[RFI] Ferrite Beads

Tom Rauch w8ji at contesting.com
Fri Oct 22 13:50:39 EDT 2004

Hi Ford,

One thing we ALL have to remember is typing doesn't convey
our emotions or intentions very well. Engineers are
generally accustomed to just saying things and being direct
is often mistaken for being a horse's rear.

   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. >>>

Well, you are not alone. Most of the people who work with RF
don't understand common mode currents or beads. It is an
esoteric concept.

Imagine the wiring in your car as an antenna and
transmission line combined. Like an antenna or transmission
line the RF current distribution and impedance in a system
has very little to do with the DC characteristics.

We are between a rock and a hard place with ferrites,
because materials that provide high impedance with a high
loss tangent don't work at higher frequencies when made
larger than certain sizes, so naturally no one builds or
sells them. That's why you can't buy 73 materials in
anything but small cores or beads.

When I trouble shoot a problem like yours I first make sure
the antenna is installed properly. that means a good ground.
Then I try bonding things like exhaust systems and the
engine to the sheetmetal. I always make sure the feedline
has a common mode choke near the antenna, like a string of

I try bypass capacitors also on "safe lines". Like you, I
won't slap a large capacitor across the pulsed terminals of
a 1200 dollar module.

I even try shielding noise sources. I find the sources with
a probe connected to a receiver.

The LAST thing I do is add cores. Why last? As a general
rule I know it's largely luck if they make a substantial
difference at HF.

The best thing they ever did was invent snap-on beads. The
worse thing we ever did is start thinking they are high
impedances that provide the best solution to unwanted HF RF.

73 Tom

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