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December 10th 03, 12:57 AM

Bill Turner wrote:

On Tue, 09 Dec 2003 04:59:29 GMT, wrote:

I maintain it does. Otherwise the formula X=2piFL is invalid.

NO! In the above equation, X varies when F varies. The equation
does NOT mean that L varies as F varies.

__________________________________________________ _______

Perhaps an example will make it clear.

Suppose you have a coil which measures 1 uH at 1 MHz. It is known to
have a self-resonant (parallel) frequency of 100 MHz.

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!

Here are your own words:
"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."

Your example ignores the capacitance, which you have stated
exists. There is nothing in your formula that addresses it.
You cannot use the formula or the math above (in your post) to
support your point of view, because it does not contain any
term for capacitance. The capacitance exists, and exhibits
a larger and larger affect on the circuit as the frequency
increases from 1 - 99 mHz.

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?

--
Bill, W6WRT

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