ORIGINAL MESSAGE:
On Wed, 31 Mar 2010 04:06:27 -0700, "Jim Thomson" <Jim.thom@telus.net>
wrote:
>
>### I tried using the inductance measuring capability of the MFJ-259B.. with
>dismal
>results. A 10 uh commercially made roller inductor, wound with 1/4"
>tubing... measures
>exactly 10 uh on my B+K 875-B lcr meter. Same deal on my 50 uh roller. When
>the mfj
>was used, the same coil measures a LOT less than 10 uh.... as low as 5
>uh.....depending on
>what freq the mfj is on. Even with the mfj set to 1.8 mhz.... the same coil
>is way < 10 uh !!!
REPLY:
Trying to measure the inductance of coils like we are talking about at
a high RF frequency is almost always an exercise in futility because
of the parasitic capacitance all such coils have. When you get any
where near the self-resonant frequency of the coil, you are really
measuring the net impedance of the coil + parasitic capacitance, and
trying to convert that value to a pure inductance value is prone to
large errors.
I think the best way to evaluate a coil such as that is to measure its
inductance at a very low frequency and then find it's self-resonant
frequency. Knowing both of those values, a little math will get you
the equivalent parasitic capacitance present and then you can treat it
as a coil of pure inductance in parallel with the capacitance you just
found, and that should be accurate enough for ham purposes.
Even that method can fool you if a coil has multiple resonances, which
some do depending on how they are constructed. Skin effect probably
has some small influence too, but that's getting a little over my head
at this point.
You can't be too cautious with this stuff. :-)
73, Bill W6WRT
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|