In article ,
Paul Burridge wrote:

You measure its reactance at 1 MHz using the formula X=2*pi*F and find
it to be 6.28 ohms.
At 2 MHz you find it to be 12.56 ohms.
At 10 MHz you find it to be 62.8 ohms.
So far the reactance is changing linearly with respect to frequency.
(Actually it is not perfectly linear, but the difference at these
frequencies is small and probably would not be observed with run of the
mill test equipment.)

But, as you approach 100 MHz, you find the change is obviously no longer
linear.
At 95 MHz you would expect the reactance to be 6.28*95=596.6 ohms, but
much to your surprise, it measures 1000 ohms.
At 99 MHz, instead of the expected 6.28*99=621.72 ohms, it measures
50,000 ohms!!

All the above is perfectly normal and easily observable.

My point is that when a coil measures 50,000 ohms at 99 MHz, its
inductance HAS TO BE L=X/(2*pi*F), or 50,000/(6.28*99)=80.4 uH!

This is not an illusion. If you have an inductance meter which uses 99
MHz as a test frequency, it WILL MEASURE 80.4 uH.

And therefore, I maintain that inductance DOES vary with frequency.

How can it be otherwise?

As Spock said to Kirk, "You proceed from a false assumption." Or, to
put it another way, the scenario you've just laid out contains an
inherent contradiction.

The inductance meter that you are using (or assuming) is not actually
measuring inductance. It's measuring reactance, and back-calculating
to what the inductance would be *if* it were measuring a "pure"
inductance.

However, as you recognize, the component that you are measuring is
*not* a pure inductance. Its actual reactance is the result of
interaction between its inductance, its inter-winding and distributed
capacitance, and its winding resistance (at any given frequency).

So, what you're observing can best be interpreted as follows:

- At low frequencies (well below resonance), the component's
reactance is dominated by its inductive component. It's a decent
approximation of a "pure" inductance. The inductance meter gives
accurate estimate of the inductive component.

- At high frequencies (well above resonance), the component's
reactance is dominated by its capacitive component. It becomes a
decent approximation of a "pure" capacitance at some point, I
suspect.

At these frequencies, your simple inductance meter lies through its
teeth. It "tells" you that the part's inductance is such-and-
such, but it's not telling you the truth. It's hiding from you
the fact that the reactance it's seeing isn't inductive at all (the
reactance decreases as frequency goes up, and exhibits a capacitive
phase angle).

So, I think, what you're facing here is the problem which occurs when
you try to force simplifying assumptions ("the component being
measured is a pure inductance" and "an inductance meter actually
measures inductance") outside the range in which these assumptions are
valid.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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