You have hit on the heart of the reason why I am so opposed to the use
of the word "balun" to describe a half dozen different things that are
very different from each other, and why I use the words "common mode
choke" when I mean a common mode choke. :) In this respect, it's
exactly like the word "ground" (or "earth" in British English). The
word "ground" is used to describe a half dozen different things
(connections to the soil, connections between equipment for safety,
circuit common, termination of cable shields, etc.), and that leads to
MASSIVE misunderstandings about anything related to the uses of the word.
This mis-use of words leads to a LOT of confusion about what things are
and how they work. The most generic definition I've seen of the word
balun is that it is some sort of device that allows connection of a
balanced circuit to an unbalanced circuit. Some use the word to include
the matching of impedances.
An ordinary transformer allows the connection of balanced and unbalanced
circuits to each other, and, depending on the turns ratio, can also do
some impedance transformation.
Transmission line transformers of various sorts are also called baluns,
and many of those variations are VERY different from each other. Some
use transformer action with windings coupled via a ferrite core, while
others use windings configured so that the core sees only common mode
current. Some use cores that are predominantly inductive with very low
loss, while others use cores that cause the choke to function
essentially as a low Q parallel resonant circuit. The design,
effectiveness, losses, dissipation, and voltage stresses, are very
different from one type of so- called "balun" to another.
Another completely different so-called balun is a half wave length of
transmission line that allows the two sides of a tee-matched driven
element to be driven by coax. The coax feedline connects directly to one
side of an antenna and the center of the driven element, the half wave
of line connects from that point to the other side of the antenna and
the driven element so that, at resonance, the opposing sides of the
element are driven 180 degrees out of phase so that their total fields add.
If you live in the video world, you can buy a piece of electronics that
allows the 75 ohm video output stage to drive unshielded twisted pair
(like CAT5). That box is also called a balun by those who sell and use it.
73, Jim K9YC
On 11/14/2010 5:49 PM, Richards wrote:
Apropos our previous discussion of "choke baluns"...
I believe the locution "choke balun" is often used to refer to various
forms of chokes - sometimes made of ferrite beads and sometimes made
of round ferrite toroids. I presume you are talking about the more
traditional and more robust "balun" (transformer) designs which employ
large, round ferrite toroids - and not the so called "choke baluns"
comprised of a series of ferrite beads along a stretch of coax.
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