On Thu, 23 Apr 2009 12:26:14 -0400, Gary Schafer wrote:
>If the line is well balanced it will not radiate
>either forward or reflected power that may be on it.
Correct. BUT -- an important clarification about balance. It is
not the LINE that determines balance, it is what is connected to
the line -- the ANTENNA and the matching network. Virtually all
ham antennas are unbalanced by surroundings, even when fed by 2-
wire line.
By the principle of superposition, transmission line current can
be analyzed as differential mode current and common mode current.
The current due to unbalance is common mode current. Differential
mode current does NOT radiate. Common mode current DOES radiate.
Common mode current can be killed on coaxial line by a serious
common mode choke. The "coil of coax" and the traditional "string
of beads" so-called "current balun" do this in a small way (that
is, they reduce the common mode current a bit. The big ferrite
coaxial chokes I've designed do it a lot better -- that is, they
reduce it a lot more. A lot less "RF in the shack," a lot less RX
noise picked up on the line.
2-wire line is far more susceptible to RF in the shack and noise
pickup on the line ONLY because there's no practical way to choke
it to kill the common mode current. If there's no common mode
current on 2-wire line, it won't radiate any more than coax.
73,
Jim Brown K9YC
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