Don't forget that MFJ now makes a balanced tuner. Three versions - 974B, 974HB, 976. They do have what is needed - a balun at the input followed by a fully balanced tuner. Or at least they appear to be fully balanced.
73 de dave ab9ca/4 On 12/8/11 8:53 PM, Don Wilhelm wrote: > > I read the W8JI comments presented in that link on the Elecraft website. > I was familiar with Tom's work and comments from emailing with him on > this particular subject many years ago. > > The one thing that struck me in W8JI's paper is at the end and embodied > in Tom's comment that -- > > "The irony is, moving the balun to the input mostly works only when the > balun is not needed!" > > For those who want a really high efficiency balanced tuner, go to ARRL > publications back several years to find a link coupled tuner. With > switching between series and parallel tuning, it can handle impedances > from very low to very high. The input is DC isolated from the output, > and the input can be either unbalanced or balanced. The output can be > either balanced or unbalanced depending on where you connect the output > tuned circuit. The venerable Johnson Matchbox is an example of a link > coupled tuner. The taps onto the output tank circuit were created by > the differential capacitor in the output tank circuit - that allowed it > to be boxed up into a bandswitchable box without need for connecting the > antenna feeders to taps on the tuner coil. That works quite well , but > it restricts the range of matching impedances to significantly less than > would be obtained by the basic tuner with taps on the inductor. Yes, I > do use these tuners, and they are configured for single band use at the > antenna - 3 coax feeds for HF and 3 for VHF/UHF give me access to all my > antennas The nearest antenna is 200 feet of coax away from the > hamshack, and that is why I use tower mounted preamps for VHF/UHF. > > I will never give up my Matchbox until it is pried from my cold dead > hands - I use mine mostly as a test instrument - as bandpass filter as > well as an impedance transformation device. After experimentation asnd > development, the permanent tuners are designed and installed in the > antenna field - I have tuned coax fed antennas for each band from 160 > meters through the 432 MHz bands. I use the Matchbox tuner only for > experimental antennas until I can develop a permanent and dedicated > tuner for any one antenna. > > OK, those are the advantages - the drawbacks are that that the link > coupled tuner arrangement does not lend itself well to bandswitching, > but i would suggest it be the tuner of choice for situations where the > antenna is used for only a single band - put the tuner on the antenna > feedline and tune it to resonance (and minimum SWR) once and be done > with the settings. > > 73, > > Don W3FPR > > On 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 >> >> > ______________________________________________________________ > 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 > ______________________________________________________________ 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
