On Tue, 06 Mar 2007 09:33:05 -0800, Jim Lux wrote:
>Could one estimate that parasitic C by sweeping the analyzer and
>looking for the resonance?
Good question. See below.
>And, if you can assure yourself that the resonance is "far" away from
>the frequency of interest, one can ignore the C.
Yes. If you're buying surplus clamp-ons and cylinders used for RFI
suppression, it's most likely that they're #43, and a single turn (that is,
the wire going through the core once) is going to resonate somewhere above
100 MHz. So single turn data is fairly good (+/- 25%) to several tens of
MHz if the "SWR" is less than about 4:1.
>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"...
It might help, actually, if I also presented some of the data from my own
antenna analyzer measurements so that folks could see how the errors
distort the data. I'll do that when I have time.
>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?
Yes. If you're more than a couple of octaves below resonance, the materials
are pretty high Q -- that is, the impedance is strongly inductive, so you
can get good numbers for mu. You know your data is looking right when
you're well below resonance and Z is all X and increasing proportional to
frequency.
>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?
If it's a 2.4" toroid, it could be #31, #43, #61, #77, or #78. All are
commonly available. #31 and #43 would be hard to distinguish unless you had
both of them. #61 is readily identifiable by its high Q resonance, even
though it will be a LOT lower in frequency on the antenna analyzer. #77 and
#78 have a strong resonance around 1 MHz, no matter how many turns you put
on them, but the MFJ-259 doesn't go down low enough to let you see it (the
AEA does).
At some point, you need to study the Fair-Rite catalog. Like anything else
in life, you really start learning when you really start studying. :) Once
you've put in that time, then you've got the possibility of using simpler
instrumentation to figure out what you have. BUT: when I started learning
about ferrites, I was using only what I bought at hamfests. The best advice
I ever got on the subject was to quit playing around and start looking at
known parts.
73,
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