On 8/19/2013 4:50 AM, Jim Garland wrote:
In
other words, I believe a common mode inductance always reduces the common
mode currents along the feedline, as one can easily visualize by drawing out
the equivalent circuit.
Jim,
Drawing the equivalent circuit is always a key to understanding, but we
must first get that circuit right. The common mode circuit is an
unintentional antenna, formed by the feedline, the intentional antenna
to which it is connected, and the connection to the transmitter (and
from there usually to the earth).
Calling a transmission line "balanced" does not MAKE the system balanced
-- mother nature determines that, and she does so on the basis of the
balance of the impedances in the system. Parallel wire line still can
carry common mode current if the circuit has imbalance -- many (most?)
ham antennas are unbalanced by their surroundings, such as buildings and
other conductive elements, trees, ground slope, even slope of the
antenna itself. The difference in the equivalent circuit for parallel
wire line is that the connection is to both sides of the antenna rather
than one, and the Z between the feedline and earth may be higher
depending on how it is coupled to the transmitter.
Also, because the common mode circuit is part of the antenna, and
because antennas do not require a connection to earth to work, depending
on the length of the feedline, there could still be substantial common
mode current on parallel wire line even if the Z to earth was high.
W1HIS did a great job of pointing out the importance of high choking Z
on a feedline to minimize noise pickup, correctly observing that while
500 ohms of choking Z might be enough to prevent noticeable pattern
distortion, it is not nearly enough if you're trying to dig a weak
signal out of the noise, where a few dB matters. Not everything in his
app note is correct, but he definitely got that right.
73, Jim K9YC
_________________
Topband Reflector
|