> It is still common practice in bandpass filters by a new generation of
> qualified engineers.
And for a good reason: Iron powder materials span the range of
permeability needed to easily build compact, low cost, low loss,
temperature-stable, high Q inductors ranging from a fraction of one uH
to several tens of uH. These are the kind of inductors used in lowpass
filters, resonant impedance matching networks, and other such circuits.
Ferrites instead have very much higher permeability, giving them a large
advantage in broadband transformers and chokes.
If a ferrite core was used in a lowpass filter, the core would have to
be larger than a powdered iron core, and would need an air gap. This,
combined with the fact that ferrite is more expensive than iron, makes
ferrite a bad choice for such resonant applications. On the other hand,
using powdered iron in broadband circuits requires many more turns of
wire, causing additional loss, and a lot of problems from stray
capacitance and delay along the long wires.
I don't know which core is meant, but if it is used only on 80 meters,
it must be working in resonant mode, and thus it should be powdered
iron. There is pretty much a standard (even if not totally) about the
color scheme used to identify the exact sort of material. This, along
with the size of the core, allows easy identification of the core.
If instead the core is not color coded, and is really damaged to a point
that it doesn't work correctly, it might be necessary to
reverse-engineer the filter it's working in, to arrive at the inductance
value needed, and from there find the exact material mix.
In any case, iron powder materials commonly used at 80 meters tend to
have a permeability of 10 or sometimes a little more.
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