balanced tuner

[bruce bennett]

I heard some one say that a balanced tuner is very expensive to build. I used some quarter inch copper tubing from the hardware store to build a four inch coil. This is the tubing for a ice maker. I then built a four inch coil and tapped it to a multi position switch from radio shack. I had a  used large old condenser and hooked the whole thing up in a L network design. I left the ground connection floating on both input and output and connected it directly to ladder line. A current balun was made out of 25 ft of coax and put at the input of the tuner. Works great on my 330 ft loop. No second coil is necessary. Everything is balanced and nothing heats up. Kind of ugly looking, but the electrons don't seem to mind.
Bruce k2pdj


     
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[W8JI]

>I heard some one say that a balanced tuner is very
>expensive to build. I used some quarter inch copper tubing
>from the hardware store to build a four inch coil. This is
>the tubing for a ice maker. I then built a four inch coil
>and tapped it to a multi position switch from radio shack.
>I had a  used large old condenser and hooked the whole
>thing up in a L network design. I left the ground
>connection floating on both input and output and connected
>it directly to ladder line. A current balun was made out of
>25 ft of coax and put at the input of the tuner. Works
>great on my 330 ft loop. No second coil is necessary.
>Everything is balanced and nothing heats up. Kind of ugly
>looking, but the electrons don't seem to mind.>>

Bruce,

I looked at this issue extensively when the whole myth about
moving baluns came out.

Moving the balun to the input of an unbalanced network does
NOT make life on the balun or the system easier for the
truly difficult problem, common mode isolation. It does not
change a thing to the better for common mode currents or
isolation, and it actually makes the system worse on higher
bands where network physical size and unwanted stray
capacitance affects balance.

It takes exactly the same common mode impedance and common
mode current and voltage capacity in the balun if it is
located at the tuner output or at the tuner input when the
network is a floating unbalanced  network. The core (if
used) will get just as hot, and current unbalance (except
for stray capacitance or network transmission line effects)
will be exactly the same.

If you draw it on paper and trace the path from one lead of
the transmission line you will see exactly what  mean. There
is a direct connection from one side of the balanced antenna
terminals to the balun, and this means the balun  has
EXACTLY the same common mode problems. The only thing you
modify is the differential impedance, and it is extremely
easy to solve that issue with any current balun so you
really just fix something that is largely a non-issue to
start with.

Now if you used a real balanced network with series
impedances in each leg and in particular some perfectly
ground reference point for the shunt elements, you would
make common mode life easier for the balun BUT the drawback
is you now have a balanced voltage source which may or may
not supply balanced currents. The symmetry of the network is
also critical. You have, in essence, exactly the same
expense and difficulty as simply building a balanced tuner
of any standard configuration.

A manufacturer would be misleading customers if it claimed
they had a balanced tuner when using an non-symmetrical
floating network with a balun on the input. It would be no
better than the same balun on the output, and likely much
worse on upper bands.

I'm afraid there is no free lunch. It has to be a balanced
network which means at least double the cost of the
expensive components, or you can simply build a good balun
and use an unbalanced network on the balun input. Building a
very good balun for the output is less than half the overall
cost of using an expensive true-balanced network, I know
because I priced this stuff out dozens of times.

There might be a marketing or sales advantage to customers
who feel good about a balun on the input, but that would be
dishonest or incompetent engineering by the manufacturer to
claim it did anything for system performance.

73 Tom






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[Jim Brown-10]

On Sat, 16 Aug 2008 11:22:52 -0400, Tom W8JI wrote:

>Moving the balun to the input of an unbalanced network does
>NOT make life on the balun or the system easier for the
>truly difficult problem, common mode isolation.

I strongly agree with all of this. One of the most important
functions of a common mode choke is decoupling noise received on the
transmission line from antenna. Obviously, the same configuration
also is most effective at decoupling transmit RF from the
transmission line and preventing pattern distortion. It is most
effective at doing this if it is at the feedpoint (that is, up in
the air).

73,

Jim K9YC


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[Arie Kleingeld PA3A]

How about a normal L-type network? Connect one side to the balanced
line, and the other side via a current balun to the 50 coax. The tuner
is 'RF-floating'. (You can run the coax through a toroid or use two
seperate wires to make the current balun.)

