First we must define what sort of "balun" we are talking about. I know of a half dozen different devices/circuit elements that are commonly called baluns, and most are very different from each other. I abhor the word "balun," and try to avoid using it. I have done considerable research, and published what I've learned, on the subject of COMMON MODE CHOKES wound on ferrite cores, especially LOSSY ferrite cores. The efficiency of such a common mode choke is essentially independent of the SWR, except to the extent of the increased loss in the short length of transmission line wound through the core to form the choke.
Since I have no idea what sort of "balun" Elecraft plans for their tuner, I have no comments on it. I have, however, measured some common mode chokes bifilar wound using #14 and #12 THHN to form a parallel wire transmission line that have excellent choking performance for the HF bands. The data are included in the latest version of my RFI tutorial (since the summer of 2010). The concept of a totally isolated tuner with common mode chokes at input and output is so non-nonsensical to me that I've never contemplated it. It is also nonsense to consider parallel wire line as a balanced system unless everything connected to it is also balanced -- that is, tuner, line, and antenna. Since most ham antennas are at least somewhat unbalanced by their surroundings, even when fed by these parallel wire lines, there will be SOME imbalance in the line, and thus some common mode current. That common mode current causes trouble in at least three ways -- it puts RF in the shack to excite Pin One Problems (very common in ham gear); the common mode current will radiate, potentially causing RFI to consumer gear in your living room (and your neighbor's); and by reciprocity, the transmission line will act as a receiving antenna, coupling noise from your neighbor's computers and battery chargers to your receiver. THAT"S why we need common mode chokes at the feedpoint of EVERY antenna, whether fed with coax or parallel wire line. Loss in common mode chokes is addressed in the sections of my RFI tutorial where transmitting chokes are discussed. http://audiosystemsgroup.com/RFI-Ham.pdf 73, Jim Brown K9YC On 12/8/2011 4:36 PM, Ron D'Eau Claire wrote: > Is it true that the efficiency of a balun is not affected by the impedance > of the line in which it is used? > > The justification I had seen for putting the balun at the tuner input where > it will see a load very close to 50 ohms resistive (when the tuner is > adjusted for a "match") while at the output the balun may see anything over > a huge range of impedances. ______________________________________________________________ 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
