Richard Clark wrote:
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

When designing any transformer, be it audio, power, or RF, you usually
want to accomplish three things:

1. Maximize the winding impedance.
2. Minimize the leakage impedance. This is another way of saying you
want to maximize the coupling between the windings.
3. Realize an acceptably low loss.

At low frequencies, most cores primarily provide inductance, so most
literature dealing with transformers speaks of "inductance" rather than
"impedance" when discussing the basic goals. But low frequency
transformers could be made and would function just fine using resistive
core materials if they were available. The third goal can be achieved
with resistive materials as long as the first and second goals are
accomplished well enough. That is, if the winding impedance is high
enough, the magnetizing current will be low, so I^2*R loss will be low.
And if coupling is good, increasing the current in one winding won't
appreciably increase the core flux and incur extra loss.

But at RF, as Richard says, many ferrite cores are more resistive than
inductive. Not all are -- there are high frequency ferrites that remain
inductive at very high frequencies. However, for the HF range and above,
the best choice is usually "low frequency" ferrites which are primarily
resistive in that frequency range. They're best for two reasons --
first, they provide much more impedance per turn squared than high
frequency ferrites, making the first two goals much easier to achieve;
and second, there are no resonance effects. An inductive winding will
resonate with stray capacitance at some frequency. The impedance drops
above that frequency, limiting the upper range of the transformer. The
winding and leakage inductances can also resonate with external
capacitance and cause ringing in some applications. A properly designed
transformer using an essentially resistive core can operate very well
over several decades of frequency.

For most RF applications, it's not hard to design a transformer or balun
that will have well under 1 dB of loss, which is negligible again in
most applications. But if high power is involved, even a fraction of a
dB might represent several or may watts of power, resulting in
intolerable core heating. In those applications, lower loss, i.e., more
inductive and less resistive, cores have to be used, often resulting in
more difficult design and/or reduced performance in other respects.

Resistive ferrites are also often good choices for RF chokes for the
same reasons that they make good transformers.

High-Q inductors and tuned transformers are a different critter, though.
For those applications you want to use very low loss, that is, inductive
and not resistive, core materials. This often leads to confusion, since
ferrite manufacturers have traditionally specified a "recommended"
frequency range for various materials which refers to the best range for
these applications. The best material for an RF transformer is usually
one with a "recommended" (for high-Q inductor applications) frequency
range well below the transformer operating frequency.

Roy Lewallen, W7EL