[RFI] Ferrite Beads
jim at audiosystemsgroup.com
Mon Oct 18 20:52:03 EDT 2004
On Mon, 18 Oct 2004 17:30:41 -0400, Tom Rauch wrote:
>The recommended beads are 31 or 73. The problem is in a
>large size 31 and 73 have internal resonances when the
>diameter and thickness is large, so you will be stuck with
>something like a 33 material in large beads. It's only a bit
>better than 44, but at least it's better.
I have measured a LOT of ferrites, beads and otherwise, and I agree in
general. BUT: Download and study the EXCELLENT pdf catalog from Fair
Rite (google to find them), and study the app note on the use of beads for
Several points. First, yes, the 31, 73, 77, and 78 mixes are best for 160
and 80. Second, making multiple turns through any ferrite toroid or bead
will multiply the impedance AND move the effective range of the
bead/toroid down in frequency a LOT.
Those #43 mix materials that are stinko at 14 MHz with one or two turns
are quite effective with 3-5 turns. With 8 or so turns, they are effective on 80
and 160 meters, and 10-12 turns makes them effective in the middle of the
AM broadcast band! And when you do multiple turns, the impedances will
get a LOT higher.
Third, as you do multiple turns, the stray capacitance shorts the choke at
higher frequencies, so a multi-turn choke optimized for 160 or 80 will
probably be a lot less effective on 15 meters and above.
And finally, to clarify what Tom was saying, it is VERY important that you
operate the ferrite choke in the range where it is hardly a choke at all, but is
primarily resistive. Why? Simple -- any inductance will increase the current
by resonating with the capacitance of the short antenna.
I'm currently at an EMC workshop in CA lectured by Henry Ott, so don't
have time to say a lot more.
Jim Brown K9YC
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