Ie: [RFI] Ferrite Beads

[Jim Brown]

On Fri, 22 Oct 2004 11:48:35 -0500, Ford Peterson wrote:

>The fact that I do not understand surge impedances on random lines, the vagaries of
>various ferrite mixes, and the subtle nuances of EMI/RFI does not make me an idiot. 
>Ignorant maybe, but not an idiot.

I certainly did not belittle you nor anyone else. I did suggest (indirectly) that you were 
being lazy, expecting others to do your work for you. I did give you a bunch of specific 
suggestions about how to chase your problem. I DID urge you to study specific reading 
materials and technical data that will help you understand things that you need to know 
to solve your problem. Those who know me (and those who have read this and several 
other lists for a while) know that I have spend thousands of hours teaching and sharing 
what I know. I have spent thousands of hours learning it, and I continue to study. But like 
Tom,  I am unwilling to spend days on YOUR engineering problem. I have plenty of my 
own to work on. 

You need to understand how ferrite materials work to solve interference problems. The 
Fair-Rite catalog does a very good job of explaining that. That's why you need to study 
it. The executive summary is that it turns the cable it is applied to into a series R and L, 
both of which vary with frequency. That R and L can help reject interference it they can 
be made to form a voltage divider with some "load" impedance. That load impedance 
may already exist in the circuit, or you may need to add it (usually in the form of a 
capacitor to ground). All of this is stuff that is VERY well covered in the ARRL 
Handbook, and which you have to know to pass the ham exams. The Fair-Rite catalog 
will give you the values of R and L for various parts that they make. The email I posted 
several days ago gave you a lot of specific information about how to increase the series 
R and L from a ferrite. 

Another point. If you are a ham, you need to understand surge impedance of lines. It is 
VERY well covered in the ARRL Handbook and the ARRL Antenna book. Surge 
impedance is simply another name for characteristic impedance.


Jim Brown  K9YC