More Balanced Current Measurements - Voltage Balun

[John W2XS]

I had previously posted some RF Current data using an MFJ-835 Balanced RF Current meter.  I measured the amount of current flowing in each side of my ladder line and compared the Johnson Matchbox, the internal KAT3 antenna tuner with the Elecraft BL2 balun in both the 4-to-1 and 1-to-1 positions, and a direct connection from the KAT3 to the ladder line with no balun.  Tonight, I came across a 4-to-1 voltage balun left over from the days of the Ultimate Transmatch (QST July, 1970 - wow! - that is a long time ago but seems like yesterday).

Anyway, I put the voltage balun in line and repeated the measurements.  Here are my conclusions based on my results:

1) Without the MFJ 835 current meter, I would have been happy with any of the configurations.  All of them had a perfect 1-to-1 SWR when matched. All of them resulted in good QSOs.
2) The Johnson Matchbox has the best balance and least loss on all bands.
3) A balun on the output of an unbalanced tuner is a compromise and there is loss there.
4) The BL2 has good balance on all bands 160m to 6m but gets very hot on some bands indicating loss. I have received reports that the Johnson MB is several S units better than the balun on some bands.
5) The voltage balun works pretty well on 160m, 80m, and 40m but is poor on 30m and higher. It is useless on 6m.
6) The direct connection had very poor balance on some bands.

So, IMHO, a balanced tuner is the best matching device for a balanced antenna fed with ladder line. We must find a way to convince Elecraft to offer a balanced version of the KAT3. Maybe something along the lines of Zack Lau's QRP Balanced Transmatch on the ARRL website (in the TIS section. The balun is on the input side and operates near the design impedances).

If you want to peek at the data itself, it's on my website.  The link is on QRZ.COM. The "sum" column is an indication of the total current in both sides.  The higher the number, the better.  The "difference" column is an indication of balance.  The smaller the number, the better.  

73,

John W2XS

[Craig Smith]

<> So, IMHO, a balanced tuner is the best matching device for a balanced
<> antenna fed with ladder line. We must find a way to convince Elecraft to
<> offer a balanced version of the KAT3.

Better yet, we must convince them to make the balanced tuner in a remote
configuration so it can be outside in the elements with 50 ohm coax between
it and the K3.

My BL2 also gets mighty hot on some bands at 100W when feeding ladderline.
Of course the line length can be adjusted, but an external automatic
balanced tuner would be MUCH nicer!  (hint, hint)

            ...   Craig  AC0DS


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[Jim Brown-10]

On Wed, 13 Aug 2008 21:32:46 -0600, Craig D. Smith wrote:

>My BL2 also gets mighty hot on some bands at 100W when feeding ladderline.

Heat means LOST POWER. Virtually all baluns like the BL2 use Fair-Rite #61
(or something quite similar). Download

http://audiosystemsgroup.com/RFI-Ham.pdf 

and study Fig 6, the loss characteristics of Fair-Rite #61. The loss begins
to rise above 10 MHz, and it isn't pretty above about 20 MHz.

73,

Jim Brown K9YC



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[W8JI]

> I had previously posted some RF Current data using an
> MFJ-835 Balanced RF
> Current meter.  I measured the amount of current flowing
> in each side of my
> ladder line

I'm pretty sure that unit doesn't actually indicate balance.
I'm pretty sure it only indicates equal currents by sampling
scalar currents in each conductor. I'll look into that, but
everything in the manual seems to indicate it is a scalar
measurement of current in each conductor.

Anyway the important point is that it is possible to have
equal currents in each conductor and have perfect UNbalance.

To measure balance we have to measure phase, not just scalar
currents. We also have to either measure current balance at
two points some large fraction of a wave apart (like 1/8th
or 1/4 wave), or measure BOTH voltage and current balance at
one point.

