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Re: [TowerTalk] beads for keyboard or microfone cable

To: "Tower and HF antenna construction topics." <>
Subject: Re: [TowerTalk] beads for keyboard or microfone cable
From: "Jim Brown" <>
Reply-to: "Tower and HF antenna construction topics." <>
Date: Wed, 10 Feb 2010 08:26:53 -0800
List-post: <">>
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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