Bill Turner writes:

On Sun, 07 Dec 2003 10:24:02 +0000, John Devereux
wrote:

Well, just about anything is "non-linear" if you measure it accurately
enough! But is it really true that the *inductance* of a "small air
coil" is "dramatically" non-linear with frequency as you stated?

__________________________________________________ _______

Yes, it really is true. If you graph the reactance vs frequency of any
coil, starting just above DC, it will rise in a near-linear fashion for
a while, but will begin to steepen and when approaching the
self-resonant frequency, will quickly rise to maximum, and at that point
will suddenly drop to the opposite (negative, or capacitive) extreme and
then diminish back to near zero as the frequency continues to increase.

No, you are talking about the *reactance* ("reactive impedance"). We
have been talking about the *inductance* ! They are not the same
thing.

If you model a real-world "coil" as a perfect capacitor in parallel
with a perfect, *fixed*, inductor, it will behave as you
describe. (Well you need a resistor too if you don't want infinite
"Q"!)

At that self-resonant frequency, the coil is behaving like a parallel
resonant circuit, which of course it is, due to the parasitic
capacitance between each winding. This parasitic capacitance is
unavoidable and ALL coils exhibit this characteristic. The truly
strange thing is that above the self-resonant frequency, the coil
actually behaves exactly like a capacitor, believe it or not.

Real "Inductors" do indeed have a self-capacitance too, which will
make the component deviate from that of an ideal inductor in the way
that you describe. But this in itself does not make the inductance
(i.e. the inductive part of the reactance), vary.

SNIP

--

John Devereux