Why the 4:1 or 9:1 baluns?
 

People keep saying that they use 4:1 or 9:1 baluns.
Often I can see no reason for this (such as when simply feeding a dipole
with coax, or on the output of an ATU).

I'm obviously missing the point. Why the impedance transformation?
Ian.
--

"Ian Jackson" wrote
People keep saying that they use 4:1 or 9:1 baluns.
Often I can see no reason for this (such as when simply feeding a dipole
with coax, or on the output of an ATU).

I'm obviously missing the point. Why the impedance transformation?

=====================================

Nearly always there IS no point to be missed.

Reasoning seldom enters an argument.

Which of the infinite number of impedance ratios on one antenna are to be
favoured?

Personally, I have never used a balun except of the choke variety which can
be used between indeterminate impedances of any ratio to serve a
recognisable purpose.

But the right to free speech must be held.
----
Reg, G4FGQ

Ian Jackson wrote:
People keep saying that they use 4:1 or 9:1 baluns.
Often I can see no reason for this (such as when simply feeding a dipole
with coax, or on the output of an ATU).

I'm obviously missing the point. Why the impedance transformation?
Ian.

The feedpoint impedance of a dipole varies with frequency from a
low value of about 50 ohms to a high value of about 8000 ohms.
In between, it can look like 1000(+/-)j1000 ohms.
Baluns with a high transforming ratio try to take the impedance
lower so, for instance, with a 4:1 balun, the above values would
ideally go from 50 ohms to 12.5 ohms, 8000 ohms to 2000 ohms,
and from 1000+j1000 ohms to 250+j250 ohms. You can see that the
second set of values have a smaller absolute range than the first
set, thus making them an easier match for the tuner.

That's the simplified version. There are disadvantages to using
baluns with a high transforming ratio.
--
73, Cecil http://www.qsl.net/w5dxp


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In message , Cecil Moore
writes
Ian Jackson wrote:
People keep saying that they use 4:1 or 9:1 baluns.
Often I can see no reason for this (such as when simply feeding a
dipole with coax, or on the output of an ATU).
I'm obviously missing the point. Why the impedance transformation?
Ian.

The feedpoint impedance of a dipole varies with frequency from a
low value of about 50 ohms to a high value of about 8000 ohms.
In between, it can look like 1000(+/-)j1000 ohms.
Baluns with a high transforming ratio try to take the impedance
lower so, for instance, with a 4:1 balun, the above values would
ideally go from 50 ohms to 12.5 ohms, 8000 ohms to 2000 ohms,
and from 1000+j1000 ohms to 250+j250 ohms. You can see that the
second set of values have a smaller absolute range than the first
set, thus making them an easier match for the tuner.

That's the simplified version. There are disadvantages to using
baluns with a high transforming ratio.

But how good is the efficiency when the balun/transformer is working
between completely the wrong impedances (which will be most of the
time)? My understanding (and limited experience) is that most of your
power simply heats up the ferrite.
Ian.
--

Ian Jackson wrote:
But how good is the efficiency when the balun/transformer is working
between completely the wrong impedances (which will be most of the
time)? My understanding (and limited experience) is that most of your
power simply heats up the ferrite.

Sorry, I don't know the answer. I operate my W2DU choke into close
to a pure resistance that varies between 28 ohms and 90 ohms. I
once measured the temperature of a #2 iron-core 4:1 commercial
voltage balun and got no appreciable temperature rise under varying
conditions but I don't know what conclusion can be drawn.

How to operate your all-HF-band dipole with a purely resistive
load of 28-90 ohms is described on my web page below.
--
73, Cecil http://www.qsl.net/w5dxp/notuner.htm


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But how good is the efficiency when the balun/transformer is working
between completely the wrong impedances (which will be most of the
time)? My understanding (and limited experience) is that most of your
power simply heats up the ferrite.
Ian.
==============================
Ian, both your understanding and experience of how a balun works are
limited. But don't be disheartened - you are in good company! ;o)

They are not simple transformers. The windings are in fact transmission
lines consisting of a pair of wires running in parallel, either coaxial or
balanced-twin - it doesn't matter which although balanced-twin is slightly
more efficient and physically more easy to construct.

