[RFI] Common mode choke on 4 Square feedlines without skewing phasing?

[Jim Brown]

Hi Andy,

See comments below.

On 1/22/2019 10:50 PM, Andy KU7T wrote:
> Hi,
>
> I am constructing an 80m 4 Square using DXE phasing unit. The instructions clearly say to not add any matching components or decoupling devices like baluns or coils between the phasing unit and the verticals.
>
> I am using 4 elevated radials at about 9 feet height (I would rather not debate that decision further…). It is usually suggested to use a common mode choke on any antenna feed point, especially for elevated radials.
Yes, that's a great move to make the antenna quieter on receive.
>   Adding an conventional choke would likely change my phasing.

It can. This is discussed in my latest work on chokes, 
k9yc.com/2018Cookbook.pdf   However, if you are able to shorten the 
electrical length of the coax to each vertical by the electrical length 
added by the choke, phasing would not be disturbed. There's enough 
information in the new cookbook to figure that out.  For 80M, the 
recommended 50 ohm choke is 15 turns of RG400, which with leads adds 
about 4 ft.  It's velocity factor is 0.703, which is 0.0205 wavelength. 
So to add the choke, you simply need to subtract that much coax from 
each antenna.

>
> So, I am looking for more information about making a current choke that does not change the electrical length of the phasing coax cables. Is making a stack of a few ferrite cores right on the feedline (maybe with a few coils if I have the length available) ok to do?
>
> I have read http://www.nu9n.com/images/RFI-Ham.pdf, and current chokes are extensively discussed there.

It greatly angers me that NU9N has put my work on his website. That is a 
violation of my copyright. AND, more important, it caused you to study 
an 8-year old version of my work, which I have updated significantly since.

> My question is whether winding on the phasing coax or sliding over the coax will change phasing or not. I am thinking it does not, but wanted to check here…

The only form of choke you could add that would not change the feedline 
length would be a LOT of clamp-on cores to the existing coax, and it 
would be a very inferior choke.  But if you can subtract that same 
electrical length that you add with the RG400 choke, as I've described 
above,  you haven't disturbed the phasing at all.

73, Jim K9YC