> > 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.
>
> Hi Larry,
>
> My shack is also an "elevated" shack on the second
floor... I have more than
> 20m of the feed line that is not buried, beacuse it goes
trough whole my
> house.
Having the feedline in the air really doesn't matter unless
you already have shield or end connection problems.
Neither does a lack of earth ground unless there is a
problem with the shield connections that allows signals on
the outside of the shield into the center.
A coaxial cable hanging from an airplane or satellite would
have no significant leakage over any other cable if the ends
didn't allow shield common mode into the inside.
If the ends are fed with exactly equal and opposite currents
all the energy is confined inside the cable. It's when
things slightly unbalance currents that the outside of the
shield has an "antenna" effect.
What burying or grounding the cable can do is reduce the
amount of signal conducted along the shield. If you have a
shield integrity problem having the cable suspended above
earth (so it acts like a better antenna) can increase
signal pickup, but the effect isn't noticeable if the rest
of the system is designed or constructed OK.
A good example of this is a Beverage feedpoint. There is
actually less shield antenna effect if the cable is floated
and connected only with an isolated link. The shield doesn't
require a ground. As a matter of fact a shield ground
probably makes very little difference (except for lightning)
while connecting the shield to the Beverage ground can make
a large difference.
I can't think of a good reason to ever switch a shield. Any
resistance or impedance on the shield connection path might
become a big problem. Removing a ground on the other hand
only has an effect if something else is wrong.
A shield works because electrical rules cause the opposing
currents to flow on the inside. Skin effect isolates the
outside of the shield. Parallel currents or common mode
currents flow on the outside. While it is a good idea to
keep outside currents and voltages as low as possible, if
you don't let the outside currents into the center conductor
at a break in the cable you don't have a problem.
73 Tom
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