On 8/21/2023 4:06 PM, David Eckhardt wrote:
Sorry, this is not what I measure with good equipment. The coaxial
chokes come out second on CMRR to the bifilar wound CMCs.
And to perfectly balance both sides of the wires (mine are 450-feet in a
doublet configuration), the surrounding soil to a reasonable depth must
be treated such that each side of the doublet or dipole present
identical dielectric constants and identical conductivity to each side.
No ham does that, not even Uncle Sugar in the vast majority of cases.
True, my CMRR measurements are made in a 50-ohm system. I would observe
EVERY antenna installation the amateur installs is different. There is
no single, one, CMC, current balun, or other balun functions that suit
every installation. Your data should show that. As a matter of fact,
your data is also taken in a 50 ohm system. True, this does not
necessarily indicate how the choke(s) will perform in practice.
I don't measure CMRR, nor do I have any idea how a MEANINGFUL
measurement could be done.
I measure the complex impedance of the choke vs frequency, placing it
between input and output at the measurement plane, measuring S21, and
computing that impedance from the voltage divider equation. The math is
done in DG8SAQ's software for the VNWA3 analyzer that he designed, and
that is built by hams in the UK. I posted a link to a description of the
test jig.
HOWEVER, if one measures the complex impedance in which the choke is
embedded, then a good solid conclusion as to CMRR can be made based on
the 50-ohm CMRR. At that point, it's just an application of Ohm's Law.
I have done so and know the complex impedance presented in the shack.
The vast majority of hams do not make this measurement. I have so I
know what to expect.
Sorry, no, it's not Ohms Law, it's the far more complex analysis of how
the choke changes the common mode circuit that includes the antenna.
The bifilar wound CMCs perform better for my purposes. They provide a
high-Z against CM currents AND force balance in the two conductors at
the shack. In addition, I really don't care about added SWR losses with
the parallel conductor transmission line. Coax can not perform in my
application as such. Nor can coaxial current chokes (coax wound on a
toroid) force a balance. They function only as a current choke to
currents on the outside of the coaxial braid.
More correctly, a choke with sufficiently high resistive impedance
forces the common mode current to near zero, preventing noise induced
onto the feedline from coupling to the differential circuit at the
antenna.
Every installation is unique. I have made the required measurements and
adjusted variables to best suit my and only my installation.
Indeed it is.
And, BTW, I'm well aware of the "third wire" or whatever it's termed in
professional circles.
I think our disagreement about this comes down to understanding that the
common mode circuit is an antenna. And I do NOT view common mode chokes
as a "balun" -- indeed, I don't even like to use the word "balun,"
because it is fundamentally confusing. Over the years, I've encountered
near ten things described as a "balun" that are VERY different from each
other.
73, Jim K9YC
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