Posted by
Jim Brown-10 on
Jul 23, 2010; 4:38pm
URL: http://elecraft.85.s1.nabble.com/Balun-Questions-tp5328810p5330342.html
On Fri, 23 Jul 2010 06:34:54 -0400, Tom W8JI wrote:
>The 100 ohms is the differential impedance, or transmission
>line impedance
Tom is the balun expert around here, but he isn't telling you
everything he knows, so I'll float a simple 4:1 balun design
that should work quite well up to moderate power levels. As
Tom's analysis shows (on his link), there are conditions of use
where high power can overheat it.
So to respond to your question with a direct answer, for a 4:1
balun, I would build it from two common mode chokes, each choke
wound on its own 2.4-inch o.d. #31 toroid. Each choke needs 14
bifilar turns of #14 THHN. As you have noted, the chokes should
be wired in parallel on the 50 ohm side and in series on the
200 ohm side. One of the important conditions that makes this
work (or not work) is that the common mode impedance must be
quite high to prevent the choke from overheating, and to
minimize power loss. These chokes provide nearly 5000 ohms
common mode impedance from 2-30 MHz. The impedance is
predominantly resistive.
Bifilar means that you form a transmission line from two equal
lengths of the #14 THHN, tightly spaced so that their
insulation touches, and either tape them together or hold them
together with Ty-wraps, then wind that parallel wire
transmission line around the toroid. You'll need 2.5 inches of
each wire per turn, plus 5 inches for connections at the ends,
then cut off the excess after winding. Sevick said that such a
line is pretty close to 100 ohms, and my measurements suggest
that he was right. Two of these in series/parallel gives you a
pretty good 50:200 balun.
What are conditions that can overheat it? Running high power
(greater than about 500 watts) AND in a condition that places
high common mode voltage on it. You might also get into trouble
at somewhat less power with key-down modes like RTTY. High
common mode voltage will be present if the antenna is severely
unbalanced (for example, an off-center fed antenna), or if the
transmission line is close to a half wave (or multiple of half
waves). At low power, you'll never run into a situation that
will overheat a balun like this, but you could when running
high power.
There's a tutorial on my website that talks extensively about
common mode chokes. It is NOT about baluns, and I don't pretend
to be an expert on baluns, but I do know a lot about common
mode chokes. :)
http://audiosystemsgroup.com/RFI-Ham.pdf
73, Jim Brown K9YC
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