On Sat, 06 Dec 2003 17:32:58 -0800, Bill Turner
wrote:

Actually, one does come across such coils. All coils have a frequency
where they become a parallel resonant circuit, due to the capacitance
between windings. And oddly enough, *above* that parallel resonant
frequency, they become capacitive. Yes, you read that right, they
actually act like a capacitor, believe it or not.

This is only an artefact if you try to determine the inductance of an
inductor by measuring the reactance of that component at some
specified frequency. The inductive reactance (Xl=2*pi*f*L) will grow
in a linear way towards a positive value depending of the frequency.

Since the parasitic capacitances are present, the negative capacitive
reactance (Xc=-1/(2*pi*f*C) will complicate the situation. When
approaching resonance in a parallel resonant circuit, the reactance
goes to +infinity, switching rapidly to -infinity as the resonance
frequency has been passed and slowly approach the linear drop of the
negative capacitance at frequencies far above resonance.

One can still argue that the inductance and inductive reactance are as
well as the capacitance and the capacitive reactance are still there
as separate entities, but we can not measure them separately from
terminals of the coil. Thus, this is an artefact of the measurement
method.

Thus, the inductance should be measured at a low frequency to avoid
the capacitive reactance. On the other hand the capacitance should be
measured at a high frequency well above resonance to avoid the effects
of the inductance. Or just measure the inductance at a low frequency
and determine the capacitance from the resonance frequency and
inductance.

While inductance and capacitance are frequency independent, the
resistance of a coil will vary with frequency due to the skin effect,
since at higher frequencies, the conductivity of the inner part of the
conductor is not used.

Paul OH3LWR