73,
Arie PA3a


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[AC7AC]

CONTENTS DELETED

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[W8JI]

> How about a normal L-type network? Connect one side to the
> balanced
> line, and the other side via a current balun to the 50
> coax. The tuner
> is 'RF-floating'. (You can run the coax through a toroid
> or use two
> seperate wires to make the current balun.)

Arie,

Respectfully that does not work at all for common mode
problems, which are the only problems that are difficult to
solve anyway. It is quite evident why this will not work if
you simply draw it out on paper and plot the voltages.

The problem is one terminal of the balun is connected
directly to the balanced feedline. The flux levels in the
balun core, the balun core losses, the amount of unbalance
are all unaffected by the balun move because one terminal of
the balanced line is always connected to the balun.

I forgot that I actually put measurements of this, along
with spice models, on my website. You can see why it doesn't
work at this link:

http://www.w8ji.com/tuner_baluns.htm

A balanced network will eliminate the need for a balun on
the tuner output, but a truly balanced network works nearly
the same with or without any balun!!! The ONLY time moving
the balun to the input could help the balun core stay cool
is when you don't even need the balun in the system.

Ironic isn't it? It only significantly could help when it
isn't needed.

73 Tom

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[Don Wilhelm-4]

Arie,

An L network with the inductor in series with the line can be easily
made into a balanced tuner - use half the inductance and put the result
in series with each leg of the load.  I have often thought about
creating a tuner like that by altering a KAT100 (using a second board
and set of relays, but the same controller), but have never gotten
around to it.  The disadvantage of the KAT100 for me is that it cannot
be used remotely, and my need is for a tuner 150 feet of coax away from
the shack because of XYL restrictions.

That still does not solve the problem for what happens to the balanced
RF inside the coax when it comes to the end of the coax and finds two
paths - one onto the intended conductor and the other onto the outside
of the coax shield.  In this case, I believe a balun at the tuner input
would be appropriate.

73,
Don W3FPR

Arie Kleingeld PA3A wrote:

> How about a normal L-type network? Connect one side to the balanced
> line, and the other side via a current balun to the 50 coax. The tuner
> is 'RF-floating'. (You can run the coax through a toroid or use two
> seperate wires to make the current balun.)
>
> 73,
> Arie PA3a
>
>
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[list1]

Hi All,
Just finished mine.
Works Well so far.
I have some pictures here:
http://www.pituch.net/Steve%27s%20Page/Radio/Link%20Coupled%20Tuner/tuner1.h
tml
or just drill down from:
http://www.pituch.net/
No documentation yet......later.

Elecraft should make it a kit... hi  hi.

Steve, W2MY

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[Don Wilhelm-4]

Steve,

Very nice work, especially on the inductor.  Is that #10 wire or small
refrigeration tubing?
Now all you have to do is implement an easy method of bandswitching it
and operating it remotely :>)

73,
Don W3FPR

list1 wrote:

> Hi All,
> Just finished mine.
> Works Well so far.
> I have some pictures here:
> http://www.pituch.net/Steve%27s%20Page/Radio/Link%20Coupled%20Tuner/tuner1.h
> tml
> or just drill down from:
> http://www.pituch.net/
> No documentation yet......later.
>
> Elecraft should make it a kit... hi  hi.
>
> Steve, W2MY
>
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[KBG Luxford]

So, Tom, how would you comment upon a transistorised rig of 50 ohms
impedance unbalanced output connected to an unbalanced  pi configuration
antenna coupling unit (variable C1 between input and ground, variable
series L in the hot lead, and variable C2 between output and ground)
followed by a balun consisting of about 20 or so turns of coax around a
large diameter coil former and the output of the balun connected to a
450 ohm window line running to an inverted vee antenna slightly longer
than that required to be resonant at 80 metres and used on 160 metres
and all the HF bands?

Does this sound a reasonable set-up?  I wanted to avoid iron or ferrite
cored toroids because of perceived difficulties in getting low loss
performance across such a wide frequency range.

