[TowerTalk] Tower location and house noise

[Jim Brown]

On Thu,5/25/2017 9:57 AM, Wes Stewart wrote:
> Not sure what you mean either. "Inside" means... well.... inside, i.e. 
> a differential-mode path consisting of a center conductor and the 
> inner surface of the outer conductor.

Common mode current, whether on the outside of the coax shield or on the 
two conductors of parallel wire line, is the result of un-equal currents 
on the two halves of the antenna, and the common mode current makes the 
feedline part of the antenna. With no choke at the feedpoint, received 
noise current on the feedline couples to the "intentional" antenna, so 
it adds to what the intentional antenna couples to the feedline in 
differential mode. By reciprocity, the mechanism is present both on TX 
and TX.

Common mode current on the feedline is the result of any imbalance in 
the antenna system. A common mode choke at the feedpoint effectively 
decouples the feedline from the antenna by presenting a high impedance 
to common mode current, reducing the common mode current to near zero, 
thus forcing the current in the two halves of the antenna to be more 
nearly equal. Without the choke, currents in the two halves are not 
equal at the feedpoint, with the difference flowing as common mode 
current on the feedline.

This mechanism has been understood for a LONG time.

73, Jim K9YC