Topband: -77 ferrites

[Rudy Severns]

Mike Tope's measurements are exactly what you would expect for a
manganese-zinc ferrite.  If you take a look at the complex
permeability versus frequency graph for this or similar MnZn ferrites
you will see that the permeability peaks at the ferro-magnetic
resonance point and then falls rapidly.  This means that at HF the
inductance goes very low as the permeability drops.  Even for the
highest frequency MnZn ferrites this is less than 2 MHz.  When you use
this material as an inductor and measure its impedance above the
resonance point what you get is basically a resistor which is what
Mike sees.

When you use this material in a transformer the effect is that the
shunt magnetizing inductance is replaced with what you might call a
shunt "magnetizing resistance" which is actually just a core loss
term.  That doesn't mean you can't use the material in a transformer
at HF but you do have to realize that the equivalent circuit for the
transformer is somewhat different from LF.  For small signal
applications it may be perfectly acceptable.

-43 material is a nickel-zinc ferrite which is an HF material.

If you are interested in more details, get your hands on a copy of
E.C. Snelling, Soft Ferrites-Properties and Applications.  This book
has a lot of detail but is actually quite readable.

73, Rudy N6LF