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.
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