[TowerTalk] FW: Balun/Choke power ratings

[Steve Hunt]

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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