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Date: Wed, 10 May 2000 13:20:42
To: rsgb_lf_group@blacksheep.org
From: "Rik Strobbe" <rik.strobbe@fys.kuleuven.ac.be>
Subject: Re: LF: RF Ammeters, results, and a new subject
In-reply-to: <UVYnHQA$mSG5Mwfj@telemetry.demon.co.uk>
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In addition to Tom's suggestion :
In order to check the 'power behaviour' of a bunch of unknown toroids I
took my old 136kHz TX (40W output), made a series circuit of a 1.5nF silver
mica cap (actually 3 x 470pF parallel), an inductance (wound on the 'toroid
under test') and a 50 Ohm dummyload. As the loss in the capacitor is
neglectable, the TX output current gives an idea of the loss in the toroid.
The toroid was also checked for heating after a few minutes of 40W carrier.
Of course the L/C has to be brought to resonance, but as I modified the VFO
of the old TX to cover 130-150kHz it was rather easy done by tapping the
windings.
A red Amidon Toroid I had as a reference did not get warm and the TX
current was 
only about 5% less than the current I could get into the 50 Ohm dummy
(without any L/C in series).
Unfortunately all my toroid stock was unusable for LF (at least for +100W).
I found out that any toroid that gets just a little warm at 40W will get
far too hot at 400W.
BTW : I also found out that a lot of toroids that are useless for resonant
appliactions (LPF etc..) work fine for broadband applications
(transformers, BALUNs etc...), don't trow them away too fast.

73, Rik ON7YD

At 10:19 10/05/00 +0100, G3OLB wrote:
>Larry
>
>Again, at a mobile rally (hamfest) I bought a box of assorted ferrite
>and iron dust rings. There were about a hundred, various colours and
>sizes all for about a quid! My method of establishing their
>characteristics was to take one of each type, wind 30 turns of 30 gauge
>and measure the inductance. 
>
>I made up a table of the inductance of each, and this gives me a good
>indication as to their suitability for LF, MF, HF or VHF. Inductance
>varies with the square of the number of turns so, starting with the 30
>turn measured value, it is easy to work out the required turns for a
>given inductance. In use, if a core gets hot, I use a bigger one! Not
>very scientific but it works.
>
>73, Tom G3OLB
>
>
>>The Question:
>>
>>What are the techniques that one might use to take a sample core and
>>determine the RF capabilities of the core at different frequencies?  I have
>>had some blind luck once finding some red coloured cores that performed
>>exactly the same as the expensive ones I bought from Amidon.  A more general
>>solution of measurement would be just great - who has some ideas on this?
>>
>>Larry
>>VA3LK
>>
>>
>>
>>
>
>
>