Cecil Moore wrote:
John Popelish wrote:

At any point along the wire, and at any particular instant, whether as
a result of a standing or traveling wave, the current flows in the
direction of the wire, one way or the other. Such current does not
have a phase. It has a direction.


In what direction is the RMS value of standing wave current flowing?

That's easy. RMS current is an AC measurement of current along the
conductor. Over any integer number of cycles, the total movement of
charge is zero. The current spends half the time going one way, and
half the time going the other way. This applies to both standing and
traveling wave induced currents. The only current that describes a
net movement of charge in a single direction is DC.

Those wave currents could not travel at nearly the speed of light
without the displacement currents.


You and I are talking about different displacement currents.
I'm talking about displacement current between a transmission
line and the ground.

If the transmission line is a center conductor in a grounded shield,
then that is what I am talking about. If the transmission line is
balanced, then the displacement current is mostly the current between
the two lines.

We had previously been talking about
displacement current between a loading coil and ground.

That's also included in what I am talking about. Ant current caused
by a voltage swing in a conductive surface along the wave path is
included.

Then you are silly. You cannot describe the reason for wave velocity
of a conductor, transmission line or EM wave without displacement
current.

How does the displacement current get to ground when it's
inside a shielded piece of coax with no common mode current
on the outside braid?

The shield is the ground potential for the center conductor. If you
could mount a tiny current pickup loop between the center conductor
and shield, surrounding nothing but the coax dielectric (so you could
look through the hole, if you were an observer on the center
conductor), that pickup loop would measure a current if there is any
voltage wave on the center conductor. That is a displacement current.

Displacement current into the space around the node.


How does the displacement current get past the coax shield
to the outside world?

As far as the center conductor is concerned, the shield is the entire
universe.

So please stop saying that displacement current is negligible in some
cases of traveling or standing waves.


I'll even say it again. Inside a piece of shielded coax, the
displacement current to ground is negligible yet the standing
wave still exists.

You have a blind spot as to where there is current in a coax. Do you
deny current through any other capacitor that has voltage swing across
it? Then why deny that the capacitance between the center conductor
and a carefully surrounding grounded surface separated by a dielectric
conducts current when there is voltage swing on the center conductor?

If the coax carries a standing wave, then at the points on the center
conductor that are current nodes (for current along the conductor) the
capacitive current to the shield is at its peak, because that is where
the voltage peaks of the standing wave occur.