[RFI] BIFILAR COMMON MODE CHOKES

[Jim Brown]

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