> I find this question very interesting. I've found ON4UN's
comments on
> minimizing common-mode signal leakage, and a number of
others (notably
> John Doty and John Bryant) all advocate the same approach
towards
> sanitizing an unbalanced coax feedline connected to an
unbalanced wire
> antenna (i.e. Beverage, Inverted L, Random, etc.).
The problem is almost always that an antenna is not
perfectly unbalanced, even when we call it unbalanced. If
the system has a connection path for the shield to the
antenna "ground" side, the outside of the shield becomes
part of the ground system and can couple shield current into
the antenna.
A groundplane with four 1/4 wl radials is is a good example.
Despite the fact the antenna has four radials the common
mode impedance of those radials is not zero ohms. This means
any coax shield connected to those radials will have
"antenna currents". As a ground system is made smaller
shield currents increase.
Some people think an end fed half wave requires no ground.
That isn't true either. At the feedpoint there always has to
be equal currents flowing into a ground as into the antenna
from any antenna that isn't perfectly balanced.
A Beverage has some RF current that flows to ground at the
feedpoint. That current causes the ground rod voltage to
"pump" up and down because of resistance. If the ground rod
has 80 ohms of resistance and the shield has 8 ohms there
will be ten times the current on the outside of the shield
as flows into the ground rod. It's always an impedance ratio
issue.
> What I'd like to know is how to deal with an elevated
shack like mine.
> I get similar results to Mac with respect to receiving
signals off the
> shield, but I'm not surprised since a good 10-15 m of the
feedline is
> up in the air.
If you have a coaxial or open wire feeder with the end open,
shorted, or terminated in a load resistor at HF it should be
essentially dead when connected to a receiver. Even very
poor shields you can see through (like Radio Shack coax)
will behave this way at HF. Quad shield or single shield
makes very little difference at HF. Neither do grounds along
the cable unless the antenna feedpoint has an electrical
problem or the shield has a connection problem someplace.
It is a little difficult to measure common mode ingress when
an antenna is connected. You could inject current by placing
a toroid over the shield with a few turns of wire connected
to a signal generator or antenna analyzer and see how strong
that signal is. There is no reference point, however, unless
you compare it to other systems.
You could also inject RF into the antenna and measure common
mode current of that injected RF.
Common mode current is easy to measure.
73 Tom
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