[RFI] Yet another balun question

[Jim Brown]

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