On 8/20/2023 1:54 PM, David Eckhardt wrote:
I put this out earlier, but have made some changes and additions. Please
see the attment for measurements of CMRR (Common Mode Rdjection Ratio) made
in a 50-ohm system for each choke.
The common mode circuit is an antenna, not a transmission line. If the
line is coax, it's the outside of the shield. Common mode rejection
depends entirely on the antenna into which it is inserted, and must be
computed based on measurements of the choke, inserted as a load into a
model of the antenna using NEC or other software. By the antenna, I mean
the feedline with attachments on both ends, one to the antenna, the
other at/near the shack. And like any antenna, voltage and current
varies along it, depending on its geometry and terminations of each
element.
What we can measure in the lab is the common mode impedance of the
choke. Before he died, G3TXQ published a piece with RSGB showing an
excellent test jig. His measurements look very good to me, but his
guidance for choke designs were faulty, because he failed to understand
the significance of mfg tolerances on the chokes themselves, so I
suspect that he measured only a few samples of the cores. This is
something that engineers working in mfg learn -- the design has to work
with every one of the parts from which it is built, including their
individual mfg tolerances.
73, Jim K9YC
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