>and compared the Johnson Matchbox, the internal KAT3
>antenna
> tuner with the Elecraft BL2 balun in both the 4-to-1 and
> 1-to-1 positions,
> and a direct connection from the KAT3 to the ladder line
> with no balun.
> Tonight, I came across a 4-to-1 voltage balun left over
> from the days of the
> Ultimate Transmatch (QST July, 1970 - wow! - that is a
> long time ago but
> seems like yesterday).

There are only a few cases where we might be better off
using a 4:1 balun rather than a 1:1 balun to transform
random impedances to a tuner. Those cases would be where the
antenna system (at the balun connection point) presents a
reasonable impedance to the balun, the balun is designed for
that frequency and impedance, and the antenna system balance
presented to the balun is pretty good.

It would be even rarer to need or want a voltage balun. I
can't think of many of any cases where I would want to use a
balanced voltage source to feed a simple antenna. As a
matter of fact I virtually never use a 4:1 unless I'm
matching a folded dipole to coax.

> Anyway, I put the voltage balun in line and repeated the
> measurements.  Here
> are my conclusions based on my results:
>
> 1) Without the MFJ 835 current meter, I would have been
> happy with any of
> the configurations.  All of them had a perfect 1-to-1 SWR
> when matched. All
> of them resulted in good QSOs.
> 2) The Johnson Matchbox has the best balance and least
> loss on all bands.
> 3) A balun on the output of an unbalanced tuner is a
> compromise and there is
> loss there.
> 4) The BL2 has good balance on all bands 160m to 6m but
> gets very hot on
> some bands indicating loss. I have received reports that
> the Johnson MB is
> several S units better than the balun on some bands.
> 5) The voltage balun works pretty well on 160m, 80m, and
> 40m but is poor on
> 30m and higher. It is useless on 6m.
> 6) The direct connection had very poor balance on some
> bands.
>
> So, IMHO, a balanced tuner is the best matching device for
> a balanced
> antenna fed with ladder line. We must find a way to
> convince Elecraft to
> offer a balanced version of the KAT3.

As a general rule it is far less expensive to build a good
1:1 balun to use on the output of a unbalanced tuner than it
is to build a balanced tuner. This is especially true if the
load is not perfectly balanced, or on higher frequencies.

When I built high power tuners for a 25 kW AM SW BC station,
I looked at all options and used 1:1 baluns on the matching
system output. It was several thousands of dollars cheaper
than using a balanced network and worked just as well.

I'd encourage them to build a good 1:1 balun, or perhaps
two...one for lower bands and one for higher bands. The
expense of a balanced tuner generally just isn't worth it,
and a 4:1 is almost never needed.

I'll look at that balance indicator if I can get my hands on
one. It might not be, but from the manual it sounds like it
is a scalar device that does not consider phase.

73 Tom

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[Phil Salas]

"Heat means LOST POWER. Virtually all baluns like the BL2 use Fair-Rite #61
(or something quite similar). Download
http://audiosystemsgroup.com/RFI-Ham.pdf
and study Fig 6, the loss characteristics of Fair-Rite #61. The loss begins
to rise above 10 MHz, and it isn't pretty above about 20 MHz."

The figure referred to is using the material as part of an RFI suppression
choke where you want attenuation at RF frequencies.  I think you'll find
that when the Type 61 material is used as a transformer, it will have very
low loss if designed properly.  See Sevick's book on baluns and ununs.  But
I think that even a well-designed transformer will have maybe 1-2% loss.  At
100 watts, that is 1-2 watts which will cause the transformer to heat up.
Just dissipate 1-watt in a resistor with your DC power supply and feel the
heat.

Phil - AD5X

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[Don Wilhelm-4]

Phil,

Phil,

A current balun *is* a choke, not a transformer.  That is true for both
the 1:1 and 4:1 baluns.  OTOH, a voltage balun is a transformer, but I
am not sure why anyone would want to use one except for special
applications.

