On Thu, 6 Mar 2008 11:39:47 -0800 (PST), Rick Karlquist wrote:
>Jim Brown wrote:
>> Another VERY important point. Voltage baluns (transformers) put all
>> transmitted power in the core of the ferrite. If you're running much
>> that's going to heat the ferrite. If you're running enough power, it
>> 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
>> impedance, they reduce the current (and their dissipation) to a very
>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.
You are mistaken.
>If you assume that the balun
The word "balun" in this context is far too broad.
>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.
I am specifically talking about common mode chokes formed by winding
turns of coax through toroidal cores (or the big clamp-on described in
my tutorial). These chokes see ONLY the common mode voltage on the
line. The same is true of a W2DU choke (what he called a current
>This assumes that the current choke is wound with a given number of
>turns of bifilar wire
This form of choke is very inferior to a choke wound with coax. The
core will typically see 40-50% of the transmitted power in the form of
leakage flux of the bifilar winding (really a parallel 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.
Some voltage baluns operate as autotransformers, coupling the
differential signal (or power) through the core. An autotransformer IS
a transformer in the classic sense, but it does NOT provide DC
isolation. A voltage balun IS a transformer (or autotransformer) in the
classic sense, because it uses inductive coupling between windings to
carry the differential signal.
A common mode choke wound with coax is NOT a transformer in any sense
because it sees ONLY the common mode field, not the differential field,
which is confined entirely within the coax. This is true because of the
unique nature of coax, which confines virtually all of the differential
signal within the dielectric. Rather, a common mode choke it is a
simple lossy inductor that self-resonates with its own stray
>A classic transformer is unrealizable at ham radio frequencies
>and power levels.
It depends on how you define "power." #61 and #67 cores of appropriate
size could certainly be used at the 50-100W level.
>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.
To know for sure, you've got to shove power to the circuit for a while
and by one means or another look for excessive heat or non-linearity or
both. Your bifilar winding on a #43 will generate significant heat in
line with a 100 watt radio, while the same turns on a #61 core will
not. The only time I've observed significant heating in a common mode
choke wound with coax is when I've used it in the very unusual and
stressful application -- as the end insulator for a vertical dipole
that uses the shield of the coax as the lower half of the dipole. And
even in that very stressful application, I was able to come up with a
choke (actually two chokes in series) that would handle 1.5kW under
contest-like conditions. You and I discussed this on the NCCC list.
BTW -- you were right and I was wrong several months ago when I said
that all "baluns" that transform impedance being voltage baluns. I took
apart a couple of DX Engineering baluns and studied them. These ARE
transforming impedance, and they ARE current chokes wound with coax,
NOT parallel wire transmission line. I showed these photos as part of
presentations to PVRC, NCCC, and NCDXA, and they're on my website.
The 4:1 "balun" is two chokes wired in parallel on the low-Z side and
in series on the high-Z side. But they are separate chokes, and there
is no coupling between them. There IS some significant voltage across
the windings of these current choke "baluns" but they dissipate very
little power because their impedance is quite high, so the common mode
current (and thus the power in the cores) is quite small. And because
it's coax, the transmitted power is all within the coax.
That W8JI guy is mucho smart.
Jim Brown K9YC
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