> 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
>>>
>>
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