[Elecraft] Balanced solution for KAT500 tuner?

Phil Hystad phystad at mac.com
Sat Sep 29 22:07:39 EDT 2012 

I have a Comtek 4:1 and a Comtek 1:1 BALUN (from DXEngineering) that I
have where I can easily hook up my 450-ohm ladder line to either one.

I have tested both across all the bands that I use on that particular antenna
which is 80, 40, and 30.  

In every case, the Comtek 4:1 gives the better solution as measured by my
MFJ 459 antenna analyzer.  I do agree that it is not always true that a given
4:1 is better then a 1:1 because on a previous antenna (a delta loop), the 1:1
just so happened to be a little bit better then the 4:1.

In all cases though, I measured impedance right from the shack where about
30 feet of coax ran up to the baluns which were mounted outside.  On the
current antenna, I have close to (but not exactly) 100 feet of 450 ohm ladder
line to the antenna feed point.  My current antenna is center-fed dipole of about 
130 feet in total length.

73, phil, K7PEH

On Sep 29, 2012, at 6:40 PM, Don Wilhelm <w3fpr at embarqmail.com> wrote:

> 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:vk4tux at bigpond.com]
>> Sent: Sunday, 30 September 2012 9:34 AM
>> To: elecraft at 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-bounces at mailman.qth.net
>> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of David Gilbert
>> Sent: Sunday, 30 September 2012 8:37 AM
>> To: elecraft at 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-bounces at mailman.qth.net
>>> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Robert G.
>>> Strickland
>>> Sent: Sunday, 30 September 2012 4:26 AM
>>> To: elecraft at 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
>>> 
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