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

[Ron D'Eau Claire]

Yes, I think you may be absolutely right, Don. I measured a maximum effect
of 9% in the one example I tested - a random core lying on my bench that had
about a dozen turns on it. That agrees with your experience of finding up to
a 10% change available. 

I saw your suggestion before, and it certainly sounds plausible. I wonder if
something else might be at work here too. Or perhaps I'm looking the same
effect you are in a different way. Here's my thought. The increased
inductance/turn of the toroid compared to ordinary air-wound coils is caused
by the very high permeability of the core compared to air. That makes the
inductance fairly independent of the spacing between turns since spacing the
turns hardly reduces the magnetic flux in the core that is available to each
turn. But I wonder if crowding the turns together doesn't slightly increase
the inductance by providing a lower permeability path for the magnetic flux,
since the distance the flux must travel along the toroid before it passes
through all the turns of the coil is shorter. In other words, the same
effect one sees by using closer spacing in an air wound coil, only much,
much less due to the efficient magnetic path provided by the core.

Does the formula you are using account for a reduction of flux density
around the distance of the torus due to the losses in the core, or does it
assume a constant flux at all points? 

It's always interesting when experiment fails to support predictions. True,
it most often turns out to be an invalid experiment that causes that result,
but I can't see the problem here, especially considering that the effect of
added distributed capacitance has an inverse effect on the reading on this
type of 'meter'. 

Ron AC7AC
 

-----Original Message-----
From: W3FPR - Don Wilhelm [mailto:w3fpr at earthlink.net] 
Sent: Monday, August 29, 2005 9:22 PM
To: Ron D'Eau Claire; elecraft at mailman.qth.net
Subject: RE: [Elecraft] OT: Effect of Compression and
ExpansionontheInductanceof Toroids?


Ron,

I have the same inductance meter, and it does a good job because it does
measure using the LR time constant rather than frequency, but 'physics is
physics', and with a true toroidal inductor, there is still no dependency on
the turns spacing (the math says so).

Now, for the practical side of things, I do believe that your results (and
others with similar findings) deviate from the 'classic physics' treatment
of the ideal (ignoring practical behavior) - and when the toroid turns are
irregular (not evenly spaced around the circumference of the core), there is
some part of that coil behaving as a solenoidal inductor where turns spacing
IS a factor.

Remember that a toroid is simply a solenoidal coil formed into a circle with
the ends of the coil meeting.  This ideal toroid has equal turns spacing all
the way around.

So my current conclusion is -- IF the toroid is equally spaced around the
core, the inductance is not dependent on the spacing of the turns, BUT, if
the turns are compressed anywhere around the core, additional factors rear
their ugly heads because the inductor is a combination of a solenoidal coil
and a toroid coil, and the math becomes quite complex - just how much of
each effect depends on just how much deviation from an ideal toroid shape
exists in the configuration at hand.

In a frequency dependent tuned circuit, how much of the frequency shift is
due to the inter-turn capacitance or a change in inductance is (to me) only
a matter of curiosity - the plain fact is that some change in the resonant
frequency (or inductance) can be achieved by changing the turns spacing, but
we all know that the adjustment range is small - the really big determining
factor is simply the number of turns.

As an example, I often improve the 10/12 meter Low Pass Filter
characteristics of a KPA100 by squeezing the toroid turns "just the right
way", but I determine what is the 'right way' by monitoring the impedance
with my MFJ259B as I do it.  Sometimes it is 'this way', while other times
it is 'that way' - the batch to batch change in the permeability of the
cores likely accounts for more variation than the turns spacing itself. BTW,
this change does not really help the KPA100 output, but it improves the base
K2 10 meter efficiency at 10 watts or lower when the KPA100 is installed.

The overall inductance change that I have experienced is about 10%, so that
is within the normal design tolerances using 10% resistors and capacitors -
so except for satisfaction of the curiosity factor, I would say just to wind
the toroids with the proper number of turns, and 'tweak' them in-circuit as
required and as close as is permitted by your measurement capability.

73,
Don W3FPR