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Re: [RFI] Bulk order source for Ferrite beads?

To: rfi@contesting.com
Subject: Re: [RFI] Bulk order source for Ferrite beads?
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Sun, 3 Feb 2019 14:02:03 -0800
List-post: <mailto:rfi@contesting.com>
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.


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