[RFI] Old wives tail, or true?

[Jim Brown]

On 4/9/2021 10:19 AM, David Eckhardt wrote:
> I see we can attach something to email within this group.  

Sorry, we cannot.

> So, one afternoon on a whim, I made a test first with the CMC inline and
> then w/o CMC on my parallel conductor transmission line to / from my
> 450-foot long double. 

Losses can get quite high in a common mode choke in a line that is 
mismatched to it's load. Count on destructive failure with high power.

  My CMCs are constructed using either 240-31
> (2.4-inch OD) or 400-31 (4.0-inch OD) 

I really object to the 240 and 400 nomenclature, which was conceived 
several decades ago by unscrupulous vendors to sell standard Fair-Rite 
products to hams at obscene markups.

material and are wound in a bifilar
> manner using AWG #12 Teflon insulated stranded conductors.

Do you mean as a 2-wire transmission line, or as a transformer? 
"Bifilar" implies a magnetically coupled transformer.

   I do not use
> coaxial cable wound on these cores due to constraints the long doublet plus
> feedline places on impedances in the shack (7.000 MHz measures 3.6 - j 62
> with the coaxial choke inline). 

A closely spaced insulated 2-wire line has Zo on the order of 100 ohms.

  The CMC choke was placed between the
> output of the homebrew L-network matching network and the input to the
> parallel conductor feedline in the shack. 

The most effective location for a common mode choke (Please use WORDS, 
not abbreviations) is at the antenna's feedpoint, because noise current 
on the feedline is coupled to the antenna because the feedline becomes 
part of the antenna without the choke).

  The results are documented in
> the attachment.  Let the data speak for itself.
> 
> And I should add there are good and sound engineering reasons for this
> noise getting onto the feedline, but not the subject (could be rather long
> with math) of this email.

Little math is required, only an understanding of fundamental concepts. 
EMC guru Henry Ott takes pride at breaking complex problems into simple 
concepts, and is a master at it. In a three-day seminar I attended in 
2005, there was almost no math at all.

73, Jim K9YC