Let's say we have two large, square parallel copper planes separated by a dialectric.

Then let's say that there is an AC current running between two opposite ends of the top plane and the bottom plane is connected to ground.

Since the voltage potential is different at each unit of space within the top plane, how does this affect the effective capacitance and more importantly, the signal current of the capacitive coupling? The voltage potential across the top plane will increase/decrease as it moves along the plane.

  • 2
    \$\begingroup\$ Capacitance is not a function of the applied voltage but of the geometry of the configuration, i.e. only on the distance between the plates, the area of the plates and the dielectric constant of the insulator. \$\endgroup\$
    – sarthak
    Commented Jan 30, 2022 at 13:02
  • \$\begingroup\$ So the signal current at the bottom plane will be the same as if the top plane had a constant voltage cross it? \$\endgroup\$
    – coinmaster
    Commented Jan 30, 2022 at 13:10

1 Answer 1


The situation you describe is only possible when the frequencies are so high that your capacitor is no longer a lumped element.

A capacitor plate in a lumped element view will always short circuit all different potentials distributed over it. So under this perception your setup won't work.

However if the frequencies are high enough a situation like what you describe is possible. In fact that's a waveguide. You will then see a superposition of the field generated by the DC-component of the potential between the two plates and the AC-component between the different potentials on the upper plate.

The capacitance is still fixed assuming the isolator is homogeneus. If it's inhomogeneus things will get difficult. The signal transmission to the bottom plate (displacement currents) will be proportional the mean potential of the upper plate.

  • \$\begingroup\$ I have metal configuration with a 0.5uf capacitance between two planes where the top plane is the main conductor. I do not want the bottom plane to interact with the top plane very much so I need to know what the capacitive signal is going to look like on the bottom plane. Ohms law says that the top plane will have about 30mv across it's length, which will have different voltage potentials across it. Are you saying that the signal current at the bottom plane will be the same either way? \$\endgroup\$
    – coinmaster
    Commented Jan 30, 2022 at 13:30
  • 1
    \$\begingroup\$ You have two very large pieces of metal or a very good dielectric \$\endgroup\$
    – Kartman
    Commented Jan 30, 2022 at 13:34
  • \$\begingroup\$ I have large thin pieces of metal with a dielectric value of 3.2 at 0.5 micron distance between them. As I said I want to minimize the signal current passing between them but I am unsure how much current flows when the voltage potential on the signal line is not constant. \$\endgroup\$
    – coinmaster
    Commented Jan 30, 2022 at 14:49

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