[RFI] Yet another balun question

[Hare, Ed W1RFI]

>From what I know, a common-mode choke can control common-mode currents in a combination of two ways. The most effective is to absorb the unwanted signal and dissipate it as heat in the core. It can also be reactive, presenting an impedance that returns the current back down the line, to its source, where some of it will be lost as heat in the inefficiencies of the source, some lost as the signal getting into the source is propagated back onto the AC mains or to ground and some of it radiated along the way back, presumably farther away from the victim.  In any case where the victim does not rely on a signal received on an antenna, keeping the common-mode signal from getting to the victim by any means will reduce the potential of overload causing interference.

-----Original Message-----
From: RFI [mailto:rfi-bounces at contesting.com] On Behalf Of Jim Brown
Sent: Tuesday, June 28, 2016 6:29 PM
To: rfi at contesting.com
Subject: Re: [RFI] Yet another balun question

Hi Paul,

First, let's call a spade a spade -- it's not a "balun" or even a "current  balun," it's a common mode choke.  What we CALL it helps us understand what it is and what it does.

See more comments below.

On Tue,6/28/2016 1:47 PM, N1BUG wrote:
> I am sorry to ask this, but the probably obvious answer isn't obvious 
> to me.
>
> Assume I want to make a current balun consisting of two (or more) 
> chokes in series. Assume each choke will consist of a single 2.4"
> toroid core wound with RG-303 coax (RG-58). Question: What should I be 
> looking at for physical orientation of one choke to another, and 
> distance between them? I am assuming that if nothing else, capacitance 
> between turns on one choke and turns on another choke would be a 
> consideration. Any guidance?

Orientation of one choke to another is not critical. For all practical purposes, the field is confined to the ferrite core .

> Second question: I think I read somewhere that there is little 
> difference in performance between all turns on a single such choke 
> wound "sequentially" around the core vs, winding half, then passing 
> the coax through and 180 degrees across the core, then winding the 
> remaining turns such that the coax leading into the choke and the coax 
> leading out can be 180 degrees opposed to each other.

As far as I know, this method was proposed by W1JR for a choke he wound on a #61 core. I've wound such a choke and measured it, and I've never been able to see a difference between it and a sequential choke. Joe is a fine engineer, but I've shown (in my tutorial) why #61 is a terrible material for a common mode choke for the HF bands.

You didn't say how much power you want to run through it or what the antenna is.  For 80M to about 15M, one of the bifilar chokes I've described wound with 12 turns of a pair of #12 enameled wire or #12 THHN should provide about 5K ohms of choking Z. If the antenna is reasonably well balanced and resonant, it should be good for at least 600W. For higher power and/or significant imbalance, two such chokes in series would be a good solution. With enameled wire, Zo is about 50 ohms. With THHN, it's closer to 80-90 ohms. Jerry Sevick described chokes like this in one of the later versions of his classic work, and noted these values for Zo. I've made both types and measured them, and got the same result. 
This is for closely spaced pairs.

73, Jim K9YC
_______________________________________________
RFI mailing list
RFI at contesting.com
http://lists.contesting.com/mailman/listinfo/rfi