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

Dave Cole dave at nk7z.net
Sun Aug 20 18:31:58 EDT 2017

Just re-read your paper Jim, very useful, thanks again!  Every time I 
re-read it, I get something new.

73s and thanks,

On 08/20/2017 02:16 PM, Jim Brown wrote:
> Thanks Chuck.  A couple of points of clarification.  First, the link to 
> the tutorial is
> k9yc.com/RFI-Ham.pdf
> 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.
> http://k9yc.com/KillingReceiveNoise.pdf
> 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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