Let me rephrase what Joe is stating - in an unmatched feedline situation, the feedline characteristic impedance is not to be considered. It does not matter whether it is 50 ohms, 300 or 450 ohms - that characteristic impedance is NOT what is being matched. The feedline acts as an impedance transformer.
As a result, a 4:1 balun may do more harm than good. A simple common mode choke (usually a 1:1 current balun) will do the job of reducing the feedline radiation (and noise pickup on receive). A 4:1 balun may actually make the antenna/feedline combination difficult to match - most tuners do not match well into extremely low or high impedance loads. 73, Don W3FPR On 9/30/2012 9:28 AM, Joe Subich, W4TV wrote: >> However if you have an antenna such as a loop which has a feedpoint >> impedance of closer to 200 ohms then a 4:1 would make sense to me. >> The reason is I need to reduce the feedpoint resistance of the >> antenna as I do not change the resistance based on the feedline. > The "problem" with this logic is that parallel wire line is generally > not the same impedance as the antenna it is feeding - e.g., "450 Ohm" > line feeding a full wave loop (ca. 100 Ohms). As such, the impedance > the tuner sees can be *anything* due to the transformation effect of > the unmatched feedline. > > This is particularly apparent if one feeds a full wave dipole (e.g. > 130' on 40 meters) at the center with an odd number of 1/4 open > wire feedline (e.g. 100'). The feedline will transform the high > feedpoint impedance to a very low value (<10 Ohms) ... when that is > further transformed down by the 4:1 balun, the tuner will have a lot > of trouble (and loss) with the 2 Ohm or so load. > > Within limits, most tuners are better able to handle high impedance > mismatches than low impedance mismatches because losses tend to be > resistive in nature and the lower impedance results in lower currents > and thus lower (I^2*R) losses. The lower current is also why parallel > wire feedlines tend to have lower losses (particularly with high SWR) > - again there is much less resistive loss due to the lower current. > > > 73, > > ... Joe, W4TV > > > On 9/30/2012 8:37 AM, gold...@charter.net wrote: >> As I am not sure of this stuff I need to ask the following question. >> >> The example below is based on a doublet antenn which has a feed point of >> close to 50 ohms. So the ladder line is not what you want to transform >> so I understand that a 1:1 is correct. >> >> However if you have an antenna such as a loop which has a feedpoint >> impedance of closer to 200 ohms then a 4:1 would make sense to me. The >> reason is I need to reduce the feedpoint resistance of the antenna as I >> do not change the resistance based on the feedline. >> >> The rational for the balun is to transform the feedline from balanced >> line to unbalanced line which is the 50 ohm coax. Thus you want the >> currents matched in the coax thus the balun. Conclusion balun to >> transform balance to unbalanced and ratio for feedpoint impedance >> matching if required by antenna type. >> >> Anyway I might have my terms for impedance and resistance slightly off >> but please verify if I am correct in thinking that you need a balun >> ratio based on the antenna not the type of feedline. >> >> So am I correct in my understanding or just totally out in the weeds. >> >> ~73 >> Don >> KD8NNU >> FH#4107 >> >> >> On Sat, Sep 29, 2012 at 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 >> > ______________________________________________________________ > 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
