what a 1:1 choke balum used for
 

Roy Lewallen wrote:

Broadband transformers, which can operate well over several decades of
frequency, commonly use ferrite cores which are essentially resistive
over most of the operating frequency range. The sign of the impedance is
unimportant to the transformer's operation; all that's necessary is that
its magnitude be adequately high over the operating range (and of course
that the core's permeability be adequately high). The wide band high
impedance requirement is virtually impossible to meet with an inductive
core whose impedance is approximately proportional to frequency, but
easily done with cores whose impedance is essentially resistive.

As long as the impedance is high enough, what is the need for it to be
constant (or even nearly so) over a useful frequency range?

what a 1:1 choke balum used for
 

On Wed, 29 Mar 2006 17:30:54 -0500, John Popelish
wrote:

Richard Clark wrote:
(snip)
#64 material is going to offer about 3 Ohms per bead;
#43 material is going to offer about 8 Ohms per bead;
#73 & 77 material is going to offer about 18 Ohms per bead;
#75 material is going to show offer 28 Ohms per bead.
(snip)

What dimension cores produce these impedances at 75 meters?

Hi John,

It is called a 101 sized bead:
.138" OD
.051" ID
.128" high

73's
Richard Clark, KB7QHC

what a 1:1 choke balum used for
 

Richard Clark wrote:
Breaking magnetic lines (flux) is a commonplace ...

If magnetic lines can be broken, doesn't that imply
the existence of magnetic monopoles which have never
been found to exist in reality? :-)
--
73, Cecil http://www.qsl.net/w5dxp

what a 1:1 choke balum used for
 

John Popelish wrote:
You'll learn a lot by reading the information in the Fair-Rite
catalog, available at their web site.

I have a hard copy right beside me, and refer to it often.

I also have a fold-up brochure from Amidon dated March 1991
and titled: "Iron-Powder and Ferrite Coil Forms".
--
73, Cecil http://www.qsl.net/w5dxp

what a 1:1 choke balum used for
 

Roy Lewallen wrote:
John Popelish wrote:

. . .
I appreciate you taking the time and effort to try to straighten me
out on this, but if there is no magnetic lines broken (whatever that
means) why use a magnetic core? Why wouldn't disks of carbon work
just as well. They are certainly resistive.


But resistive impedance isn't the only characteristic of ferrite. Disks
of carbon won't work because they have a relative permeability of one.

I know that. I wanted to see if Richard does.

(snip)

You'll learn a lot by reading the information in the Fair-Rite catalog,
available at their web site.
(snip)

I have a hard copy right beside me, and refer to it often.

what a 1:1 choke balum used for
 

Richard Clark wrote:

Hi John,

Breaking magnetic lines (flux) is a commonplace of fields, motors, and
generators. A single wire that passes through a bead, torus, or core
will build a magnetic field concentrated within that structure when
the circuit is completed outside of it. The flux lines of half the
loop will penetrate the core to reach the other half of the loop. The
core breaks the magnetic line of flux. The dissymmetry of penetration
builds a magnetic field in the core.

I cannot picture what you are saying. I guess I need a picture. The
way I understand flux, is that, any current has flux wrapped around
it. Putting a core material around the current allows the flux to
increase in magnitude in proportion to the permeability of the material.

However, when the complete current loop is within the same structure,
the flux lines do not fulfill that same function.

The simplest way to say that is that two equal currents going in
opposite directions have magnetic fields that cancel well outside the
pair of conductors. It doesn't make a lot of difference if the two
currents are coaxial or side by side, as long as the flux path
surrounds both of them.

The flux lines that
do emerge from the tightly bound wires can be said to penetrate the
torus, but here the symmetry creates bucking fields, the net effect is
as though there was no core at all (except to add capacitance).

Since the magnetic fields from the two currents cancel (at sufficient
distance), there is no flux to enter a surrounding core. The only way
bucking takes place is if the two currents are side by side. Then
there is some flux leakage near the two current paths, but the fields
these create buck each other in the two halves of the core, so there
is only local flux fringing into and back out of the core.

