[Elecraft] Balun Questions

[Jim Brown]

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