From: "Allan Henry Kaplan" <w1ael@mindspring.com>
> W4EF wrote,
> "....the ferrite
> material in the KLM balun to overheat beyond its Curie
> temperature. Although a postive result from this test
> wouldn't necessarily entail permanent degradation of
> the balun,...."
>
> Sorry, Mike,
> A magnetic change resulting from exceeding the Curie temperature DOES
> definitely entail permanent degradation of the balun for all practical
> purposes.
> Allan, W1AEL.
Allan,
I think what your saying is true for permanent magnets.
In that case the orientation of the magnetic domains in
the material are randomized by the elevated temperature,
causing the net magnetic field from the material to average
to zero. For a ferrite material, the magnetic domains
are not arranged in any particular order until the material
is subjected to an external field. It is my understanding
that above the Curie temperature, the thermal agitation
prevents the magnetic domains from aligning with an
externally applied field. Thus the permeability of the
core is reduced significantly. Assuming you don't continue
heating the material until it goes up in smoke, this
process should be reversible. From the Ferronics website:
>>>ENVIRONMENT:
Ferrites are inert ceramics free of any organic
substances. They will not be degraded by most
environments, including temperatures up to a few
hundred degrees centigrade. Magnetic properties vary
somewhat with temperature. Generally, inductance
increases with increasing temperature while the effect on
resistance is small. Above the Curie temperature the
bead is non-magnetic and no suppression can be
expected. This effect is completely reversible and once
the temperature is reduced below that point, normal
performance is regained.<<<<<<<
Permanent degradation can occur however if the
core gets too hot too fast. About 20 years ago when
I got my Ten-Tec 4229 antenna tuner kit, I cooked
the tiny little core in the 4:1 balun after about 2 minutes
on 75 meters driving it with a Heathkit SB-221. The
core got so hot that it cracked in two pieces :):)
Dick Frey, K4XU the then chief RF guy at Ten-Tec
gave a then young kid a long lecture about the
impedance matching limitations of ferrite baluns
when I called to complain about the failed balun.
73 de Mike, W4EF..........................................
----- Original Message -----
From: "Allan Henry Kaplan" <w1ael@mindspring.com>
To: <W4EF@dellroy.com>; <tentec@contesting.com>
Sent: Tuesday, March 30, 2004 8:20 AM
Subject: Toasting those cores....
> W4EF wrote,
> "....the ferrite
> material in the KLM balun to overheat beyond its Curie
> temperature. Although a postive result from this test
> wouldn't necessarily entail permanent degradation of
> the balun,...."
>
> Sorry, Mike,
> A magnetic change resulting from exceeding the Curie temperature DOES
> definitely entail permanent degradation of the balun for all practical
> purposes.
> Allan, W1AEL.
>
>
>
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