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
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