On Mon, 19 Jan 2009 09:43:27 -0800, Jeff Liebermann
wrote:

The over-simplified idea behind the ferrite (or powdered iron for HF)
cores is to increase the inductance of the windings so the xformer
works at lower frequencies. At VHF and UHF frequencies, the core
mostly disappears and you're relying totally on the coupling between
the bifilar windings.

Actually that is not the mechanism at all, simplified or complexified.

Ferrite compounds for BalUn application principally add resistance to
the common mode path and have (as a design goal) absolutely no
magnetic influence whatever. The increase of inductance is haphazard
at best, and some ferrite compounds actually increase the capacitive
reactance. As such, ferrites are not principally employed in building
inductors. Further, iron cores don't serve much purpose for choking
which is the principle requirement for BalUn input to output
isolation.

If you want to build an iron core, magnetic linkage transformer (AKA
audio or power transformer), this is a very different beast from a
BalUn, and a miserable replacement.

73's
Richard Clark, KB7QHC