Many thanks and 73
Kevin
VK3DAP / ZL2DAP


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[W8JI]

> So, Tom, how would you comment upon a transistorised rig
> of 50 ohms impedance unbalanced output connected to an
> unbalanced  pi configuration antenna coupling unit
> (variable C1 between input and ground, variable series L
> in the hot lead, and variable C2 between output and
> ground) followed by a balun consisting of about 20 or so
> turns of coax around a large diameter coil former and the
> output of the balun connected to a 450 ohm window line
> running to an inverted vee antenna slightly longer than
> that required to be resonant at 80 metres and used on 160
> metres and all the HF bands?

Kevin,

We are a bit off topic for this reflector and I am severely
time limited but I'll try to answer.

Regardless of which side of the unbalanced network the balun
is on, you will need the same common mode impedance in the
balun. It is ONLY the common mode that heats the core. The
differential mode only places an electric field between the
conductors, the magnetic flux all cancels fully for
differential modes, so the core is totally unaffected by
normal "push pull" transmission line currents. It is the
impedance from end-to-end of the winding that matters in a
true transmission line balun.

That impedance will have to be high enough to have minimal
current flow, otherwise the output terminals will never be
able to "float' to the proper voltages with respect to earth
at the tuner.  The required impedance has nothing at all to
do with the output impedance of the rig. The impedance and
length of the transmission line to the antenna and the
antenna certainly affects it, but the transmission line
impedance does NOT determine the balun impedance you need in
almost any case.

For example if you have a dipole 1/4 wave above earth and
bring a transmission line directly down to earth where one
side is grounded, you would have virtually no unbalance at
all in antenna or feedline currents. This would be true if
the feedline was coax or open wire line. The common mode
impedance of the line itself would act like a nearly perfect
balun.

If you added a balun at the earth, you could easily make the
system worse of it was fed with coax! The better the balun
at earth with coax, the worse the feedline radiation would
be in this case. This is why a balun has to be at the
transition point between balanced and unbalanced parts of
the system.

You could, with the "wrong' length of feedline in
combination with a certain antenna, need many thousands of
ohms of common mode impedance in the balun. You will never
get that with an air core balun on any more than one band,
and that would be the band where the balun is self-resonant.
Air core baluns are fine when dealing with low controlled
impedances like the feedpoint of a dipole,  but they aren't
very good in multiband systems with uncontrolled impedances.

You would probably need a low loss tangent or "high Q" core
(at the operating frequency range) with very high impedance
for high power. That might be 20 turns on a 3 inch high
stack of 61 material (Q=350 at 1 MHz). For really high
power, higher frequencies, or higher  impedances you might
want 67 materials, with a Q of 400 at 2 MHz.    65 and 43
materials are generally adequate at lower power and can use
a smaller core size.

I used a binocular stack several inches tall of 68 material
(Q=350 at 7 MHz) with just a  few turns (maybe 5 or 10, I
can't recall) of HV insulated twisted pair for high power SW
BC baluns.

The only place I ever use air cores are on my yagis or
dipoles. Monoband applications with controlled modest
impedances.

One way to solve all this without a special balun design is
a true balanced tuner, but that is expensive. The other
thing is most people don't know and don't care if the system
radiates from the feeder or not. If nothing smokes or arcs
and people answer them...they are happy.

Sorry I'm out of time for now, but I hope this helps get
some experimenting with the right cores. You absolutely do
NOT want 73, 65, 33, or 43 cores at high power or with high
voltage from the feeder to earth, and while an air core
might not heat it likely won't be doing much balancing
either unless in a 50 ohm system right at the balanced to
unbalanced transition.

73 Tom

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[AC7AC]

CONTENTS DELETED

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[KBG Luxford]

Tom, thanks for this.

Actually, I do not think it OT.  My KAT100 is an unbalanced coupling
unit, and my antenna is as described, so I am looking to match it
properly!  At present I have a 4:1 balun (relieved from duty at the font
end of an HF LPDA) connecting the 450 ohm feeder to a short length of
coax to the K2/KAT100.  I have been told that this is not a very good
set-up especially for 160 metres.

I have been reading transmission line and antenna coupling unit theory,
but nothing beats theory plus practical knowledge which you seem to possess.

