Jim Brown wrote:
> Another VERY important point. Voltage baluns (transformers) put all of the
> transmitted power in the core of the ferrite. If you're running much
> power,
> that's going to heat the ferrite. If you're running enough power, it will
> saturate the ferrite and cause DISTORTION. That means harmonics and
> splatter. Current chokes do NOT see the transmitted power, only the
> unbalanced voltage/current, and if they have a sufficiently high choking
> impedance, they reduce the current (and their dissipation) to a very low
> value.
What you say is correct only if you have a balanced line feeding
a balanced antenna, and the current mode choke is simply used
to "enforce" balance.
If you assume that the balun is connected between coax with no RF
voltage on the shield and an antenna with balanced voltage, then
the magnetic flux in the ferrite is the same whether it is
a current balun or a voltage balun. This assumes that
the current choke is wound with a given number of turns of bifilar
wire and the voltage balun is wound with the same number of turns
of trifilar wire on the same core. Volts per turn is the same.
In fact, if you disconnect one end of the third conductor in a
voltage balun, you convert it to a current balun. Neither of
these is a transformer in the classic sense, with galvanic isolation.
A classic transformer is unrealizable at ham radio frequencies
and power levels.
Also, I think you will find that in most cases, ferrite cores
will not saturate when operated at the maximum power permitted by
thermal considerations. I've never seen it happen.
Rick N6RK
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