Yet another BALUN questions
 

A current balun (as described by W2DU in his book ''Reflexions''), put at
the center of a dipole, prevents HF current from flowing in the outer shield
of the coax. This is due to the high impedance caused by the ferrite rings.
Now, if instead, I put a voltage balun, what should prevent the current from
flowing in the outer shield of the coax? Am I missing something?

Thanks for helping.

Chris

W2DU's device is not a balun. It has no ratio - it is just a choke.
It behaves in the same way as a single bifilar winding on a ferrite
rod, or on a ring.

But a choke does not necessarily prevent longitudinal standing-wave
currents on the line. It merely shifts standing waves to other
positions along the line.

If a choke is located at a current minimum it may serve no useful
purpose at that operating frequency.

A true balun transformer, whatever its ratio, by itself provides no
obstruction to longitudinal currents. There can be a direct connection
between the line and antenna.
----
Reg, G4FGQ.

On Thu, 28 Apr 2005 14:19:18 +0000 (UTC), after apparently consuming a
750 ml bottle of California red, "Reg Edwards"
wrote:

W2DU's device is not a balun. It has no ratio - it is just a choke.
It behaves in the same way as a single bifilar winding on a ferrite
rod, or on a ring.

Wherever did you get the idea that a balun "must have a ratio?"

"Balun" is a contraction of 'Bal'anced to 'Un'balanced. Walt's device
certainly fulfills this function.

[remaining bafflegab snipped]

Balanced to Unbalanced WHAT ?

"Balun" is a contraction of "Balanced to Unbalanced "TRANSFORMER".

What is the transformation ratio of the W2DU "balun" and other devices
which serve a similar purpose? Between which pair of impedances does
it match?

If YOU cannot provide answers then there are many old-wives who can.

There is a program named BALCHOKE available free from my website.

I have no wish to participate in futile arguments about the meaning of
words. They can drive one to drink
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

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

"Wes Stewart" wrote:

W2DU's device is not a balun. It has no ratio - it is just a
choke.
It behaves in the same way as a single bifilar winding on a ferrite
rod, or on a ring.

Wherever did you get the idea that a balun "must have a ratio?"

"Balun" is a contraction of 'Bal'anced to 'Un'balanced. Walt's
device
certainly fulfills this function.

[remaining bafflegab snipped]

You are correct on this point, it is actually an "UnUn."
(UNbalanced-to-UNbalanced--don't confuse this with a child's pronouncing of
"onion.") grin

Regards,
John

Reg Edwards wrote:
What is the transformation ratio of the W2DU "balun" and other devices
which serve a similar purpose? Between which pair of impedances does
it match?

Resorting to passive voice - The W2DU choke is considered to
be a 1:1 balun. Each bead is considered to be one turn. If
one mounts the beads on a piece of 50 ohm coax, it becomes
a 50 ohm balanced to 50 ohm unbalanced balun. With 50 ohms
on both sides of the balun, the Z0 of the transmission line
used should be SQRT(50*50)=50 ohms.
--
73, Cecil, http://www.qsl.net/w5dxp


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The difference between a 1:1 current balun and a 1:1 voltage balun is
that the latter has a third (tertiary) winding. I'll assume for a moment
that the baluns are perfect, i.e., have an infinite common mode
impedance. If the load is balanced, that is, if the two load terminals
have equal impedances to the "cold" side of the balun input, the third
winding of the voltage balun carries no current, and there will be no
current on the outside of the coax. Because the third winding has no
current, it can be removed with no effect, so the voltage balun acts
exactly like a current balun when the load is balanced. However, if the
load isn't perfectly balanced relative to the input terminal of the
balun, a current balun will still prevent current from flowing on the
outside of the coax. The voltage balun, however, will force the voltages
at the two output terminals to be equal and opposite relative to the
cold side of the balun input. This will cause unequal currents from the
two terminals. The difference flows along the outside of the coax.

In summary, the best a voltage balun can do in preventing current flow
on the outside of coax is to be as good as a current balun, and this
happens only if the load is balanced. In all other cases, using a
voltage balun will cause current to flow on the outside of the coax
while a current balun will prevent this flow.

For more information, see the article posted at
http://www.eznec.com/Amateur/Articles/Baluns.pdf.

Roy Lewallen, W7EL

C.Aymon wrote:
A current balun (as described by W2DU in his book ''Reflexions''), put at
the center of a dipole, prevents HF current from flowing in the outer shield
of the coax. This is due to the high impedance caused by the ferrite rings.
Now, if instead, I put a voltage balun, what should prevent the current from
flowing in the outer shield of the coax? Am I missing something?

Thanks for helping.

Chris

Wes Stewart wrote:

On Thu, 28 Apr 2005 14:19:18 +0000 (UTC), after apparently consuming a
750 ml bottle of California red, "Reg Edwards"
wrote:



W2DU's device is not a balun. It has no ratio - it is just a choke.
It behaves in the same way as a single bifilar winding on a ferrite
rod, or on a ring.



Wherever did you get the idea that a balun "must have a ratio?"

"Balun" is a contraction of 'Bal'anced to 'Un'balanced. Walt's device
certainly fulfills this function.

[remaining bafflegab snipped]


Does that make it a 1:1 balun? I'm asking serioulsly, I'm not smart
enough to figure these things out. If I can't picture it in my head, I
can't figure it out!

Butch Magee KF5DE

On Thu, 28 Apr 2005 18:03:40 +0000 (UTC), "Reg Edwards"
wrote:

What is the transformation ratio of the W2DU "balun"

1:1

Think, "One-to-one isolation transformer."

Regards,
John