On Wed, 18 Mar 2009 13:19:12 -0600, Dr. Gerald N. Johnson wrote:
>> Another VERY different thing that is CALLED a balun is really a common mode
>> choke. Put a bunch of ferrite cores around coax, or wind a bunch of turns of
>> coax through one or more ferrite cores. Those are common mode chokes,
>> commonly (wrongly) called "current baluns." But when done well (enough turns
>> to have a high enough choking impedance), they work VERY well.
>And if they don't have enough core cross section, they too can saturate.
>But saturation is more difficult achieve with only one turn per core.
Saturation is VERY unlikely in a coaxial choke if the choking impedance is high
enough, because the core sees ONLY the common mode component. It sees NONE of
the
differential field -- it's all confined to the dielectric of the coax. That is
NOT true in a bifilar winding, which has a lot of leakage flux. So the core in
a
common mode choke that is bifilar wound (that is, a pair of wires) IS likely to
saturate if you run enough power through it.
>Their big claim to fame is that the allow the antenna to set the balance
>where the transformer forces voltage balance.
That's the 30 year old version of why common mode chokes are much better than
voltage (transformer) baluns. That's certainly true, but they have another HUGE
advantage -- they prevent the feedline from becoming part of the antenna. This
keeps RF out of the shack and your neighbor's living room, and it also prevents
NOISE received on the feedline from coupling to the antenna and making it
harder
to hear weak signals. That is, the only noise you hear is what's picked up on
the
antenna, which is usually further from the noise source (in your home and your
neighbor's) than the feedline.
73,
Jim Brown K9YC
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