W2FMI's book is heavy on Differential-Mode performance, but says little
about Common-Mode performance; some of what he does say on CM
performance is plain wrong! It's relatively easy to build a choke balun
that will handle high Differential-Mode currents and voltages - _but the
"killer" is the Common-Mode current_.
As Jim already said, for some applications it's almost impossible to
build a choke that will handle QRO. Take, for example, a 1:1 choke at
the feedpoint of a coax-fed resonant 80m dipole that is well balanced;
at 1500W the CM voltage across the balun is a worst-case 137v rms. Now
place the balun at the output of an unbalanced tuner feeding the same
dipole through 600 Ohm open-wire line, and the worst-case voltage across
the choke rises to over 1.6kV rms. The choke dissipation will be 144
times as great !!! A choke with an excellent 10kOhm resistive CM
impedance would be dissipating 270W in that application ..... but not
for long :)
Steve G3TXQ
On 29/04/2014 03:30, Matt wrote:
IMHO - W2FMI's book on baluns (& ununs) offers much insight on the subject
of power ratings associated with different Balun construction - highly
recommended reading. One may also find some good info on W8JI's web site as
well.
Depending on application, it is apparent that a well-designed balun need not
necessarily to be physically large. From my limited recollection, for a
well-designed balun, power is generally limited by the dielectric strength
of the insulation and the ohmic heating of the conductors. If the core is
getting hot - then something is probably wrong with the design. For some
low impedance high power balun applications, the challenge of close-spaced
windings becomes one of dielectric strength. Baluns wound with home-made
low impedance coax cable apparently offer some opportunity for advantage
here. For high impedance high power applications, the challenge of
controlling the stray capacitance associated with the wide spacing may lead
one to consider achieving the transformation in multiple stages using two of
more baluns connected in series.
Hope this helps answer at least part of your question. Perhaps someone will
chime in with more info on choke baluns.
Matt
KM5VI
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