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

Roy Lewallen wrote in
:


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.

Yes Roy, and none of this has visited impedance transformation... or the
departure from ideal transformation that is envisaged by many if not most
buyers. Choices for magnetising current and the TL Zo in my model of the
R balun affect transformation at the upper and lower frequencies.

I know you have pointed out many times that shunting an antenna feed
point with a resistor may improved system efficiency. At least one
antenna manufacturer uses a lossy ferrite transformer to tame system
efficiency in a wideband antenna... the CHA250 comes to mind.

Owen