On Wed, 10 Feb 2010 08:08:44 -0800, Dan wrote:
>The impedance to RF is proportional to the number of turns of cable
>through the core center hole squared ( 3 turns gives 9x impedance).
That's only PART of the story. What you're actually doing when you wind
multiple turns through a ferrite core is multiplying both the resistance
and the inductance by N-squared, AND adding parallel capacitance.
A wire passing through or around a ferrite core is fundamentally a parallel
resonant circuit. A single turn around or through #43 or #31 material
places that resonance around 150 MHz. 2-3 turns pulls the resonance down to
30-50 MHz, depending on the size of the part and the size of the cable, AND
it increases the impedance at resonance. It is the RESISTIVE component of
that impedance that is most useful in RFI suppression (including coaxial
chokes -- so-called current baluns). If you wind enough turns, you can
pull that resonance down to our HF bands.
My RFI tutorial includes a LOT of measured data for selected chokes wound
on selected ferrite cores that are useful for ham applications. As a result
of my work, Fair-Rite now publishes impedance curves vs frequency for most
of their "suppression" parts for the HF sprectrum (mostly #31). To find
this data in their on-line catalog, search their suporession part numbers
for one that looks promising based on its dimensions, then click on the
part number to see the detailed data sheet, then scroll down for another
link to detailed data. For most of these parts, you'll get their measured
data for 1, 2, and 3 turns. For more than 3 turns, you'll need to study my
data and interpolate from it and Fair-Rite's data.
Link to my tutorial in previous post.
Jim Brown K9YC
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