On Mon, 27 Dec 2004 14:10:47 -0800, Chuck Hutton wrote:
>I've seen a peak like yours when sweeping cores, but can't reconcile the
>Fair-Rite charts with that. I think their chart doesn't mean what I think
it
>means. Can you explain?
I'll take a stab at it, Chuck. I've also measured a bunch of ferrite
chokes, both single turn and multiple turns. It is VERY important to
understand that these are not transformers, they are chokes, and what is
being measured is the series equivalent impedance of R, X, and Z. The X
starts out as L at low frequencies, then becomes C as the stray
capacitance of the coil dominates. This resonance is quite visible in
Michael's data.
The complex Z of these chokes will vary with the material and the number
of turns. See Fig 22 on page 181 of that same Fair-Rite catalog you
referenced. This is the more relevant plot.
I find Michael's data quite consistent with mine -- although I haven't
measured any 77 material, I have measured the rather similar #78. Also, I
suspect that Michael's instrumentation is considerably better than mine.
Remember that with suppression, you want lots of R but relatively little X
(because you don't want the X to resonate with the cable that you are
choking, causing current to increase rather than decrease). Transformers
are different, especially if you are putting power through them -- you
want lots of X but not much R.
To convince yourself of this, wind a 1:1 balun with the toroids that
Amidon sells made from different materials and hook them up to your rig.
These baluns will get hot pretty quickly with 100 watts at frequencies
where the R is high, but not when R is low. Obviously, you want that power
in the antenna, not the balun.
Jim Brown K9YC
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