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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