[RFI] Snap-on RFI suppression core size vs cable size

Jim Brown jim at audiosystemsgroup.com
Sun Aug 20 17:16:56 EDT 2017

Thanks Chuck.  A couple of points of clarification.  First, the link to 
the tutorial is


Second, there's a much more recent piece on identifying, finding, and 
killing RF noise that I wrote for NCJ a year or so ago that has more 
practical advice.


A few general statements.

A ferrite choke works by adding RESISTANCE, inductively coupled from the 
lossy core, to conductor into which it is inserted. That resistance is 
from the broad (low Q) parallel resonance of the choke with its stray 
capacitance, and the resonance of most clamp-ons designed for 
suppression is around 200 MHz (this can clearly be seen from the plots 
of Z vs frequency on the data sheets for each individual part on the 
Fair-Rite website.

Clamp-ons without multiple turns are only effective above about 100 MHz 
(and #61 is effective at low UHF).  At 6M and below, we must wind 
multiple turns to move their resonance down in frequency to the range 
where we want to use them.

Inductance and resistance are proportional to the length of the ferrite 
core, and proportional to the square of the number of turns. Stray C is 
roughly proportional to the number of turns.

When counting turns, we count the number of times the conductor passes 
THROUGH the core, so a single pass through the core is one turn. In 
other words, the number of turns is one greater than the number of loops 
in a multi-turn choke.

For a LOT more detailed discussion see the tutorials, which, among other 
things, discuss WHY resistance is what matters most, and why #31 is  the 
most universally effective for hams.

73, Jim K9YC

On 8/20/2017 1:40 PM, chuck.gooden wrote:
> Torid cores with multiple turn are more effective.  K9YC has written a very good paper about this.  It can be found on his website.  The file is publications/rfi-ham.
> Chuck Gooden - K9LC
> -------- Original message --------
> From: N1BUG <paul at n1bug.com>
> Date: 08/20/2017  3:27 PM  (GMT-06:00)
> To: "rfi at contesting.com List" <RFI at contesting.com>
> Subject: [RFI] Snap-on RFI suppression core size vs cable size
> I suspect this is a stupid question but I am not finding a concise
> answer or I am not recognizing it when I see it.
> How important is it to fit the I.D. of snap-on RFI suppression
> ferrites to the size of the wire or cable they are used on? At what
> point does the core I.D. being larger than the wire begin to reduce
> its effectiveness? Surely this must happen at some point? Larger
> cores tend to have higher equivalent series resistance than smaller
> ones, but I wonder how much of that may be given up if the wire
> diameter is much smaller than the core I.D.
> I ask because I need to purchase a significant number of these in an
> effort to minimize RFI at 144 MHz. Obviously there is a significant
> price break when buying large quantity of a given part, but I have
> cables of differing diameters to deal with.
> 73,
> Paul
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