On 1/21/2011 10:08 AM, Andy wrote:
> "Two terminals" is fine on paper. But you never have two terminals.
> You always have "ground", and who knows what else.
More correctly, we have WIRES of finite length (called a feedline),
often connected to the earth via the transmitter. Those wires do NOT
represent, nor do they act, as GROUND. Rather, they are simply wires
that become part of the antenna. When we add a common mode choke to
that line, we are adding SOME impedance in series with those wires. The
BEST common mode choke is carefully wound to provide a high resistive
impedance at the operating frequency(ies) by placing its self resonance
in that frequency range. That's easy to do if it's the RIGHT core
material, because the RIGHT core material typically yields a Q on the
order of 0.5. These "BEST" chokes can easily provide an impedance on
the order of 5K ohms or more at resonance, so they are close to being
insulators in the common mode circuit.
My tutorials (text and power point pdf) show how to determine the values
of parallel R, L, and C that comprise the parallel resonant circuit
formed by a choke on the basis of the measured impedance curve, and
values are shown for some typical chokes. These values can easily be
plugged into the NEC model (or at least it's easy in W7EL's EZNEC).
As G3TXQ has observed, a resonant feedline can still interact with the
antenna (or another antenna nearby), even if there is a BEST choke at
the feedpoint. That can be prevented by one or more additional chokes
along the line, just as we add "egg insulators" on the guy wires for our
towers.
Another point. The BALANCE of a circuit or system is defined by
IMPEDANCE with respect to the reference plane, not voltage or current.
A circuit imbalance CAUSES voltage and/or current imbalance, which in
turn causes common mode voltage and current on the line.. Adding a
common mode choke to the line places nearly all of the common mode
voltage across the choke, reducing the current to near zero, which
reduces the radiation from (and reception by) the feedline to near zero.
73, Jim Brown K9YC
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