73,
Don W3FPR

Phil & Debbie Salas wrote:

> The figure referred to is using the material as part of an RFI
> suppression choke where you want attenuation at RF frequencies.  I think
> you'll find that when the Type 61 material is used as a transformer, it
> will have very low loss if designed properly.  See Sevick's book on
> baluns and ununs.  But I think that even a well-designed transformer
> will have maybe 1-2% loss.  At 100 watts, that is 1-2 watts which will
> cause the transformer to heat up. Just dissipate 1-watt in a resistor
> with your DC power supply and feel the heat.
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[W8JI]

> The figure referred to is using the material as part of an
> RFI suppression choke where you want attenuation at RF
> frequencies.  I think you'll find that when the Type 61
> material is used as a transformer, it will have very low
> loss if designed properly.

Even 73 materials can have low loss if the transformer has
high enough impedance compared to the load impedance and it
is a true transmission line transformer, but materials with
a lower loss tangent are generally better if the transformer
has enough impedance across each winding.

> I think that even a well-designed transformer will have
> maybe 1-2% loss.  At 100 watts, that is 1-2 watts which
> will cause the transformer to heat up. Just dissipate
> 1-watt in a resistor with your DC power supply and feel
> the heat.

It's all about the surface area and the dissipation. Many
even wrongly call heat "saturation", when it is almost
always nothing more than dissipation from the resistive part
of the impedance and the voltage across (or current through)
that resistance.

As a matter of fact most baluns and transformers operating
at more than a few watts heat long before they come close to
saturating. For example the balun in the ATR30 tuner
overheats to the point where it reaches the core temperature
limit at just 100 watts of dissipation for about one minute.
The heat is not caused by saturation unless you are running
a 30 kW pulse transmitter with low duty cycle into a
particular load. Heat is almost always caused by resistive
losses, and the balun will typically handle 2-3 kW of CW
without undue heating. 50 watts of loss out of 2 kW will
make it get VERY warm, but it is insignificant loss and not
saturation.

Put heat it into perspective of the applied power and the
area dissipating heat. Think of how hot a 7 watt light bulb
gets and the surface area of the bulb. Now think of a
standard 100 watt bulb, or a standard 40 watt florescent
lamp.  All of them mostly make heat  from the applied power.
It is a surface area and power dissipation problem so we
have to use common sense by comparing it to similar size
things.

A final thought. The worse way in the world to determine
balun efficiency is with a signal report. First, there is a
time problem on skywave. By the time we change baluns or
tuners propagation can change. Second, either locally or on
skywave any change in balance can change antenna patterns.
Groundwave is particularly USELESS with a horizontally
polarized antenna. I know of at least three tests where
people compared tuner or balun configurations on groundwave
using horizontally polarized antennas. The results of those
tests are meaningless, because a change in feeder unbalance
would significantly affect ground wave field strength. This
is because only the vertical component can propagate over
any distance along the earth, or does well near the earth's
surface. The worse system for balance will produce the
strongest local signal when using horizontally polarized
antennas at HF.

We have to be careful what we conclude or assume. Even when
we are careful and use good methods, there can be mistakes
or things we miss.

73 Tom




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[Jim Brown-10]

On Thu, 14 Aug 2008 06:37:15 -0500, Phil & Debbie Salas wrote:

>"Heat means LOST POWER. Virtually all baluns like the BL2 use Fair-Rite #61
>(or something quite similar). Download
>http://audiosystemsgroup.com/RFI-Ham.pdf
>and study Fig 6, the loss characteristics of Fair-Rite #61. The loss begins
>to rise above 10 MHz, and it isn't pretty above about 20 MHz."

>The figure referred to is using the material as part of an RFI suppression
>choke where you want attenuation at RF frequencies.  

NO!  The figure shows the FUNDAMENTAL PROPERTIES OF THE MATERIAL as published
on the data sheet by the manufacturer, Fair-Rite. See the text of the tutorial
for a discussion of what those data mean.

>I think you'll find that when the Type 61 material is used as a transformer,
>it will have very low loss if designed properly.

That can be a big if. It depends on frequency, the parts used, the windings,
and the impedances. And as Tom observes, it's not easy to measure with
instrumentation commonly available to hams.

73,

Jim Brown K9YC




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