73
Kevin
VK3DAP / ZL2DAP


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[Jim AB3CV]

since i just run qrp i have an elecraft t-1 remoted 60ft from my shack at
the base of an oak tree at the midpoint of my dipole.

a simple injection circuit provides for remote tune activation.

73

jim ab3cv

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[Jim AB3CV]

forgot to add that i'm using a ferrite choke on the input of the t-1 current
on the outside of the coax which is buried all the way back to the house.

tunes 80m to 10m, need to try it on 6m now that i have a k3.

73

jim ab3cv

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[David Woolley (E.L)]

Tom W8JI wrote:

>
> It takes exactly the same common mode impedance and common mode current
> and voltage capacity in the balun if it is located at the tuner output
> or at the tuner input when the network is a floating unbalanced  

Probably true of the 1:1 configuration, but not of the 1:4
configuration.  If you analyze the latter in terms of chokes, you have a
  choke connected across the differential signal, so if the differential
impedance is high, most of the current would bypass the antenna.

Having a high impedance (short) antenna, is precisely when you might
think in terms of using the 1:4 configuration.

As an extreme case, consider what happens if you feed such a balun from
a high current DC source.


> network. The core (if used) will get just as hot, and current unbalance
> (except for stray capacitance or network transmission line effects) will
> be exactly the same.
>

--
David Woolley
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[W8JI]

> Probably true of the 1:1 configuration, but not of the 1:4
> configuration.  If you analyze the latter in terms of
> chokes, you have a choke connected across the differential
> signal, so if the differential impedance is high, most of
> the current would bypass the antenna.


Why would anyone ever put a 4:1 voltage balun on a tuner
input? Bad enough to use one on the output!

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[David Woolley (E.L)]

Tom W8JI wrote:
>
>
> Why would anyone ever put a 4:1 voltage balun on a tuner input? Bad
> enough to use one on the output!

Who said voltage balun?  The Elecraft 4:1 baluns are current baluns.

One wouldn't put them directly on the input, but the point was that it
is not sufficient to say that a current balun never has an impact on
differential signals.  The real implication here is that 4:1 current
baluns have rather a narrow range of applicability.

Actually one might put them on the input, in the sense that one might
have a balanced feeder at close to 200 ohms and a balanced tuner at the
antenna end.  That's probably the only case in which they would work
well.  (Given that good balanced feeders are rather more than 200 ohms
(although twisted pair is less), 4:1 is a compromise between easy
engineering and optimum match.

--
David Woolley
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[Geoffrey Mackenzie-Kennedy]

David Woolley wrote on Sunday, August 17, 2008 at 1:02 PM

> Actually one might put them on the input, in the sense that one might have
> a balanced feeder at close to 200 ohms and a balanced tuner at the antenna
> end.  That's probably the only case in which they would work well.  (Given
> that good balanced feeders are rather more than 200 ohms (although twisted
> pair is less), 4:1 is a compromise between easy engineering and optimum
> match.

Good 200 ohm open wire line can be built using four wires in the
cross-connected configuration, I have been using such feeders for long runs
(kept under tension) at HF and 6m / 2m during the past 50 years. Leaves and
small twigs can get caught between the wires, especially in the Fall, so the
occasional inspection is required. Flashover has never been a problem in my
experience even while running a kW when living in Canada.

73,
Geoff
GM4ESD

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[W8JI]

> Actually one might put them on the input, in the sense
> that one might
> have a balanced feeder at close to 200 ohms and a balanced
> tuner at the antenna end.  That's probably the only case
> in which they would work well.

The basic rule still applies. We can't move any balun to the
input of an unbalanced network and expect less problems.
This is because one terminal of the balun always connects
directly to the load, so the common mode voltages or
currents are not reduced or transformed **by the network**.

The only time moving the balun helps is if the network is a
balanced network. Even your example shows with the 4:1 balun
you still had to use a balanced tuner. With an unbalanced
tuner the feedline could have serious radiation problems
(depending on feeder length).

Interesting isn't it? The idea of placing a balun on the
input of an unbalanced network took off like wildfire but no
one actually took the time to look at what really happens in
the system.  Many just accepted it without question.

The choice is build a good 1:1 for the output and use an
inexpensive unbalanced network, or use a much more expensive
balanced network (in which case we might not even need a
balun at all).

73 Tom



.

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