Their full name is "Transmission Line Transformers".

They have a wider bandwidth and a higher HF response than ordinary
primary-and-secondary HF transformers. This is because the normal
capacitance between turns and between windings is incorporated in the
transmission lines instead of being directly in shunt with winding
inductance.

They are inherently very low loss devices because there's only short lengths
of high-conductivity copper involved.

In theory, there is no loss in the ferrite core because the currents in the
transmission line wires flow, adjacent to each other, in oposite directions
and the flux cancels out in the core. But cancellation is never completely
perfect and HF-quality ferrite core material should be used. However, LF
grade materials found in the junk box often work quite satisfactorily.

Cancellation is better with coaxial lines than with balanced twin but this
is hardly a matter of importance. Ferrite rods require more turns to achieve
the same inductance than rings at the low frequency end.

The ordinary inductance of the windings considered as single wires, in
conjunction with the nominated terminating resistances, sets the low
frequency response of a balun transformer in the usual way.

The high frequency end of the range is limited by the physical length of the
line winding. Line length should be small in relation to the wavelength
along the line. Upper frequency response and ratio begin to fall off when
line length exceeds about 1/8 or 1/10 of a wavelength at its own velocity.
Not free space velocity. This allows a frequency range of 1.8 to 30 MHz on a
core of ordinary proportions with permeability in the range 200 to 400.

------------------------

The performance and behaviour of an X-to-Y ohm balun, as with ordinary
transformers, goes haywire if it is not operated between its designed-for
resistances. Or at least resistances in the right ball-park.

Hence the futility at HF of using one between a multi-band antenna and any
sort of transmission line or tuner. The range of impedances to be covered
is even greater than those listed by our good friend Cecil.

A choke balun, a single short length of transmission line wound on a ring or
rod, is a different kettle of fish. Its range of impedances is unlimited.
----
Reg, G4FGQ

On Sun, 9 Jan 2005 15:06:59 +0000, Ian Jackson
wrote:
But how good is the efficiency when the balun/transformer is working
between completely the wrong impedances (which will be most of the
time)? My understanding (and limited experience) is that most of your
power simply heats up the ferrite.

Hi Ian,

In a current BalUn it is the Common Mode current that is absorbed by
the BalUn as a choke (one purpose of two purposes to use it). The
current BalUn is designed to add R rather than X to the Common Mode
path (although, any additional X that comes down the pike is not
shrugged off). Thus the product of that current and that resistance
is heat. However, if you have a sufficient R, it snubs the I before
you get to many Watts. A string of 20 beads can present upwards to
1000 Ohms resistance, but I wouldn't try to dissipate more than 20W
throughout.

Now, as to efficiency. I presume the load is a nominal 50 Ohms, and
that it is paralleled by that choke's 1000 Ohms. If we were in a
condition where that same 1000 Ohms were dissipating 20W then it
follows that the load is dissipating 400W. This is about the time
when some Hams begin to tear their hair and fulminate at the BalUn
(which is doing its job faithfully) rather than asking themselves the
question:
"why is my antenna system trying to pump
that current down the transmission line?"

If they solved that question, then they would have more power
available for their intended load. Others who would have reflected on
that question naturally, they would have shrugged their shoulders and
figure chasing that last 5% efficiency exceeded the small price they
paid for the BalUn - which was working quite well.

Now, consider if that BalUn were to suddenly vanish. The same current
which formerly saw 1000 Ohms, would see none. We can all work out the
equivalent resistance of 0 Ohms paralleled to the intended load of 50
Ohms to observe where the power would eventually go. This is so
extreme as to be absurd (or at least rare). Such a radical
possibility necessarily presumes a rather disastrous implication in
the state of the antenna's design - or damage.

