Karl Beckman wrote:
First thing to remember is a really radical thought: The power rating of a
voltage balun has to be decreased by the highest SWR factor existing on the
line. That is, a 2 kW rated (at 1:1) balun is only good for 1 kW at 2:1
VSWR or 200 watts if your wattmeter shows a 10:1 VSWR looking at the
antenna. The reason is that the current (heating) losses in the windings go
up as the square of the maximum current - remember hearing about "I squared
R". A 4:1 VSWR means the current max value is twice the minimum, therefore
you'll be heating up the core four times as much as if you had a 1:! VSWR on
the line. It doesn't matter whether the line is unbalanced coaxial or
balanced open wire.


The other factor is core saturation.. flux goes as the voltage*frequency
.... so 10:1 VSWR means you might have 10x voltage..

On Tue, 22 Sep 2009 09:52:46 -0700, Jim Lux
wrote:

The other factor is core saturation..

Hi Jim,

The only reason why a common mode choke (a more appropriate BalUn
construction for any application) would encounter "core saturation" is
due to the presence of a very significant common mode current - the
thing the choke (or BalUn) is supposed to suppress. If the "core
saturates" this is an indication that a common mode choke is very,
very necessary.

The better the choke (the higher its common mode Z), the more it will
snub the current. The more it snubs the current, the less chance of
"core saturation."

The solution to a hot BalUn is suppression of the current that is
energizing the "core."

As for:
Karl Beckman wrote:
First thing to remember is a really radical thought: The power rating of a
voltage balun has to be decreased by the highest SWR factor existing on the
line.

I have NEVER heard this apocryphal "rule of thumb" before. For one
thing, it doesn't make sense on the face of it as it defines a linear
relationship between current (or voltage) with power.

73's
Richard Clark, KB7QHC

Jim Lux wrote:

The other factor is core saturation.. flux goes as the voltage*frequency
... so 10:1 VSWR means you might have 10x voltage..

At HF, the ferrites which are best to use for baluns will go up in
flames due to loss at flux densities well below saturation.

Roy Lewallen, W7EL

Roy Lewallen wrote:
Jim Lux wrote:

The other factor is core saturation.. flux goes as the
voltage*frequency ... so 10:1 VSWR means you might have 10x voltage..

At HF, the ferrites which are best to use for baluns will go up in
flames due to loss at flux densities well below saturation.

Roy Lewallen, W7EL


Well then, same general idea.. lots of volts = lots of flux = lots of
dissipated energy from the core loss...

And, of course, not all baluns are made with appropriate materials (e.g.
using a lossy EMI suppression mix might be fine in a "choke"
application, where high Z is keeping the current low.. but terrible in a
transformer type situation, where you have a lot of flux in the core)

On Tue, 22 Sep 2009 14:44:39 -0700, Jim Lux
wrote:

And, of course, not all baluns are made with appropriate materials (e.g.
using a lossy EMI suppression mix might be fine in a "choke"
application, where high Z is keeping the current low.. but terrible in a
transformer type situation, where you have a lot of flux in the core)

Hi Jim,

A BalUn IS a transformer, and many of the lowest loss, widest
bandwidth ones are specifically designed to choke common mode
currents.

Such BalUns are NOT flux linked transformers and thus avoid working
(transverse) currents inducing loss and saturation issues.

It appears that the inference from your flux transformer terminology
transformer type situation, where you have a lot of flux in the core)
is at cross purposes with BalUn best design.

73's
Richard Clark, KB7QHC

Jim Lux wrote:
Roy Lewallen wrote:
Jim Lux wrote:

The other factor is core saturation.. flux goes as the
voltage*frequency ... so 10:1 VSWR means you might have 10x voltage..

At HF, the ferrites which are best to use for baluns will go up in
flames due to loss at flux densities well below saturation.

Roy Lewallen, W7EL


Well then, same general idea.. lots of volts = lots of flux = lots of
dissipated energy from the core loss...

Not exactly. The dominant loss mechanisms in "low frequency" ferrites
are essentially linear, and don't cause distortion. Saturation occurs
during a portion of each cycle, resulting in distortion. And saturation
doesn't always imply high loss.

