Topband: Common mode chokes

[Jim Brown]

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