On Sun, 07 Dec 2003 04:31:46 -0800, Bill Turner
wrote:

On Sun, 07 Dec 2003 13:55:35 +0200, Paul Keinanen
wrote:

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.

Not only can you *not* measure them separately, they can not be
physically separated either, since the parasitic capacitance is always
present between adjacent windings. I would not call it an artifact of
the measurement method, but rather an artifact of the coil itself.

The problem with circuits containing both inductances and capacitances
is that in one kind of reactance, there is a +90 degree phase shift
and the other with -90 degree phase shift. Thus, when these are
combined, they partially cancel each other, producing different
magnitudes and some phase shift between -90 and +90 degrees. If only
the resultant magnitude is used (and the resultant phase is ignored),
this would give the false impression that the inductance changes with
frequency.

Instead of using the resultant reactance on some specific frequency,
the inductance could be measured in a different way.

When a DC current I is flowing through and inductance L, the energy
stored in the inductance is W = I*I*L/2. This could be used to
determine the inductance L.

One way to measure the energy W would be to cut the DC current through
L and after disconnecting I, dissipate the energy in some kind of
integrating load across L. Even if there is a significant capacitance
across L, no energy is initially stored in C, since during the steady
state condition, the current I would be flowing through L, but there
would be no voltage difference between the ends of L (assuming R=0),
thus all energy in this parallel resonance circuit is stored in L.

After disconnecting the DC current I, the energy would bounce back
between L and C, but finally it would be dissipated by the external
load. The same energy would be dissipated in the external load even if
C did not exist (assuming zero losses).

Thus using this measurement method, the value of L would be the same
regardless if C is present or not.

Thus, getting a frequency dependent L, is a measurement artifact in
the method that you are using.

Paul OH3LWR