This probably has a limited audience but I do know that there are some
readers on this list interested in fabrication of high impedance common mode
chokes for their stations.
I built and measured 18 of this type of choke during the past week. There is
a write-up on my web site which shows how they were fabricated, how they
were measured and the measurement results. I used Fair-Rite 2.4 inch
diameter type 31 toroidal cores and small diameter [RG 59 size} coax. I
measured the magnitude of the impedance for each of the chokes on the 160,
80, 40, 20, 15 and 10 meter bands. There are also attenuation values for
each choke [specific to the 50 ohm measurement circuit]. The results are
shown as a series of graphs for two cores, three cores and four cores with
3, 4, 5, 6, 7 and 8 turns on the core bundle.
These graphs should provide guidance to those fabricating their own common
The site is
The article is under Experiments & Measurements
"Measurement of Chokes made using type 31 material cores"
My work is an extension of material found in the excellent paper on this
subject written by Jim Brown, K9YC, which has been mentioned here before.
Tod Olson, K0TO
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