 4:1 (or did
you notice?) with it reversed.

That is obvious, but in common designs there is no provision (e.g. a relay) for
reversing the transformer when operating on frequencies where the antenna
resistance is lower. So the transformer remains there as it is, doing the
opposite job of what it should do.

Working with, designing, and building BalUn applications demands a
good tool for validation. Do you have something that will measure Z
with accuracy?

No, I am not building that antenna or doing any experiment. I am only trying to
understand the rationale behind what people proposes and I am seeking advice
from people having specific experience on the issue.

73

Tony I0JX.