Bill Turner wrote...

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.
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.

Bill, it's one thing to say a coil's reactance is non-linear, but it's
another to assert its inductance varies with frequency. As I responded
before, the inductance of air coils varies very little with frequency.
I know this having made many types of air coils to verify the standard
inductance formulas, and precisely measured them over a 60Hz to 50MHz
range. Earlier in the thread I pointed out the effects of SRF (self-
resonant frequency), due to the coil's parallel capacitance. It's not
useful to my thinking to characterize those two components as one part,
and it's little surprise one gets into trouble when attempting to do so.
A similar statement can be made at very low frequencies where the dc
resistance exceeds the reactance, and the coil is best considered as
two separate parts in series.

The capacitance and dc resistance are both simple and rather obvious
considerations, with straightforward solutions. In contrast, a subtle
and difficult issue in air coils is modeling Q or loss vs frequency.

The concept of ac resistance is often used for loss, and is expressed
as a ratio to the dc resistance, Rac/Rdc. Predicting that ratio is
the tough part, including not only the well-understood skin effect,
but also the relatively obscure and often larger proximity effect.
Further complications enter if one uses multiple wires, and how they
are wound, or if one uses any of the many types of litz wire.

Thanks,
- Win

whill_at_picovolt-dot-com