[RFI] Fwd: BIFILAR COMMON MODE CHOKES

[Jim Brown]

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