[RFI] Ferrite Beads

[Ford Peterson]

Tom 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.
> 
> 73 Tom

Unfortunately, I can find NO beads or toroids for sale that are 33 or 73.  A figure in the new 2005 handbook (Figure 13.9) describes the impedance (in ohms) of a "101" size ferrite bead.

          5MHz  10MHz  Perm
Type    Ohms   Ohms      u
64          4          8        N/A (not available)
43        11        16         850
73        20        25        N/A (not available)
75/77   28        28        75=5000  77=1800

I can see that the availability of cores is going to be a problem.  You cannot buy what is not available.  Perhaps these companies have had the various mixes in the past and simply do not produce them today.  It is the initial permeability that I'm looking for?  It appears that the high 'u' or high AL numbers will work the best.  Is this true?

In this application, I will have to take a toroid and snap it in two to make a bead to encase the wires coming from the COP.  The #43 split beads are expensive.  Snapping them and keeping them in pairs will be much more economical and effective if I select the correct mix.  Based on this, I would guess that the #75 mix would be the way to go.  Does this sound correct?

Ford-N0FP
ford at cmgate.com