I have measured a big air core coil of approx. 3.37mH
- once w/o ferrites inside
- once with two identical ferrite cores (µi about 125) spaced by 2cm
- once with one core of the same type and dimensions exept
its height which is exactly twice of the cores above.
See screenshot and picture.
The red trace is w/o ferrite core(s),
the green trace is with the two ferrite cores spaced apart
the blue trace on top is with the double height core.
Interesting results indeed!
I´m not so sure about this because the axial demagnetisation factor of
one or two toroid cores with high diameter-to-hight ratio is rather
large compared with a longer "tube" of cores. The magnetic flux lines
tend to use a short and direct way from face to face around the single
core(s) thus diminishing the influence of the core material on the
inductivity of the surrounding coil even more if they are much smaller
in diameter than the coil. You can see this by comparing Stefans
measurement with your own: inserting the cores at Stefans layout
increases the inductivity with a factor of 3.8 whereas at your
arragement the influence of the core is only marginal. Therefore at
Stefans layout the effect of reducing the airgaps should be larger but
that´s just pure theory. Perhaps Stefan is willing to carry out a test -
it´s not very risky!
This is not exactly representing Stefan's coil because the ferrite cores are
much smaller in diameter than the coil but one can see a trend:
Eliminating the air gap between the cores has very little effect.