Both models attempt to stimulate a current within the toroid, the
common mode of the single wire model above is lossy, the differential
mode of the twin line model that followed sees nothing. Superpose
these two for the coaxial solution.

As I understand common mode current, it is the net imbalance between
the current through the center conductor and the current through the
shield.

To put this to a test. Load up your rig, through a SWR meter to a
dummy load using two short connection wires (this will undoubtedly
require adapters and such to break out both paths). You should note a
1:1 indication. Place two #75 beads on ONE wire. You should note a
2:1 indication. You have just inserted 40 to 60 Ohms of additional
resistance into the circuit. Now, move the same two beads to
encompass BOTH wires. This should return the SWR meter to a 1:1
indication.

Yes, obvious to the casual observer. You can string beads all along a
coax, and if it had no common mode current in it, this will make
absolutely no difference in how the coax acts.

what a 1:1 choke balum used for
 

Richard Clark wrote:
On Wed, 29 Mar 2006 17:30:54 -0500, John Popelish
wrote:


Richard Clark wrote:
(snip)

#64 material is going to offer about 3 Ohms per bead;
#43 material is going to offer about 8 Ohms per bead;
#73 & 77 material is going to offer about 18 Ohms per bead;
#75 material is going to show offer 28 Ohms per bead.

(snip)

What dimension cores produce these impedances at 75 meters?


Hi John,

It is called a 101 sized bead:
.138" OD
.051" ID
.128" high

Are these what you recommend as coax choke balun beads? That is some
fine coax.

what a 1:1 choke balum used for
 

John Popelish wrote:
rocky wrote:
To sort of sum things up, then what should I use for a choke balun on
75 meters, 77, 73 or 43 mix? More the better? I read the W2DU article
and he used 73, but I thought 77 may be better?

If you are going to have to go out and buy the cores, type 43 (and its
competitors) is way more common and cheap than any of the others. It
is also readily available in long form toroids called shield beads.

You are going to get into trouble with the W2DU balun at high power
levels or with high commonn mode voltages across the balun. Each bead
suggested is only good for about 1/2 watt dissipation in open air
during long duty cycle.

Also, a string of beads is a terribly inefficient use of ferrite
materials. If you double the expense of the balun by doubling the
number of beads, you only double the common mode impedance.

If you use a less resistive material with lower loss tangent and
multiple turns through the core, you can greatly increase power
capacity and choking impedance with much less increase in cost.

73 or 75 material beads are great in 50 ohm systems at modest power on
higher frequencies when the common mode excitation is low, but I'd stay
away from that method at kilowatt power levels especially if common
mode exciting the shield is high in level or frequency is low.

For high stress applications a string of beads is terribly inefficient.

73 Tom

what a 1:1 choke balum used for
 

John Popelish wrote:
Roy Lewallen wrote:

Broadband transformers, which can operate well over several decades of
frequency, commonly use ferrite cores which are essentially resistive
over most of the operating frequency range. The sign of the impedance
is unimportant to the transformer's operation; all that's necessary is
that its magnitude be adequately high over the operating range (and of
course that the core's permeability be adequately high). The wide band
high impedance requirement is virtually impossible to meet with an
inductive core whose impedance is approximately proportional to
frequency, but easily done with cores whose impedance is essentially
resistive.

As long as the impedance is high enough, what is the need for it to be
constant (or even nearly so) over a useful frequency range?

None.

Roy Lewallen, W7EL

what a 1:1 choke balum used for
 

John Popelish wrote:
. . .
As I understand common mode current, it is the net imbalance between the
current through the center conductor and the current through the shield.

If you consider the sum of the physically separate currents on the
inside and outside of the shield to be the current "through the shield",
that's correct. And it's equal to the current on the outside of the shield.

. . .

Roy Lewallen, W7EL