At 07:32 AM 3/6/2007, Jim Brown wrote:
>On Tue, 6 Mar 2007 09:06:45 -0500, Dan Zimmerman N3OX wrote:
>
> >measures, inaccurately, on my MFJ-259
>
>Ferrite chokes are essentially very low Q parallel resonant
>circuits. Antenna analyzers are not very useful for measuring
>ferrite chokes, for at least two reasons. First, they have a
>fairly low input Z (typically 10K resistance in parallel with 12
>pF. The capacitance is the major problem -- it combines with the
>R, L, and C of the choke to move the choke's resonance down in
>frequency to a new false resonant point, and above that false
>resonance gives an impedance that is falsely quite low.
Could one estimate that parasitic C by sweeping the analyzer and
looking for the resonance?
And, if you can assure yourself that the resonance is "far" away from
the frequency of interest, one can ignore the C.
Since there's really a limited number of possible mixes in use, it
might be useful to come up with some sort of "diagnostic method" to
identify the material using simple ham tools (like the MFJ
box). Once you know the mix, and the mechanical dimensions, then you
can go to the mfr charts and find the "real numbers"...
At a first glance, I would think you could just try and identify the
mu of the mix. Pick a low frequency to test at (so the parasitic C
doesn't bite you) and measure L and work backwards from that?
I'm sort of lazy and don't want to page through the FairRite catalog,
and K9YC probably knows these things off the top of his head, but are
all the "usual" mixes for this application sufficiently different in
mu that you could use that as a sole distinguishing
characteristic? Or, are there mixes with the same mu, but different
loss properties?
Jim, W6RMK
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