Many hams *asssume* that because the feedline character8istic impedance is 450 ohms, that it will work better with a 4:1 balun.
Nothing could be further from the truth, the impedance seen at the shack end of the feedline can vary from quite high to quite low - the feedline works as a transmission line tuner (and the ATU does too). For powers up to 100 watts, I suggest the Elecraft BL2 which provides a switch to change from 1:1 to 4:1. Use the setting that provides the best results. 73, Don W3FPR On 9/29/2012 7:41 PM, Adrian wrote: > Here: > > http://www.karinya.net/g3txq/tuner_balun/ > > "Amateur Radio (G3TXQ) - Tuner Balun: 4:1 or 1:1 ? > > Follow the discussions on any Internet Ham Radio forum and it wont be long > before someone recommends the use of a 4:1 balun at the output of a tuner > feeding a multiband doublet through ladderline. Often the recommendation is > accompanied by an explanation such as: "It helps the tuner to tune" or: "It > reduces the impedance of the 450 ohm ladderline towards 50Ω". This article > examines whether the ubiquitous advice to use a 4:1 balun in this scenario > is valid. > > The arguments put forward for a 4:1 impedance transformation often assume > that the tuner works best when operating with a load impedance close to 50Ω. > That simply isn't true! The chart on the right shows the losses vs load > resistance of a typical T-network tuner on 80m for several values of load > reactance; 80m was chosen because losses tend to be more evident on the > lower-frequency bands. As we can see the lowest tuner losses occur when the > load resistance is in the medium/high range 250Ω-2500Ω; the highest losses > occur at low load resistances, particularly where they are accompanied by a > large capacitive reactance. > > Let's now take the example of a commonly proposed multiband doublet - a > half-wave 80m dipole fed with 450Ω ladderline. At modest heights above > average ground the dipole has a feedpoint impedance close to 50Ω. That means > that the impedance seen at the tuner end of the ladderline could have a > resistive component anywhere from 50Ω to 4050Ω depending on ladderline > length; that range of impedances is indicated by the lower shaded bar in the > chart, labelled 1:1. If we now introduce a 4:1 impedance transformation, the > range of impedances will be lower by a factor of 4 as indicated by the upper > shaded bar labelled 4:1. It's clear that the 1:1 range of impedances will > result in the lower overall losses. > > In fact, no matter what the antenna impedance, the range of impedances seen > at the tuner end of the ladderline would have a "geometric mean" of 450Ω - > that is they would swing equally below and above 450Ω, but once we introduce > a 4:1 balun the geometric mean will reduce to 112.5Ω. One look at the loss > chart tells you that centering the impedances at the higher value is the > preferable option. > > > Let's now take a look at the specific losses that would occur with our > example 132 doublet fed with 450Ω ladderline. > > The chart on the right was produced by varying the feedline length from 0° > to 180° in 10° steps. At each step the impedance seen by the tuner was > calculated both with a 1:1 balun and then with a 4:1 balun, and the tuner > losses determined using W9CF's T-network tuner simulator. Of course, beyond > 180° the chart simply repeats itself. Ladderline losses were ignored. > > Apart from a small range of line lengths between 80° and 115°, where the > line has transformed the 50Ω feedpoint impedance to a very high value around > 4000Ω, the 1:1 balun is the better option; not only that, the worst case > loss never exceeds 14% with the 1:1 balun whereas it reaches 21% with the > 4:1 balun. > > But what about other bands - the doublet wont be used on just 80m! > > > This chart shows the tuner loss plotted against line length for our example > doublet on 40m. Here the doublet feedpoint impedance is around 4000Ω, so for > short ladderline lengths the 4:1 balun shows an advantage. However, as the > ladderline length increases and the impedance is transformed to lower > values, the 1:1 balun soon shows the lower losses again. Across the whole > range of ladderline lengths the 1:1 balun is twice as likely as the 4:1 to > produce lower losses. > > The conclusion seems clear: if you have to choose just one balun, unless you > know that your combination of doublet/ladderline length falls into the > minority of cases where a 4:1 balun has the advantage, a 1:1 balun is the > preferred choice. Add into the mix the fact that most 4:1 baluns are Voltage > Baluns, whereas to prevent feedline radiation