chuck wrote:
Of course not, Roy. I sure hope what I wrote did not suggest
otherwise. But I am trying to learn exactly which properties of an
antenna system cause house wiring to be "loaded up."

In particular, I am trying to establish whether a top-loaded vertical
with the same wire geometry and RF ground as the open-wire
transmission line-fed dipole would be just as likely to cause
undesirable coupling to the house wiring. It seems to me that it is.
If it is not, I'm trying to understand why not.

When you have common mode current, it not only flows on the feedline,
but continues to ground via whatever path it can. And this is usually
the house wiring. So your antenna now consists of the "antenna", the
feedline, and the house wiring. The problem here is current in house
wiring due to conduction, not coupling.

If you properly feed a vertical, no RF current is conducted to the house
wiring. Either a properly fed vertical or a radiating feedline can
induce current in the house wiring by coupling, but the amount will
depend on (among other things) proximity of the antenna or feedline to
the house. Most of us can put a vertical at least a little distance from
the house, but the feedline has to come right in.

In other words, is the problem transmission line unbalance, or simply
having a radiator with undesirable proximity to house wiring?

Again, the problems are twofold. One is conducted current, and the other
is coupled current due to proximity.

One more way to word the question: if you tie the open-wire lines
together at the tuner/transmitter and feed the antenna as a vertical,
all of the current in the line will be common-mode. Would that be
less likely to cause undesirable coupling than the exact same antenna
with transmission line unbalance.

You would have exactly the same problems in either case, assuming a
worst case of imbalance when feeding the antenna normally (which isn't
likely). Whatever current leaves the rig via the connected-together
feedline conductors (or via common mode current in a normally fed
antenna), an equal amount of current has to leave the rig via its
chassis or "ground" connection. In a properly fed vertical, this current
ends up in the ground system at the base of the antenna. In the
tied-together feed, it'll end up getting to ground however it can,
radiating and inducing other currents along the way.

. . .

Roy Lewallen, W7EL