# Re: [SI-LIST] : Upper limit of interplane capacitance

From: Doug McKean ([email protected])
Date: Fri Jun 09 2000 - 10:55:27 PDT

"Chan, Michael" wrote:
>
> I would like to point out that what would be the impedance look like when
> you looks at it from the center of the two plates viewing from the top? Can
> you still qualify it as a rectangular wave guide as the wave is spread out
> in
> 360 degree other than in one particular direction. Instead of calling it the
>
> traditional "characteristic impedance" I would prefer to see it as " driving
>
> point impedance". Any comment from any guru ????

The "equations" come out the same. Just as if you
were asking if there would be any difference with
the characteristic impedance of a one dimensional
transmission line at the end or in the center.
Reality would dictate something different with the
geometries and cutouts in the planes.

But there's a couple of different issues going on
with parallel plates structures and quite different
in many ways. One structure is that the parallel
plates make a transmission line, the other is that
they make a waveguide. The transmission line supports
electric and magnetic fields in the dielectric as
currents move in the plates. The waveguide simply
*guides* fields between them. either along the axis
of the plates or by reflecting them off the walls.

In the case of the plates constituting a transmission
line, circulating currents say in the power plane would
terminate at the edges. Since there's no termination
at the edge, this would cause theoretically a complete
positive reflection.

Like ripples on a pond, these would create nodes at
various points about the planes depending upon many
factors geometry being one. Which is in fact the case.
Termination of such could not be accomplished with
a single point connection such as a resistor.

This phenomena cannot be explained when considering
the planes as a *waveguide*.

- Doug McKean

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