On 9/24/2011 12:07 PM, Rob Atkinson wrote:
> I may have missed something but I was unable to find any sort of
> common mode choke on the DX Engineering website,
MANY of the DX Engineering so-called baluns are common mode chokes --
indeed, what is commonly called a "current balun" IS a common mode
choke. Many DXE baluns that transform impedance are ARRAYS of common
mode chokes connected in series and parallel. If you open up some of
these you will clearly see chokes would not with coax, but with parallel
wires. And DXE DOES sell a common mode choke. I haven't bought one,
because I can rolll my own that are probably better for one-sixth of the
cost.
I HAVE inserted the bifilar chokes between the output of a Titan 425 and
the antenna tuner and tested at 1.5kW keydown for several minutes from
1.8 MHz to 28MHz. At that point, the choke sees ONLY the differential
field, and there is VERY little heating because the field from one
conductor cancels the field from the other. Dissipation due to common
mode current is a very different matter, and is discussed at length in
the tutorial. In essence, if the choke as sufficiently choking high
impedance and the antenna is not very poorly balanced, the common mode
current, and thus the common mode dissipation, is reasonably small. If
conditions of the application (for example, impedance transformation)
place very high common mode voltage across a choke, the common mode
impedance must be much higher. In a testing situation, I have set up
very high common mode voltages and placed two chokes in series to
withstand them. DXE builds some of their impedance transforming arrays
of chokes that way.
As to mismatch -- a study of the fundamentals of transmission lines
would lead one to the conclusion that the loss due to mismatch in the
short length of 100 ohm line that comprises the choke is quite small.
After all, one of the most common uses of parallel wire line (notice
that I do NOT repeat the fiction of calling it a balanced line) is to
minimize the loss due to mismatch when feeding antennas that are wildly
mismatched, like the "one-size'fits-all" dipole that is nowhere near
resonance on most frequencies where it is used. Think about this --
we're connecting an antenna that could be anything from 5 ohms to 5,000
ohms, plus reactance, to a feedline that is, perhaps, 400 ohms. The
insertion of a 24 inch piece of 100 ohm line simply modifies (and not
very much) the impedance of the antenna as seen by the line. And, if
wound using #12 copper, as the chokes I have described are, the loss is
VERY VERY small, as confirmed by my tests.
Now, I'm a guy who plays by the rules, and shares my work FOR those who
play by the rules, and my testing is done at that power level, at duty
cycles consistent with serious contesting. Someone who wants to run more
than 1.5kW can design and test his own solutions. :)
73, Jim Brown K9YC
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