Just re-read your paper Jim, very useful, thanks again! Every time I
re-read it, I get something new.
73s and thanks,
Dave
NK7Z
http://www.nk7z.net
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@n1bug.com>
Date: 08/20/2017 3:27 PM (GMT-06:00)
To: "rfi@contesting.com List" <RFI@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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