However, none of this actually answers your question about "wrong
impedances," but it does respond to the worst efficiency you could
expect. By and large, when you are in the middle-to-near fringes of
the BalUn's operational range, efficiencies are exceptionally high
(95% to 99+%). Whatever power that is lost to improper matching
occurs in far greater abundance elsewhere.

73's
Richard Clark, KB7QHC

Ian Jackson wrote:

But how good is the efficiency when the balun/transformer is working
between completely the wrong impedances (which will be most of the
time)?

I use a 4:1 balun under such conditions. I use it simply to make my autotuner
happy (which makes bandhopping easier). I have run extensive tests with the
balun in and out on the HF bands and have yet to find anyone on the other end
who could tell a difference.

My understanding (and limited experience) is that most of your
power simply heats up the ferrite.

My KW rated balun does get warm, even at 50 watts on some bands.

Of course tuners can also eat up considerable power on large mismatches. If you
want (or need) a simple antenna system to work on all bands then you will likely
have to absorb some loss no matter how you do it to get there. And the reason
many don't have a big problem with this is even if you do lose *half* you power,
that's still not a real big difference on the S meter at the other end...

"Cecil Moore" wrote
Reg Edwards wrote:
A choke balun, a single short length of transmission line wound on a
ring or
rod, is a different kettle of fish. Its range of impedances is
unlimited.

The more I think about it, the more it seems likely that a common-mode
wave travels down the outside of the braid until it encounters the
choke impedance and is reflected, at least partially. That would
create common-mode standing waves. When the weather clears up, I
think I will see if the common-mode current goes through a maximum
and minimum up and down the coax.
--
==================================

Cec, its obvious even to a citizens bander that when the common-mode current
and volts encounters the choke, or anything else, it is reflected. There's
no need to risk your neck to detect it.

You won't prove anything anyway. Your particular antenna might not suffer
from noticeable common-mode effects. And you can't deliberately inject a
test signal at any place because it would upset circuit conditions.

Furthermore, the choke does NOT do what the old-wives say it does, ie., stop
radiation from the line and prevent noise pick-up. It might even make
matters worse. The choke merely shifts the volts and amps standing waves to
other places along the line.

Have I upset the apple cart again?
----
Reg.

Reg Edwards wrote:
Cec, its obvious even to a citizens bander that when the common-mode current
and volts encounters the choke, or anything else, it is reflected. There's
no need to risk your neck to detect it.

Judging from some of the assertions on the subject, some people consider
common-mode current to be a series circuit problem, not a distributed
network problem.

You won't prove anything anyway. Your particular antenna might not suffer
from noticeable common-mode effects. And you can't deliberately inject a
test signal at any place because it would upset circuit conditions.

I'm running a G5RV right now. All I have to do to gin-up common-mode
currents on at least one of the eight HF bands is to remove the choke
at the coax to ladder-line junction.

Furthermore, the choke does NOT do what the old-wives say it does, ie., stop
radiation from the line and prevent noise pick-up. It might even make
matters worse. The choke merely shifts the volts and amps standing waves to
other places along the line.

I use a choke to reduce common-mode problems in the shack and it does
that apparently by causing reflection of common-mode waves back toward
the antenna which, as you say, wouldn't decrease feedline radiation
between the choke and the antenna.

Have I upset the apple cart again?

Where does the common-mode power go? :-) If differential reflected
power can be almost 100% delivered to the antenna by matching, how
about common-mode power? What happens to the common-mode reflected
power when it gets back to the antenna?
--
73, Cecil http://www.qsl.net/w5dxp


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Cecil Moore wrote:

Furthermore, the choke does NOT do what the old-wives say it does, ie., stop
radiation from the line and prevent noise pick-up. It might even make
matters worse. The choke merely shifts the volts and amps standing waves to
other places along the line.

I use a choke to reduce common-mode problems in the shack and it does
that apparently by causing reflection of common-mode waves back toward
the antenna which, as you say, wouldn't decrease feedline radiation
between the choke and the antenna.

