From: "Jim Brown" <jim@audiosystemsgroup.com>
> No, it's not "sloppy," it's an advanced application of the physics of
> how ferrites work, and a method of getting very effective
> suppression from commonly available components. The 20-300
> MHz recommendation assumes a single turn choke. See Fair-Rite's
> applications note on the use of ferrites for suppression, which
> includes an equation, graph, and discussion showing how multiple
> turns around a toroid will increase the impedance and reduce the
> effective frequency range of the choke by a factor roughly equal to
> the turns ratio.
>
> Now, a choke made from a material like Fair-Rite #78 would be
> more effective over a broader frequency range in the MF and HF
> spectrum with fewer turns. But the #43 material WILL work, and will
> be quite effective. More to the point -- a multiturn choke of #43 can
> provide a resistive component >800 ohms over at least an octave
> centered anywhere between about 700 kHz and 30 MHz, and the #
> 78 will cover several octaves >800 ohms over that range. The loss
> of effectiveness of the multiturn choke at higher frequencies is the
> result the interwinding capacitance.
Just by chance, I happened to be designing a common mode
choke for my generator over the weekend. I had a couple of
FT-240-43 cores in my junkbox. Five turns gives about 490
ohms |Z| at 3.5 MHz (about 200 ohms resistive). I think the
main impedance peak is around 25 MHz (about 2000 ohms),
but its a little hard to measure with the MFJ-269. My cliplead
fixture has about 12pf of capacitance, so I had to
"de-imbed" the |Z| of the DUT mathematically at that
frequency which is probably not the most accurate calculation
in the world. When I get a chance I will try to measure it
on an HP network analyzer.
At 1.8 MHz, you only get about 270 ohms |Z| from a 5 turn FT-240-43
choke. You could add more turns, but I needed two #12 wires and
a safety ground to carry the generator's AC load (this size toroid
starts to get a little crowded):
http://www.dellroy.com/W4EF's-Ham-Radio-Page/Portable_Operation/EU2000i_Filt
er.htm
For this choke I was mainly interested in getting good
attenuation on 10 and 15 meters, so the FT-240-43's that I
happened to have on hand fit nicely with my design requirements.
BTW, when looking thru the Fair-Rite products catalog, it looked
like there might be other materials that would as Jim says provide
a broader impedance range, but I don't recall exactly which
ones looked the best (31 comes to mind).
73 de Mike, W4EF..........................................................
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