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From: DK1IS <DK1IS@kabelmail.de>
Message-ID: <572243AA.5000808@kabelmail.de>
Date: Thu, 28 Apr 2016 19:08:58 +0200
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Subject: Re: VLF: T106-52 cores on VLF
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Status: O

Hi Clemens,
> I have measured a big air core coil of approx. 3.37mH
> - once w/o ferrites inside
> - once with two identical ferrite cores (痣 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!
> 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.
I惴 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愀 just pure theory. Perhaps Stefan is willing to carry out a test - 
it愀 not very risky!

73,
Tom, DK1IS
www.qrz.com/db/dk1is