Topband: Common mode chokes

[Lee K7TJR]

<> 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?

     One must remember that common mode signals can also be
caused by differences in ground potentials, not just transmission
line pickup. Many forms of RX antennas require a ground return
for proper operation. Examples would be Beverages, Ewes, and
shortened verticals. Any voltage difference between the antenna
 ground connection and the receiver ground is a common mode 
signal. If you think for a moment of all the house and pole wiring
that can be flowing RF current into the house/shack ground
 connection as compared to a remote antenna ground there
 indeed can be potential differences. Any potential difference
 causes current flow on the outside of the line. Without isolation,
 this is common mode current. You may still need isolation even
 with a short transmission line.
Lee  K7TJR  OR