Hi Jim,
Thanks for responding.
Comments below.
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.
Correction noted. In theory I should know better.
Orientation of one choke to another is not critical. For all practical
purposes, the field is confined to the ferrite core .
OK. I was thinking capacitance between turns of adjacent chokes
might come into play if they were quite close, say an inch or two.
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.
Good. For some applications, depending on number of turns and
mounting, I like the visual symmetry of that approach. I know that
is being ridiculous. I am often ridiculous.
You didn't say how much power you want to run through it or what the
antenna is.
100 watts on 30 meters, 1500 watts on 40 meters. The antenna is a
rotatable two band dipole. See below.
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.
I have been using chokes of 16 turns RG-303 on 160 meters, 16 turns
#14 THHN on 80 meters, 14 turns on 40, 13 turns on 30, and 12 turns
on 20-10m, all on #31 material. I have yet to experience a failure
at 1500 watts on my wire antennas. Perhaps I have been lucky. I
considered it marginal when I built them.
I have no evidence to back this up, but I am concerned the TH-11DX
antenna below the new 40/30 meter dipole may introduce significant
imbalance. The 40/30m dipole will be parallel to TH-11DX boom. I
want to design for more choking impedance to be on the safe side. I
plan to use #43 material for these chokes.
My bifilar chokes are a little different than what you describe in
your tutorial. I made sure the wire was very straight, then bundled
into pairs with a continuous wrap of tightly wound teflon plumber's
tape.
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.
Understood. I have yet to see any objectionable affect on SWR using
#14 THHN.
73,
Paul N1BUG
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