On 12/8/2011 7:51 PM, Alan Bloom wrote: > There's a simple way to look at this that requires no high-level math or > complicated analysis. > _________ _________ > I1 --> | | | | I3 --> > --------| |--| |------- > Transmitter | BALUN | | TUNER | Antenna > --------| |--| |------- > I2 --> |_________| |_________| I4 --> > > The purpose of the balun is to eliminate the common-mode current. The > CM current is the NET current, that is, I1 + I2 at the input and I3 + I4 > at the output. The balun does not store electrons, so it must be that: > > Input common-mode current = output common-mode current. > > That is true for both the balun and the tuner. No matter whether the > balun is on the transmitter or antenna side of the tuner it does an > equally good (or bad) job of choking the common-mode currents. > > The advantage of putting the balun at the input is that the > differential-mode voltages and currents (the ones you want) are > well-controlled because of the constant 50-ohm impedance. The DM > current or voltage at the output can be much higher, depending on the > load impedance. Of course, the common-mode current and voltage are the > same at the input and output, but even so the worst-case stress on the > balun should be less when it is at the input. > > The disadvantage of putting the balun at the input is that none of the > tuner circuitry can be grounded. For example, the control shafts of the > variable capacitors have RF voltage on them, so the knobs must be > isolated.
Change the word "balun" to common mode choke, and I'm with you up to here. > If it is an automatic antenna tuner, that is less of a problem. Except that the automatic circuitry, and it's power supply must also be isolated from ground. That part of the design problem seems non-trivial. > Bottom line? The ARRL is not necessarily "wrong" to put the balun at > the tuner input. It's just a matter of whether you think the lower > worst-case current/voltage on the balun is worth the hassle of floating > the tuner ground. I agree with your analysis, Alan, with the exception noted. 73, Jim K9YC > > Alan N1AL > > > On Thu, 2011-12-08 at 18:59 -0500, Don Wilhelm wrote: >> This is of interest to me, because in my first experiences with antenna >> forums (or was it newsgroups at the time), I recall a lot of exchanges >> with Tom W8JI on exactly the "balun at the tuner input and isolated >> unbalanced tuner". I concur with Tom - it does not work -- both from a >> theoretical basis, and also from Tom's measurements. >> >> This was "way back when" - as I recall my situation when all this was >> going on, I was running Windows 95 and the year was between 1997 and 1998. >> >> In the timeframe of this discussion, Zack Lau (ARRL engineer) who had >> first published the "balun at the tuner input" concept as a QRP tuner, >> had retracted that design because it did not maintain balance, but Dean >> Straw (ARRL engineer, editor, etc.) published his design of a high power >> tuner using the same concepts, and that design can still be seen in the >> ARRL publications. >> >> Apologies for the comments into the politics of the ARRL decisions on >> what is to be published, but that is both a bit of the history as I know >> it as well as my view of the technical side of this issue. >> >> If anyone can tell me how you can run a signal through a balun - and >> have equal and opposite currents at its output, and then run it through >> an unbalanced network with unequal elements in the two series legs and >> still maintain equal and opposite currents and phase, and I will then >> concede that an isolated unbalanced tuner with a balun at the input will >> work, but until that is presented to me along with detailed engineering >> level test data (not just "it works"), I will continue to believe that >> using a balun on the input of an isolated unbalanced tuner is a "pipe >> dream" that does not mesh with reality. >> >> 73, >> Don W3FPR >> >> aOn 12/8/2011 6:28 PM, Eric Swartz - WA6HHQ, Elecraft wrote: >>> Hi Ignacy, >>> >>> This is a common misconception. (One which I held until recently. :-) It >>> turns out there is no advantage to placing the balun at the input of the >>> L-Network tuner. Since one end of the balun is grounded by the input to >>> the tuner, it is still presented with the same stresses under high SWR >>> situations. Baluns at the input and output both drive balanced loads >>> equally well. >>> >>> We've now put together a web page describing the impact of placing the >>> balun at the input or at the output of a L-Network tuner. See: >>> >>> http://www.elecraft.com/KAT500/input_versus_output_balun.htm >>> >>> At the bottom of that page are several links providing detailed >>> technical analysis of these configurations. The first two, by W8JI and >>> W7EL are very clear discussions of this issue. >>> >>> 73, Eric WA6HHQ >>> >>> www.elecraft.com >>> >>> >>> On 12/8/2011 3:17 PM, Ignacy wrote: >>>> It seems to me that the story is more complex than it sounds. >>>> >>>> The input balun always works at low SWR and at 50 Ohm. It is very easy to >>>> have such balun. A small balun would easily handle a KW without heating. >>> ______________________________________________________________ > > ______________________________________________________________ > 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 > > > ----- > No virus found in this message. > Checked by AVG - www.avg.com > Version: 2012.0.1873 / Virus Database: 2102/4668 - Release Date: 12/08/11 > > ______________________________________________________________ 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
