Yeah, well, this thread has gotten slightly corrupted. As I followed it, the
object was to place a tower well away from the house (source of "noise") and
then the discussion of transmission lines ensued. There seems to be an idea
that a long transmission line is going to "pick up" noise unless it is choked,
as if it's a beverage or something. I fail to swallow this idea, hence, my
comment about worrying about signal+noise propagating normally inside the line.
A comment was made:
"Not so sure what you mean by "noise inside the line." It has to get
"inside" somehow, from whence it is choked almost any convenient place along
the line prior to the rig connection."
Which of course makes no sense and to which I foolishly failed to respond to
with enough specificity to avoid another unneeded lesson from you.
I regret the oversight.
Wes N7WS
On 5/25/2017 10:42 AM, Jim Brown wrote:
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
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