An astute Towertalk reader asked me off-list whether the ferrite that
I had salvaged from my burned-out common-mode choke/balun was still
good. He said he had read that frying ferrite ruins it.
I replied that I did not know how ferrite could be damaged in a
garden-variety fire unless it was mechanically fractured by rapid
differential thermal expansion, because ferrite is a ceramic
material, and it is refractory.
Ferrite (like all ferromagnetic materials) loses its ferromagnetism
above its Curie temperature, but AFAIK it recovers fully when it is
cooled. (A "hard" ferrite loses its "permanent" magnetism, but it
can be remagnetized. The ferrites used in chokes, inductors, and
transformers are "soft" and do not retain much magnetism.)
But there's no substitute for a good experiment, so I rewound my
choke and measured it.
When the choke was brand-new, I had measured its inductance to be 37
microhenries and its series resistance to be zero, within the roughly
one-ohm precision of my Autek RX Vector Analyst. The measurement
frequency was 1 MHz, where the measured reactance was about one-half
of the range (500 ohms) of this instrument.
The ferrite rod that I salvaged from the incinerated choke/balun was
a blackened mess, but I had scoured and scrubbed it clean. Despite
my scouring and scrubbing, it still bore a visible imprint of the
original winding, which appeared to have had 26 or 27 turns. So I
rewound it with 27 turns, spaced like the original winding, and used
the same Autek instrument that I had used originally, to measure the
impedance of the rewound choke. I measured it at f = 0.75 MHz and
also at 1 MHz. Both measurements yielded the same inductance, of 40
microhenries, and the same series resistance, of zero.
So frying this ferrite did _not_ change its magnetic properties.
When you burn out a choke/balun/transformer with a ferrite core,
salvage the ferrite, which is usually the most expensive part.
73 de Chuck, W1HIS