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.
Second, antenna analyzers are essentially SWR analysers, and have
their greatest accuracy when the unknown Z is close to their
design impedance (that is, 50 ohms or 75 ohms), and very poor
accuracy when the SWR is high. Again, they will give an impedance
that is falsely low for any unknown Z that is much higher than
several hundred ohms.
The data presented in my ferrite tutorial was measured by another
ham working in a well equipped lab with high precision equipment,
proper test jigs, and calibration standards. I measured some of
the same parts he did with an antenna analyzer, and compared my
data with his. His data shows impedances that are typically 5X-10X
higher than mine, and resonant frequencies that are much higher
Any reasonable comparison of the two data sets confirms that the
differences are the result of these fundamental inaccuracies of
the antenna analyzers when used in this manner. In other words, if
you take my data and subtract the capacitance of the analyzer, the
resonances move up in frequency as they should.
If you want impedance data for ferrite cores not shown in my
tutorial, a far better way is to visit the Fair-Rite website and
download their excellent catalog. Fair-Rite is the mfr of
virtually ALL ferrite cores sold by ham distributors as new parts.
Find the part in their catalog, click on the part number, and
you'll get a graph of the series-equivalent impedance vs
frequency. This catalog has recently been upgraded to show not
only single turn chokes, but multiple turn chokes.
The tutorial is at
Jim Brown K9YC
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