And, of course, not all baluns are made with appropriate materials (e.g.
using a lossy EMI suppression mix might be fine in a "choke"
application, where high Z is keeping the current low.. but terrible in a
transformer type situation, where you have a lot of flux in the core)

A properly designed transformer has very little flux in the core -- the
flux is the vector sum of the flux in all the windings, which in an
ideal transformer is zero. The flux you do have is the magnetizing flux
due to the finite impedances of the windings, which you strive to
maximize just as you do in a choke (and the leakage flux which isn't
coupled from one winding to the others). So the best core material is
generally the same for a broadband choke as it is for a broadband
transformer. EMI suppression materials are very good for both, since
they're engineered to maximize impedance. An exception is where you're
dealing with enough power that the core loss is intolerable. For
example, a well designed choke or transformer might have only 0.5 dB
loss, a generally insignificant amount. But if you apply a kW to it,
you're talking about 120 watts of power dissipation, too much for a
small core. In those cases you have to use lower loss material, which
usually means lower impedance windings and consequent higher flux density.

Roy Lewallen, W7EL

Roy Lewallen wrote in
:

....
A properly designed transformer has ...

I think the most common 4:1 balun in use is the Rutroff 4:1`balun.

If it is constructed as a bifilar winding on a ferrite core, and the
distance between wires is much less than the distance between turns
(which is commonly the case), it can be though of as a transmission line
wound on a core.

Rutroff suggested the transmission line equivalent in his original
article.

I have developed a transmission line model and solution in the draft
article at http://www.vk1od.net/balun/Ruthroff/RU1-4.htm . It predicts
both low frequency and high frequency departure from ideal
characteristics, and predicts core loss with different loads. The model
suggests that balun efficiency can easily be below 50% on extreme loads
using some typical commercial constructions. The model results have been
validated on a small number of prototype baluns.

Owen

I should mention that the model and prototype measurements suggest that
lossier higher µ cores may produce a more efficient device... it depends
on ther things.

The tradeoff between core size, core material, number of turns etc for a
given load is a complex one, more complex than implied by simple rules
like "#61 is the best material for HF baluns".

Even the low frequency model of such a balun reveals this. If the balun
is analysed using the techniques common used for a 50Hz or 60Hz
transformer, the magnetising current (the current that flows into the
transformer with no load attached) is design point. If the core is a low
loss core, one could choose a relatively high mangetising current yet
still have low H+E losses because the Power Factor of that magnetising
current is quite low... or in the case of the RF transformer, one could
use a relatively lossy material (high magnetising current Power Factor),
but the higher µ of the lossier core means lower magnetising current, and
the losses are acceptable.

The model I have proposed allows exploration of these different
configurations, and the tools that I have developed allows solution of
the problem using the core material frequency dependent characteristics.

I don't want to trivialise designing with magnetics, it is a challenge...
but we can do better than simple rules like #x material is the best HF
balun material... it is a very eHam approach.

Owen

BTW, the commonly held belief that powdered iron the material of choice
for baluns is not soundly based. Such a view seems driven by the belief
that lowest loss core material assures a good outcome. It is interesting
that powdered iron has such a following, yet so little information is
published on the core material compared to the ferrite materials.

Owen Duffy wrote:
. . .
Even the low frequency model of such a balun reveals this. If the balun
is analysed using the techniques common used for a 50Hz or 60Hz
transformer, the magnetising current (the current that flows into the
transformer with no load attached) is design point. If the core is a low
loss core, one could choose a relatively high mangetising current yet
still have low H+E losses because the Power Factor of that magnetising
current is quite low... or in the case of the RF transformer, one could
use a relatively lossy material (high magnetising current Power Factor),
but the higher µ of the lossier core means lower magnetising current, and
the losses are acceptable.
. . .

As you say, though, there are always tradeoffs. A higher magnetizing
current means a lower winding impedance. In a winding connected across a
transmission line, this means adding a relatively low shunt impedance
across the line. In a series connected winding, as in a current balun,
it means less effective choking of common mode current. Maximizing
winding impedance, which also minimizes magnetizing current, is always
beneficial. But as we've both pointed out, sometimes we're forced to
choose a material that gives us less impedance in order to lower the
loss to a level that won't cause a problem at high power levels. The
price is a smaller shunt winding impedance or less effective common mode
choke, and also typically a narrower operating bandwidth.

Roy Lewallen, W7EL