Even Jasik, ed., "Antenna Engineering Handbook" mentions the balun. My
first ham antenna (1952) was a 40 meter dipole fed with 72-ohm twinlead
and a B&W 1:1 air-core balun and I once worked on an airborn military
system which used a balun.
The term "unun" has a much more dubious justification, however (my opinion).
73, Bob N7XY
On 5/22/17 10:07 PM, Wes Stewart wrote:
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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