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 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
