what a 1:1 choke balum used for
 

wrote:
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.

I assume you are talking about the beads mentioned in this document:
http://www.w2du.com/r2ch21.pdf

At the bottom of page 21-6, he says that the beads have a .197 inch
hole and are .190 inch long.

The beads I am suggesting are over an inch in diameter and over an
inch long. A half watt would barely raise their temperature
noticeably above ambient. And they will fit over coax that is a half
inch in diameter. There are other shapes that are more cost effective
for smaller diameter coax. 28B0591-200 has a .23 inch hole, but is
1.38 inches long, with a .591 inch OD. A half watt won't warm that
one very much, either. And it costs only $.76 each, if you buy 10 of
them.

My point was that there are cheap long form factor toroids available
now, that weren't available when W2DU designed and described his BalUn.

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.

But it spreads the heat out, and doesn't require you to kink the coax,
which makes it last longer in the weather. If you want to wind large
diameter toroids, they now make low cost versions of these for EMI
purposes, also. For example Steward # 28B2400-000, outside diameter
2.4 inches OD, 1.4 inch ID, .5 inch thick costs only $3.36 at Digikey,
quantity 1. Again, Steward type 28 is roughly similar to Fair-Rite
type 43.

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.

Yes, the turns squared effect makes better use of the mass of the
ferrite to produce impedance.

what a 1:1 choke balum used for
 

Roy Lewallen wrote:
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.

Yes, I was calling the sum of the currents on the inside and outside
of the shield, the current through the shield. Sorry, if that is a no no.

what a 1:1 choke balum used for
 

On Wed, 29 Mar 2006 17:50:28 -0500, John Popelish
wrote:

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.

Hi John,

I do use some small stuff, like RG-174 and precision hard line, but
you are right, this is not for RG-58 which I commonly use. The 101
size is Amidon's reference for a multiplier for other forms.

73's
Richard Clark, KB7QHC

what a 1:1 choke balum used for
 

The choke impedance is inductive at low frequencies. Loss in the
ferrite material is low.

But due to a lossy ferrite material at the higher frequencies the
equivalent circuit is the inductance in parallel with a high
resistance.

At sufficiently high frequencies the choke impedance is almost a pure
high resistance. Which chokes the current just as well as a high
inductive reactance.

This is all that's necessary to explain the operation of a choke
balun.

I can't imagine why you are all making such a fuss about it.
----
Reg.

what a 1:1 choke balum used for
 

On Thu, 30 Mar 2006 01:32:10 +0100, "Reg Edwards"
wrote:

I can't imagine why you are all making such a fuss about it.

Reggie,

There's no way you can convince us you lack imagination.

73's
Richard Clark, KB7QHC

what a 1:1 choke balum used for
 

Richard Clark wrote:

I do use some small stuff, like RG-174 and precision hard line, but
you are right, this is not for RG-58 which I commonly use. The 101
size is Amidon's reference for a multiplier for other forms.

I wind about 10 turns of RG-174 type coax on a core having an OD of a
bit more than an inch. Works fine up to at least 300 watts with a
reasonably matched antenna.

Roy Lewallen, W7EL

what a 1:1 choke balum used for
 

rocky wrote:
You would use physically larger cores of 43 material and more turns in
the core for say 75 M in a choke balun?

Sure.

The problem actually all comes down to the common mode current flowing
through the beads, and that's the same as looking at voltage across the
shield from one end to the other of the "choke".

Some antennas, even with very low power and 50 ohm feedpoints, will
produce fairly high across the string of beads. This can cause quite a
bit of heating, and cause the feedline to remain part of the radiating
system despite the use of a balun.

In controlled situations strings of beads work pretty well. In other
cases they are totally inadequate, and the problem can have nothing to
do with the impedance across the feedline (SWR).

I never use strings of beads on my antennas.

73 Tom

what a 1:1 choke balum used for
 

John Popelish wrote:
The beads I am suggesting are over an inch in diameter and over an
inch long. A half watt would barely raise their temperature
noticeably above ambient. And they will fit over coax that is a half
inch in diameter. There are other shapes that are more cost effective
for smaller diameter coax. 28B0591-200 has a .23 inch hole, but is
1.38 inches long, with a .591 inch OD. A half watt won't warm that
one very much, either. And it costs only $.76 each, if you buy 10 of
them.

My point was that there are cheap long form factor toroids available
now, that weren't available when W2DU designed and described his BalUn.

My point is dollar for dollar multiple turns always wind up being a
better investment. That's because impedance approximately follows the
square of the passes through the core.

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.

But it spreads the heat out, and doesn't require you to kink the coax,
which makes it last longer in the weather.

I've never once had cable fail from being wound on cores. I can't
imagine why someone would need to kink the cable.

The only part of the cable that significantly increases CM impedance is
the part inside the core window. Why care how tightly the cable is
pulled against the core outside??

73 Tom

what a 1:1 choke balum used for
 

what a 1:1 choke balum used for
 

John Popelish wrote:
wrote:
I've never once had cable fail from being wound on cores. I can't
imagine why someone would need to kink the cable.

I just found this rule of thumb that says coax should not be bent to a
radius smaller than 6 times the cable diameter.
http://www.wiremold.com/www/commerci...dex.asp?wpid=4

For RG-58 this comes out to about a 1.2 inch bend radius.

This spec for RG 58:
http://www.pacificcable.com/Picture_...?DataName=RG58
Specifies 1.25 inch minimum bend radius, only if there is no pull on
the cable. If the cable is tension loaded, they recommend it be
wrapped over a minimum 2.5 inch radius form.

That would get to be a pretty big coil on a toroid.