On Tue, 9 Nov 2004 06:59:50 -0500, Jeff Maass wrote:
>Isn't #43 material the *wrong* material to use for
>supressing noise on 160 meters? It's described by
>Fair-Rite as appropriate for 20 - 300 MHz for EMI
>suppression. Multiple turns may make it more effective
>at lower frequencies, but that's just sloppy.
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.
I do not speak about this from a point of conjecture -- I have done
extensive research on the use of ferrites for MF and HF
suppression, and plan to publish the results in the near future in the
form of technical papers.
Jim Brown K9YC
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