[RFI] Bulk order source for Ferrite beads?

[Jim Brown]

But designing to meet EMI testing requirements is VERY different from 
SYSTEMS design, and products that meet EMC regulations still cause and 
receive EMI, largely because the regulations fail to consider the 
SYSTEMS aspects of EMC. This is especially true below 30 MHz. I spent my 
professional life designing SYSTEMS. As a member of the AES Standards 
Committee and Vice-Chair of the WG on EMC, was principal author of all 
AES EMC Standards. WG members who worked on those Standards included 
engineers from major broadcast networks (BBC, ABC), recording studio 
designers, equipment and microphone manufacturers, and designers of 
large sound systems (my specialty). The inclusion of all of these 
disciplines forced us to concentrate on the SYSTEMS aspects of EMC, and 
to develop Standards that were based on both fundamental physics and 
practical applications. Stuff has to work across the street from a 50kW  
AM broadcast station, in the near field of big power transformers, and 
in studios in high rise buildings in the shadow of other high rise 
buildings (Hancock and Sears Tower) that house all of the TV and FM 
broadcast for metro Chicago. And it has to do this with lots of antennas 
(mic cables, etc.) connected to it.

As my tutorials clearly show, it is the RESISTIVE component of the 
common mode impedance at the frequency(ies) of interest that is most 
effective at suppressing common mode current, because the reactive 
component of the impedance, whether inductive or capacitive, can, 
depending on the electrical length of the rest of the common mode 
circuit, be cancelled by the reactive component of the rest of the 
common mode circuit, leaving only the resistive component to block 
common mode current.  And this was one of many conclusions of that DoD 
engineering report.

Yes, the resistance dissipates power, but dissipation can be limited by 
making Rs sufficiently large. The designs in k9yc/com/2008Cookbook.pdf 
provide measured Rs values in the range of 10K ohms. Stick that value in 
an NEC model of a dipole that includes the common mode circuit (a wire 
with the diameter of the coax shield and the dielectric constant of the 
outer jacket, and that follows the geometry and electrical connections 
of the feedline).

The virtue of low Q materials like #31 is that multi-turn chokes wound 
on it are predominantly resistive over one or two octaves of bandwidth, 
depending on the dimensions of the core, the winding, and the frequency 
of interest.  The shortcoming of high Q materials like #43, #52, and #61 
is that their resonance is much narrower, AND the fact that ferrite 
components have rather wide tolerances.  For most Fair-Rite components 
it's +/- 20%. I can get some very high Rs values from chokes wound on 
these toroids, but if I measure chokes on toroids that are as little as 
10% different, their resonances will be displaced enough that you'd have 
to measure every choke you wind.

Even with #31 material, I characterized more than 300 toroids, for the 
Cookbook, built and measured chokes on selected cores that were at the 
limits of what I measured, and used worst case results for 
recommendations on a band-by-band basis. That would not be possible with 
#43 material.  And #61 is much higher Q than #43 -- it's a major 
engineering project to even FIND the resonance. The hard part is that 
the stray C that forms the resonance is quite small.

73, Jim K9YC


On 2/3/2019 12:19 PM, Larry Benko wrote:
> I spent years designing EMI compliant telecom and other communication 
> equipment.