we want balanced currents; > then consider that all baluns other than a 1:1 Current Balun have the full > transmit voltage applied common-mode across one or more windings, and the > case is compelling for a 1:1 Current Balun in this situation. > > In this application any small impedance transformation caused by the Current > Balun is immaterial because the tuner will compensate, so the windings do > not need to be of any specific characteristic impedance. Typically, bifilar > windings using Thermaleze wire inserted in Teflon tubes are employed to cope > with the high differential-mode voltages present at current minima. Balun > specialists "Balun Designs" offer a nice example in their Model 1171. > " > > -----Original Message----- > From: Adrian [mailto:vk4...@bigpond.com] > Sent: Sunday, 30 September 2012 9:34 AM > To: elecraft@mailman.qth.net > Subject: RE: [Elecraft] Balanced solution for KAT500 tuner? > > A UK G call did a graphed efficiency comparison test with the 1:1 coming out > on top. I will post it when re-found. > > Also from > http://www.theladderline.com/doublets-ladder-line-and-automatic-remote-tuner > s ; (spell-checked) > > "There is also some debate about whether the balun should be a 1:1 or 4:1. I > think 4:1 has been quite popular in the past. This perhaps comes from the > thought that the ladder line is higher impedance than coax so we need to > step down to get it closer to the coax impedance. On the forums, people who > have modelled the antenna with software like EZNEC seem to make a fairly > compelling argument that a 1:1 balun is more likely to present an impedance > within the range of the tuner over a wide range of frequencies. The > impedance will depend on the length of the ladder line so it's a bit of a > gamble but I'm getting good results from a 1:1 current balun. > > After doing some reading and playing with a home brew balun, I finally took > the lazy way out and splashed out on a serious balun. It's a DX Engineering > BAL050-H10-AT. It's not cheap but I think it was a good investment. I don't > have any hard evidence to show how good it is compared to a cheaper balun > but I have a feeling that it contributes to the good performance I get with > this antenna. I've had absolutely no "RF in the shack" or similar problems > even at the old QTH when I had the ladder line coming into the shack. > > An interesting fact that I learned from the forums is the reason ladder line > works more successfully than coax in a multiband situation like this. The > common belief is that ladder line "doesn't care" about high SWR. It's true > that ladder line usually has lower loss than coax at a given SWR but that's > not the whole story. Another rather simple factor is that the characteristic > impedance of ladder line is higher than coax so therefore, for a typical > wire antenna over a wide range of frequency, the average SWR on ladder line > tends to be lower than it would be with coax and that helps keep the loss > low." > > -----Original Message----- > From: elecraft-boun...@mailman.qth.net > [mailto:elecraft-boun...@mailman.qth.net] On Behalf Of David Gilbert > Sent: Sunday, 30 September 2012 8:37 AM > To: elecraft@mailman.qth.net > Subject: Re: [Elecraft] Balanced solution for KAT500 tuner? > > > I don't believe that is necessarily true. Can you cite a reference to back > up that statement? Or at least describe in physical terms (Q, currents, > voltages, component loss, etc) why that would be so? I'm honestly curious > what the difference would be. > > 73, > Dave AB7E > > > > On 9/29/2012 2:13 PM, Adrian wrote: >> 1:1 current balun has proven more efficient in conjunction with the >> appropriate balanced (matchbox style)tuner. >> >> -----Original Message----- >> From: elecraft-boun...@mailman.qth.net >> [mailto:elecraft-boun...@mailman.qth.net] On Behalf Of Robert G. >> Strickland >> Sent: Sunday, 30 September 2012 4:26 AM >> To: elecraft@mailman.qth.net >> Subject: Re: [Elecraft] Balanced solution for KAT500 tuner? >> >> Jim... >> >> Is there anything to be gained in putting a 1:1 "balanced isolator" at >> the feed point of an antenna that is fed by a "parallel wire" feed >> line? Does such an arrangement achieve feed line isolation while >> preserving the ability of such an antenna to be driven on various >> bands other than its resonant frequency? Thanks for your input. >> >> ...robert >> > ______________________________________________________________ > 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 ______________________________________________________________ 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
