PLEASE ignore all of Jim's pontification.
I find it curious that Terman ("Radio Engineering"), Kraus ("Antennas"), Johnson
("Transmission Lines and Networks") all use the "meaningless" word "balun" in
their books. Clearly, these guys should have consulted with Jim before doing so,
because obviously they didn't know what they were talking about.
A transmission line transformer can be as simple as a geometric mean
quarter-wave line between two different impedances. No ferrites required. A
balun (pardon me, I'm with Kraus) can be a quarter-wave open stub at the
feedpoint of an antenna. Collins ("Fundamentals of SSB") calls this a
"Bazooka-type balun", but what does Collins Radio know about anything?) Or, it
could be a half-wave line connecting the two halves of a dipole. A stub balun
can be both a balancing device and an impedance transformer at the same time.
And it's nothing but coax. A two-wire line wound around a core might be a
common-mode choke, but if it's long enough and different in impedance from the
load, then it's an impedance transformer too.
In summary, just removing the term balun from one's lexicon doesn't simplify
anything.
And I almost forgot, that N6BV article Jim mentions is titled, "Don't blow up
your BALUN."
Wes N7WS
On 5/22/2017 2:11 PM, Jim Brown wrote:
On Mon,5/22/2017 12:42 PM, Bill Leonard N0CU wrote:
I am no expert when it comes to baluns
You're not the only one. :)
Some important comments. First, PLEASE strike the word "balun" from your
vocabulary. It is a meaningless word that tells us NOTHING about the device or
circuit element it is used to describe. I can think of nearly a dozen VERY
different devices that are CALLED baluns. Use the word "balun" conceals what
the device actually is and prevents everyone involved from understanding what
it does.
A two-wire line wound around a ferrite core forms a COMMON MODE CHOKE. It is
not a "transmission line transformer," nor is it an inductor, nor is it a
transformer at all! The ferrite core carries only flux due to common mode
current, and loss in the choke is I squared R, where I is the common mode
current and R is the resistive impedance of the common mode choke.
Arrays of common mode chokes CAN be wired in series/parallel combinations to
match circuits of differing impedance, but that device is NOT a transformer,
it is an array of common mode chokes. If we want to know how this array of
chokes work, we must analyze them as arrays of common mode chokes, not as a
transformer.
A transformer, is, by definition, two windings that are magnetically coupled,
and the impedance transformation ratio is the square of the turns ratio. If we
want to know how a transformer works, we must analyze it as a transformer.
It's as simple as that. The ferrite core carries ALL of the flux, and thus all
of the differential power carried by the circuit into which it is inserted.
In general, common mode chokes do NOT affect the differential signal, but
there CAN be differential mode loss in the transmission line that forms the
common mode choke due to transmission line effects. For example, if the common
mode choke is inserted in a badly mismatched transmission line, there can
excess loss due to SWR throughout the line, both in the part of the line that
forms the choke and in the rest of the line. Below UHF, virtually all loss in
real transmission lines is due to I square R; if the combination of the
antenna and the line places a current maxima at the choke, that segment of the
line can burn a high fraction of the transmitter power, greatly reducing the
transmitter power that gets to the antenna and overheating (and frying) that
segment of the line. N6BV wrote an excellent applications note about this for
QST several years ago, to which I contributed.
It IS practical to model (predict) dissipation in a common mode choke using
NEC. A single wire is added to the model with the geometry and physical
connections of the transmission line, and the known (measured) impedance of
the choke is added as a Load at the point where it is inserted in the system.
NEC is then set to model with a defined transmitter output power (for example,
1,500W), and currents are computed. NEC then provides a readout of current at
every point on every conductor, and the current in the choke is used to
compute dissipation in the choke.
Tutorials at k9yc.com/publish.htm show a practical method for measuring the
common mode impedance of ferrite chokes, and for determining values for a
parallel equivalent circuit that can provide a good first approximation of
dissipation.
73, Jim K9YC
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