It isn't really helpful to think of it as "reflecting common-mode waves
back". It is simply Ohm's law: a high series impedance reduces the
amount of current that will flow.

Forcing a current minimum at one location on the feedline (usually right
at the end of the coax) will change the current and voltage distribution
not only on the feedline, but also on the antenna proper. The whole
antenna-feedline system readjusts itself to take account of the fact
that the choke impedance is there. This readjustment will also change
the feedpoint impedance, so it's actually true that a balun will change
the SWR of the antenna - it has become a different system that no longer
involves the feedline.

Reg is right to say that the choke shifts the common-mode voltage and
current standing waves to other places along the feedline. Depending on
the location of the 'victim' equipment (TV etc) relative to those
standing waves, the change can sometimes make RFI problems worse. Even
so, a choke balun right at the end of the coax is almost always the
right place to start. If it doesn't help, it may *still* make sense to
leave it there, and try a second choke somewhere else as well.

A clip-on RF current probe can work wonders in showing you what really
is happening.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Ian White, G3SEK wrote:
It isn't really helpful to think of it as "reflecting common-mode waves
back". It is simply Ohm's law: a high series impedance reduces the
amount of current that will flow.

True for a bench circuit. Not necessarily true for a distributed
network. In fact, a choke that makes the outside braid of a
transmission look like an open circuit (best case), makes it look
like a short circuit 1/4WL back from that point. If the choke is
placed at an existing current minimum point, it will have little
effect.
--
73, Cecil http://www.qsl.net/w5dxp


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Reg Edwards wrote:
. . .
Furthermore, the choke does NOT do what the old-wives say it does, ie., stop
radiation from the line and prevent noise pick-up. It might even make
matters worse. The choke merely shifts the volts and amps standing waves to
other places along the line.

Have I upset the apple cart again?

Once again I proudly don the mantle of a Reg's Old Wife.

A common-mode choke, aka "choke balun" or "current balun" can have the
effect Reg mentions, but that's not the only possibility. Depending on
the lengths of the antenna and transmission line and the placement of
the choke, it can reduce the common mode current to a much lower value
everywhere on the line. This is done by the same mechanism as an
insulator reduces the current induced on a guy wire -- by making a
formerly resonant line non-resonant. In stubborn cases, two (or perhaps
very seldom, more) chokes placed about a quarter wave apart are required
to get a low value of common mode current everywhere on the line.

The general principles are easily illustrated by modeling(*). In
practice, the actual path along the feedline from antenna to ground
isn't well known so can't be modeled well, and some experimentation
might be necessary.

(*) Anyone having an EZNEC program, including the demo, can look in the
manual index under "Coaxial Cable, Modeling" for information. To
simulate a choke balun, insert a resistive or inductive load in the wire
which represents the outside of the coax. 500 - 1000 ohms is a
reasonable value to use.

Roy Lewallen, W7EL

On Tue, 11 Jan 2005 12:52:59 -0800, Roy Lewallen
wrote:

Once again I proudly don the mantle of a Reg's Old Wife.

Oh my gawd! Do I detect Kurt Sturba here. GG

Danny

"Cecil Moore" wrote in message
...
Sorry, I don't know the answer. I operate my W2DU choke into close
to a pure resistance that varies between 28 ohms and 90 ohms.

Cecil - I stopped using baluns for dipoles when my neighbor's TV reception
was getting killed every time I went on 10 meters. I took out my W2AU balun,
and no more problem. Whenever I need to match higher or lower antenna
impedances now, I use a 1/4-wave length of the appropriate coax line to get
close to the antenna impedance. Works great, seems impervious to the
weather, and have never found the unbalanced feedline to balanced antenna
(dipoles & full wave loops/quads) was a problem that feeding the coax away
from the antenna in a perpendicular fashion wouldn't solve. Also, two
parallel RG-59 coax lines feed grounded verticals nicely, and a tuner or the
amp pi network can easily do the load match.

ak