OT - Testing Ferrite Beads using common tools

[Tom Hammond-3]

Hi folks:

This a pretty off topic, but I hope someone might be able to shed some
light upon this question.

A couple years ago, at the Dayton HamVention, in a fit of greed, I
purchased a number of ferrite beads which, although they were NOT the FB-
73-2401 beads I usually buy, they WERE "advertised" (by the seller) as
being mix 73. These beads have a somewhat larger inside diameter, making it
easier to slip them over slightly larger diameter coax (e.g. RG-8X, etc.).
And the price was REALLY GOOD (hence the previous reference to 'greed').

I am now at the point of wanting to use these beads, but I'd like to
confirm that they might work at HF. I don't have a lot of sophisticated
test equipment, BUT I do have a sign gen, 100 MHz scope, an MFJ-259B
Antenna SWR Analyzer, and the more commonly available 'shack-type'
tools  at my discretion.

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?

Since some of our OT threads seem to linger MUCH longer than they should,
please reply to me OFF-REFLECTOR and if I can confirm that a particular
procedure works as advertised, and it repeatable, I'll summarize it (giving
full credit to whomever offered it), and post it back to the reflector.

73,

Tom   N0SS

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[Jim Brown-10]

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



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