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Re: [RFI] Ferrite Beads from DX Engineering

To: "rfi@contesting.com" <rfi@contesting.com>
Subject: Re: [RFI] Ferrite Beads from DX Engineering
From: "Jim Brown" <jim@audiosystemsgroup.com>
Date: Mon, 11 Jan 2010 11:36:34 -0800
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
On Mon, 11 Jan 2010 14:08:29 -0500, David Jordan wrote:

>"These Ferrite beads, when placed around a current carrying conductor, act
>as an RF choke. They offer a convenient, inexpensive, yet a very effective
>means of RF shielding, parasitic suppression and RF decoupling. This special
>Ferrite mix is ideal for use at HF and effectively decouples unwanted RF
>from automobile ignition systems, control lines, audio cables or other wires
>and cables suspected to be radiating interference."

DXE is only partially correct here. Ferrites add a parallel resonant circuit 
in series with whatever wire/cable passes through/around them. The NATURAL 
resonant frequency of most single beads or clamp-ons is around 150MHz. It is 
the RESISTIVE component of impedance near resonance that is most useful for 
suppression, NOT THE INDUCTANCE. To make these ferrites useful in the HF 
spectrum, we must wind multiple turns through/around them to lower the 
resonant frequency to the HF range. BECAUSE inductive coupling (of resistance 
from the core) increases as the SQUARE of the number of turns, this has the 
added benefit of greatly increasing the choking impedance.  

>I'm scratching my head trying to understand how any number of snap-on
>ferrite beads would reduce ignition noise.  I scratch further where they
>think one should install these beads.

Ferrites suppress noise and RFI by reducing the RF current in the 
wires/cables on which they are used. EVERY wire that carries RF current acts 
as an antenna, both for transmitting and receiving. The outside of the coax 
feeding your antenna acts as a long wire antenna, receiving trash from your 
vehicle (computers, data sensors, ignition), and radiating some of your 
transmitted RF where it is more likely to be picked up by the car's computer 
and data sensors. 

>I asked DX Engineering for a quick explanation and I am waiting for their

DXE is a SALES company. You would learn more by studying the ARRL Handbook, 
the ARRL Antenna Book, and the RFI tutorial on my website. You can also learn 
about these issues from W8JI's website. I believe that Tom is the principal 
designer/engineer for most DXE products, and he's a very good engineer. 


>Has anyone tried this or tired it and experienced success?  I tend to think
>the beads don't do a damn thing to the RFI flying off the spark cables,
>radiating from the igniter or traveling down the low voltage input.

For the reasons noted, DXE is correct that ferrite chokes can significantly 
reduce noise and RFI in a vehicle. I believe that most of the ferrite parts 
they sell (and use in their "baluns") are Fair-Rite #31 material. Study my 
tutorial to understand what that means. 


Jim Brown K9YC

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