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Re: VLF: T106-52 cores on VLF

To: [email protected]
Subject: Re: VLF: T106-52 cores on VLF
From: DK1IS <[email protected]>
Date: Thu, 28 Apr 2016 19:08:58 +0200
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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 (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!
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´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!
Tom, DK1IS

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