[Amps] LCR meter.

Bill, W6WRT dezrat1242 at yahoo.com
Wed Mar 31 11:28:37 PDT 2010


ORIGINAL MESSAGE:

On Wed, 31 Mar 2010 04:06:27 -0700, "Jim Thomson" <Jim.thom at 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




More information about the Amps mailing list