On Mon, 31 Jan 2011 21:20:57 +0100, "Antonio Vernucci"
wrote:

Instead of directly telling which it is, just for fun I wonder whether anyone
can figure out a case in which a bipole may not be reversed without
consequences. Not difficult, but it anyway requires some thinking.

Hi Antonio,

This would fall into the area of a hidden variable (a ghost terminal,
so-to-speak) and is called Common Mode. This occurs in the situation
that includes the proximity of ground which is often neglected as an
influence.

The dipole could be unbalanced forcing currents to be out of balance.
This is notorious with cabled down leads which preserves the sense of
two terminals, but the imbalance with ground forces a third terminal
into the reality of implementation.

If we were to divorce ground from this (free space) and simply looked
at the two lead impedance at the drive point, then adding a cable will
force the same imbalance (albeit to a different degree, as found with
the proximity of earth). If the dipole is off-center fed, this will
be more profound (even though when it is measured at the feed point,
the measurement is immune to pole switching).

Reversing the "polarity" of the cable connection will create a new
topology (although the Common Mode will persist with new
characteristics).

Although probably unnecessary, let me recall that a filter is typically a
THREE-terminal device (IN, OUT, GROUND), not a TWO-terminal one.

Well, even though I have anticipated ground, a filter can easily be a
two terminal device without need for ground - being a series
component. This two terminal description is quite typical too.

73's
Richard Clark, KB7QHC