[RFI] Bulk order source for Ferrite beads?

[Larry Benko]

I also designed some "Tempest" type communications stuff.  The EMI from 
it was required to be low enough that foreign spooks couldn't detect and 
decode what was happening.

I have no interest in debating this further.

Larry, W0QE

On 2/3/2019 3:02 PM, Jim Brown wrote:
> 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. 
>
>
> _______________________________________________
> RFI mailing list
> RFI at contesting.com
> http://lists.contesting.com/mailman/listinfo/rfi