Balun at Input or Output of Antenna Tuner? Dean Straw, N6BV (Senior Assistant Technical Editor, Retired) December 12, 2011 I have been lurking on the Elecraft Reflector monitoring the animated discussion about where to place a balun -- at the input or the output of a tuner. I was going to jump into the discussion after I read comments claiming that a balun at the input of an unbalanced tuner feeding a balanced transmission line (and balanced antenna) simply "doesn't work." On Dec. 8 Alan Bloom, N1AL, wrote such an elegant analysis that I didn't have to.

If I might paraphrase Alan, the stress across the balun due to unwanted common-mode current is the same whether the balun is placed at the input or at the output of an unbalanced antenna tuner network. Other investigators have come to the same conclusion and have argued that the complexity of "floating" the antenna tuner components doesn't warrant installing the balun at the input of the antenna tuner. As Alan pointed out, when a balun is placed at the input of the tuner, the differential-mode currents/voltages are well controlled, since the tuner's input impedance when tuned is 50 ohms. When the balun is placed at the output of the tuner, the differential-mode currents/voltages are determined by whatever impedance the load happens to be at the shack-end of the transmission line feeding the antenna. And that impedance can range very widely. For my own curiosity, I wanted to get a handle on the amount of loss incurred by the differential-mode loss (i.e., the loss "inside the coax") for a common-mode choke constructed with coaxial cable. (Google Jim Brown, K9YC's excellent paper "A Ham's Guide to RFI, Ferrites, Baluns, and Audio Interfacing" Revision 5a, 5 Jun 2010. The section on transmitting baluns starts on page 25. Photos of typical coaxial CM transmitting chokes appear on page 29.) By the way, like Jim Brown, K9YC, I prefer the name "common-mode choke" rather than the term "balun," whether it is used at the input or at the output of an unbalanced tuner feeding a balanced transmission line and balanced antenna. I shall use the abbreviation "CM choke" in the rest of this paper. I assumed that the CM choke consisted of three feet of RG-213 wound through ferrite torroids of the appropriate material and size to achieve K9YC''s target common-mode impedance of 5000 ohms. At that choke impedance the effects of unwanted common-mode current would be low and the effect of differential-mode loss could be explored from a conventional transmission-line perspective. A quick review: a CM choke exhibits two main modes: (1) an impedance to common-mode currents flowing on the outer surface of the coaxial shield and (2) the effect of differential-mode currents/voltages - that is, matched-line loss and the additional loss due to SWR of a three-foot length of RG-213. My tool of analysis is my own TLW program, but there are other programs out there that can do the analysis. I assumed that the load presented to the output terminals of the common-mode choke balun at the output of the tuner was 3000 + j0 ohms at 28 MHz. This amounts to an SWR of 60:1. TLW says that the total transmission-line loss in the three feet of RG-213 under these conditions is 0.373 dB. This is comprised of 0.034 dB of matched-line loss if the RG-213 were matched and an additional loss due to the 60:1 SWR of 0.339 dB. A total loss 0.373 dB doesn't sound like much, but when you pump 1500 W into such a tuner this translates into a lot of watts in a confined space. For 1500 W into the input of the CM choke, 123.5 W must be dissipated safely in the three feet of RG-213. This is 41.2 W per foot and at that level the RG-213 would get very warm and could even melt, especially if the choke were confined in a small box with no circulation of cooling air. Times Microwave has a convenient calculator on their web site, where the power-handling capability of various types of coaxes may be calculated. 100 feet of RG-213 is rated to handle 2.02 kW at 28 MHz with a 1:1 SWR. For a matched line there would be no "hot spots" along the line due to SWR. The average power-handling capability of RG-213 is thus 2020 W divided by 100 feet, or 20.2 W per foot. Interestingly enough, this short length of coax transforms the 3000 + j 0 load to 1.72 - j 47.3 ohms at the output terminals of the tuner, making the task of the antenna tuner a little more difficult at such a low resistance level. If this 3000-ohm load were presented to a tuner with the CM choke at the input, the loss would be the matched-line loss only of 0.034 dB, which is a power loss of 11.7 W for 1500 W input to the tuner. The same kind of TLW analysis can be done for a low-impedance load of 3 + j 0 ohms, for an SWR of 16.67:1. For a CM choke balun at the output of a tuner this results in an additional loss due to SWR of 0.402 dB and a total loss of 0.436 dB. This is a power loss of 143.3 W in the three feet of RG-213, meltdown city again. While we're at it, let's look at the potential loss due to line losses at a CM choke balun placed in the wrong place in an antenna system. Assume the common scenario where a balanced antenna is fed with open-wire transmission line to a 1:1 common-mode choke balun located at the shack window. From the balun at the window the ham uses, say, a 20-foot section of RG-213 to the antenna tuner (which in this case is an unbalanced tuning network). Assume again that the CM choke balun uses three feet of RG-213 wound on the appropriate ferrite donuts to achieve the target common-mode impedance of 5000 ohms so that common-mode currents are choked off properly. The total length of RG-213 is now 23 feet. Again, we'll present the balun at the windowsill with a load of 3000 ohms. The overall differential-mode loss in 23 feet of RG-213 is 4.534 dB, nearly 4 dB worse than connecting the open-wire line directly to a tuner with a CM choke balun at its output! Ouch, that's a lot of wasted power. 73, Dean, N6BV ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html