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Re: [RFI] Ferrite Loops

To: "rfi@contesting.com" <rfi@contesting.com>
Subject: Re: [RFI] Ferrite Loops
From: "Jim Brown" <jim@audiosystemsgroup.com>
Reply-to: Jim Brown <jim@audiosystemsgroup.com>
Date: Sat, 14 Aug 2004 22:00:14 -0500
List-post: <mailto:rfi@contesting.com>
On Sat, 14 Aug 2004 20:45:06 -0400, Tom Rauch wrote:

>If you have access to an antenna analyzer like the MFJ259B,
>you could measure them. Otherwise, you may just be wasting

It is also important that they be measured over the range of frequencies at 
which you hope to use them. A wire running through a ferrite (or wrapped 
around it) can be analyzed as if it were a series R and L (or XL),  but for any 
given ferrite material, and any given physical size, the values of R and L (or 
will vary with frequency.  That's why you must measure over a range of 

There's another important set of issues. In general, you are using a ferrite to 
make a choke to eliminate RFI at low power levels, you want the ferrite choke 
look mostly resistive. That prevents the inductive reactance of the choke from 
resonating with the capactive reactance of a short line and making the problem 
worse .

But that may not work well if you are using the choke with a transmitter -- the 
ferrite material may overheat, and ferrite materials don't react well to 
heat. So ferrites that are used with transmitters, at least at high power, 
look mostly reactive with relatively small resistive component to their 

The Fair-Rite website (they make most of the ferrite materials that are sold in 
the ham marketplace) has their entire catalog online as a pdf. It is a wealth 
information, both data and tutorials, on ferrites. It is required reading for 
trying to understand how these very unique and useful components work. 

The obvious question is, how do you measure them?  Easy. Simply pass a 
single piece of wire through the ferrite loop. Connect the two ends to the 
terminals of the analyzer (MFJ or AEA), set the analyzer to the frequency, read 
the result, and write it down (R and X). Do that for the range of frequencies 
care about -- I typically measure in steps of 1 MHz or so up to 10 MHz, then 2 
MHz up to 20, then 4 or 5 MHz steps to 40 MHz. 

Then I put the values in a spreadsheet and plot them vs frequency. I also let 
spreadsheet compute Z (the phasor sum of R and jX). As Tom has noted, 
neither the MFJ analyzer nor the AEA analyzer knows what the sign of the 
reactance is. You have to guess (and the AEA guesses for you, sometimes 

In general, the impedance of a wire passed through any piece of the same 
ferrite material is proportional to the length of the ferrite and the square of 
number of turns (that is, the number of times the same current-carrying wire 
goes through the ferrite). 

One thing bothers me about the original question, which stated that he hoped to 
reduce RFI/TVI caused by the transmitter. I'm not at all sure that a choke 
(which is more or less that this would be) will do a lot about that (although 
it may 
help to the extent that it reduces common mode shield current).  

On the other hand, chokes on the wiring to the equipment receiving interference 
may be more effective, and will probably be easier to implement.  Multiple 
around the ferrites sold by folks like Amidon can be quite effective in 
common mode current on the wiring of audio systems and on telephone lines. 

Another important point. It is VERY important to use TWISTED PAIR cable for 
loudspeaker wiring if you have an RFI problem. Twisted pair cable reduces RF 
transferred from the cable to the equipment a LOT as compared to parallel wire 
cable (zip cord). Yeah, I know the exotic/expensive speaker cables are zip 
but the people who market it only know how to SELL it, they don't know anything 
about how it works.  

Hope this helps. 

Jim Brown  K9YC

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