On Thu, 9 Jul 2009 14:34:18 -0600, Craig D. Smith wrote:
><> If I connect my receiver
><> with a very short feedline (say 1m), to the antenna, I should
><> not be receiving much common mode noise. Is that true?
Not necessarily. There is a common mode circuit through the coax
shield to the chassis of the equipment to which it connects, and
from there to the station ground, which itself can be noisy.
It is important to understand that a terminal that we CALL
"ground" is not at zero potential. It is simply a point in a
circuit that can be carrying current, and that current can be
anything from DC to daylight. Power systems induce noise currents
on terminals that we CALL "ground." Radio signals induce noise
currents on terminals that we CALL "ground."
The EARTH is NOT a sump into which noise is poured, nor is it at
zero potential with respect to all other points. The EARTH is
LOSSY, which means that it's equivalent circuit contains
RESISTANCE and CAPACITANCE. There is even INDUCTANCE (especially
in the conductors connecting to it). The EARTH is NOT a POINT, it
is a large mass that is non-homgeneous and acts as a circuit
element! By virtue of its size, the earth can conduct a lot of
current, but by virtue of its resistance, that current produces a
voltage drop, which makes any "ground" terminal noisy!
Bottom line -- you DO need a good common mode choke on even a
short cable connected to that antenna.
73,
Jim Brown K9YC
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