Topband: Common mode chokes

[K4SAV]

Thomas Damboldt wrote:
> if I understand the common mode discussion correctly, a short
> (not necessarily active) antenna and a much longer feedline
> act together as kind of a dipole. 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?
> Certainly worth trying.
>   
Common mode noise gets into the antenna from the feedline due to an 
unbalanced to balanced transition that may not be sufficiently choked.  
The noise comes from mainly two places, either radiated onto the coax 
line or conducted thru it from a noisy AC ground.  How much gets in 
depends on the amount of noise on your AC ground, how much noise is 
picked up out of the air, how much is capacitively or inductively 
coupled onto the feedline from other cables, and the common mode 
impedance of the feedline.  If you can evaluate all those then you can 
figure out if shortening the feedline will result in more or less noise.

Jerry, K4SAV