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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