[Elecraft] Balanced solution for KAT500 tuner?

Joe Subich, W4TV lists at subich.com
Sun Sep 30 10:22:49 EDT 2012 

> 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.

That's incorrect.  The feedline characteristic impedance *must* be
considered because it effects the *transformation* that occurs.  The
tuner is not matching the feedline impedance or the antenna impedance
- rather it is matching the *transformed* antenna impedance at tuner
end of the feedline.

Because of the transformation when trying for multi-band operation, it
is generally best to use completely non-resonant antenna lengths, feed
line lengths that avoid even quarter wave lengths on *all* bands, and
the highest possible feedline impedance.

73,

    ... Joe, W4TV


On 9/30/2012 9:47 AM, Don Wilhelm wrote:
> 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, goldtr8 at 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: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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