Topband: T-200 vs. T-300

[Rick Karlquist]

Martin wrote:

>
> BUT if you go QRO, you should stick with the iron powder cores.
>
> I once replaced the T-200-2 in my homebrew s-match with a ferrite. I
> changed back the same day, cuz after a few seconds with 750W the ferrite
> got so hot you would not want to touch it. The T-200-2 runs cool over an
> entire contest. Next step is to replace it with a T-300A-2.

I don't agree with this generalization.  I have built many legal
limit transformers and baluns with ferrite, and have not had
overheating problems.  Of course it is necessary to use the correct
type of ferrite for the application and the right number of turns on
it, etc.  It is hard to comment specifically on your anecdotal data
on your ferrite failure without additional details.  The main
misunderstanding I have encountered is that users sometimes choose
a ferrite with a very high permeability, but which has a high
loss tangent at the frequency of interest.  It is often better to
use a lower permeability material with a low loss tangent.  Although
the shunt impedance may not be as high in the latter case, the
parallel resistance (Rp) will be much higher and hence the loss
will be much lower.

Powered iron cores have a very limited permeability range, well below
100.  They are cheaper than ferrite cores and can be made to work
by using a sufficient number of turns.  They are mainly useful for
making inductors rather than transformers, because the Al value is
more closely specified than ferrite.  Also, you want a low Al value
when making an inductor because you get improved inductance resolution
(since the number of turns must be an integer).

Rick N6RK