On 9/24/2011 3:47 PM, Peter Voelpel wrote:
> How could common mode current appear on a near or perfectly balanced
> parallel wire feedline?
Common mode current results from imbalance at either end of the
feedline. Most ham antennas are unbalanced to at least some extent by
their surroundings -- trees, buildings, sloping earth, sloping antennas,
off-center-fed antennas are all common causes of unbalanced antenna
current in a transmission line. The DIFFERENCE between the current in
one conductor and the current in the other is called common mode
current. That common mode current radiates from the line, AND that
imbalance will cause the feedline to act as a receiving antenna.
There are at least three important PRACTICAL reasons to kill common mode
current. First, to prevent RX from coupling into the antenna (like from
your neighbor's computer, battery chargers, etc.). Second to kill
conduction of RF into your shack, where it can excite Pin One problems.
Third, to prevent radiation from the feedline, which is often closer to
your neighbor's stereo rig than your antenna.
The RX noise issue is one important reason for having a really good
choke on the line -- a mediocre choke may be good enough to eliminate
pattern distortion, RF in the shack and interference to your stereo for
100W, but if you're trying to work the weak ones, every few dB can make
a difference. How much difference a better choke will make depends on
your antenna, proximity to noise sources, etc. 4-6dB reduction is not
uncommon.
73, Jim K9YC
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