steveeh131047 wrote:
It's interesting to model this sort of arrangement, note the wide
range of feedpoint impedances that will be seen on the different
bands, and observe the feedline losses that might be incurred when it
is directly fed with 50 ohm coax. If you then introduce an ideal
transformer at the feedpoint and repeat the exercise you will
generally find that feedline losses increase on some bands and
decrease on others.

With the vertical length I tried, the effect of a 1:9 transformer was
to limit the more extreme losses at the cost of making some very low
losses higher. I guess over the several bands I tried you could say
there was a net improvement with the transformer.

But one question would be how to build this ideal 1:9 transformer
which maintains its transformation ratio and exhibits zero loss across
the wide range of impedances and frequencies involved.

Steve G3TXQ

Some time ago I made a series of careful measurements of a transformer
which was at the feedpoint of a multiple band antenna. At frequencies
where the feedpoint impedance was very much different from the (purely
resistive) design impedance, the transformation wasn't equal to the
design transformation, and the transformer introduced both series and
shunt impedance. At some frequencies, these effects were extreme, and
the transformer acted nothing at all like an ideal transformer.

Modeling a system like this with an ideal transformer might be an
interesting intellectual exercise. But that's all it is -- the real
system won't behave anything like the model. You can extend a
transformer's range of impedances and frequencies by using great care in
the initial design and construction, then adding compensating circuitry.
The job gets more difficult as the transformation ratio increases. I
seriously doubt you'll ever come close to making a transformer anything
like the one described in the last paragraph.

The amateur way is to build a system with a transformer, then figure out
how to live with whatever you get. An engineering approach usually
involves designing a system with predictable and repeatable performance,
and that precludes depending on a transformer over a wide impedance range.

Roy Lewallen, W7EL