[Elecraft] OT: Effect of Compression and ExpansionontheInductance of Toroids?

[Ron D'Eau Claire]

Eric, KE6US wrote:

I don't have your experience or background, Ron, but the L-meter is
measuring inductance indirectly and ignoring the presence of distributed
capacitance. Maybe it isn't as negligible at the frequency of the meter as
you think.

But in the end, it doesn't really matter. The real test is how a particular
toroid reacts in the circuit it was intended for. If adjusting the turns
spacing changes circuit resonance, then...it changes circuit resonance.
That's the result we would have been looking for. Hi.

--------------------------

I'm fond of saying that the only dumb question is the question not asked, so
your question is an excellent one, Eric. As for lots of experience, for me
that's too often just an opportunity to remember more things incorrectly.

About your question, my L-meter evaluates inductance using a low-frequency
square wave at about 60 kHz that goes to a differentiator consisting of a
200 ohm resistor in series with the unknown inductance. The waveform at the
junction of the resistor and inductor is a series of spikes at the frequency
of the input square wave. The spikes decay at a rate proportional to the
time constant of the resistor and inductor. This decay rate is converted
into a voltage in a simple logic circuit. 

The ARRL Handbook has featured this circuit in a number of editions. It's a
simple and surprisingly accurate meter, depending upon the quality of the
inductors used to calibrate it. 

The way the circuit works means that any significant capacity in parallel
with the inductor would tend to cancel the effect of the inductor, and make
the inductance value displayed read lower than it really was, not higher. 

Again, at the bench I tested that conclusion by adding capacitors in
parallel with the inductor. Sure 'nuf, adding capacitance decreased the
reading. Recall that squeezing the turns together raised the test inductor
value read on the meter from 3.1 to 3.3 uH. Now I added fixed capacitors
across the inductor to simulate added distributed capacitance caused by
squeezing them together. Adding 10 pf of capacitance across the inductor had
zero effect on the reading. Adding 33 pf across the inductor *lowered* the
reading by 0.1 uH. We're dealing with a very small toroid in this test whose
inter-turn capacitance isn't going to be as much as 2 or 3 pf when squeezed
together. 

So I'm confident that the added capacitance by squeezing the turns together
is not what is causing my L-meter to show increased inductance. If anything,
the capacitance would tend to cause the L-meter to show lower inductance. 

Ron AC7AC