On 9/22/2017 5:31 AM, Wes Stewart wrote:
This thread started with a question about whether to just coil up some
coax or to wind a balun at the feed point to "keep RF off the coax."
This sounded like a valid transmitting concern to me.
It IS a valid concern. Coiling some coax at the feedpoint IS a common
mode choke. Winding a "balun" at the feedpoint IS a common mode choke.
It then morphed into something else completely when the salesmen for
CM chokes hijacked it.
My contribution to the thread was to correct misconceptions about how
these methods work (or don't work). hardly 'hijacking, and I don't
sell anything, I'm simply giving away what I've learned.
The fundamental principle of common mode chokes to "keep RF off the coax
" is that pure inductance (that is, just winding turns of coax) is a
lousy solution because L can resonate with the rest of the feedline in
the common mode circuit at frequencies where that feedline is
capacitive. This makes its effectiveness strongly dependent on feedline
length and geometry.
Ferrite chokes work NOT by virtue of their inductive reactance, but by
virtue of that inductive reactance forming a low-Q resonance at
frequencies of interest with the parasitic capacitance of the choke,
resulting in a high resistive Z in the common mode circuit. The
resistance in the circuit is coupled from the ferrite core, and appears
in series with the inductance, and that series combination is in
parallel with the capacitance. In the octave or so around resonance, the
circuit devolves to parallel RLC. That "coil of coax" (with no ferrite
core) is a high Q inductor, with self-resonance far above the frequency
where it is used.
All of this is discussed in the two links I posted much earlier in this
thread. I read your stuff, find it useful, and have said so on many
occasions. Maybe you can take the time to read mine!
73, Jim K9YC
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