[Elecraft] OT - Testing Ferrite Beads using common tools

[Jim Brown]

On Wed, 21 Jul 2004 07:17:46 -0500, Tom Hammond wrote:

>Does anyone happen to know of a way to do some cursory testing on these 
>beads (possibly using the MFJ-259) to tell if they might offer a decent 
>impedance at HF? And if, how?

I don't believe in "off-list" replies except for buying/selling stuff. I have done a fair 
amount of work on this, and can give you some very simple methods using the 
MFJ-259B or the AEA CIA-HF. So here goes.

The equivalent circuit of a choke formed by passing a wire through a bead is 
simply a series R and L (or R and X L). Both R and L vary widely with frequency. 
That is, there is not a single L value, nor is there a single X value that describes 
the choke at all frequencies. Rather, there is a set of curves that describes both R 
and X.  In general, R and  X both start off small at low frequencies, both increase 
with increasing frequency, and at some frequency X (or L) will drop off to a pretty 
small value leaving only R. 

To measure these curves for any ferrite, simply take a short piece of wire, pass it 
through the ferrite, and connect the wire to the analyzer. Then read the impedance 
over a range of frequencies, put the values of R and X in a spreadsheet, and 
compare them to the datasheet for the material. There are excellent data for Fair-
Rite beads in the Fair-Rite pdf catalog on their website. I strongly recommend 
this catalog -- in addition to the data, there are some excellent applications notes 
that talk about how ferrites work and how to use them. 

You can also wind multi-turn coils through many ferrites (depending on their size 
and shape). This will increase the impedance (both R and X) at any frequency, 
but will also shift the peak in both R and X to a much lower frequency that is sort 
of proportional to the square of the number of turns.  For example, the #43 mix 
peaks somewhere around 100 MHz for a single turn, but 8-10 turns will pull the 
peak down around 80 meters, and 14 turns or so will get it down in the AM 
broadcast band (you can easily do this with the 2.4" OD toroids commonly used 
for baluns.

The limit of measurement for the MFJ-259B is 650 ohms, and for the CIA-HF is 
1K ohms. Many multi-turn chokes will result in impedances well above this. 

Jim Brown K9YC