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In a pcb stripline with ground reference planes above and below the signal trace, I understand that the return current will split between both planes and return to the supply.

What would happen if one of the planes was not connected to ground or anything else in the circuit (the plane is essentially floating)? Would there still be a return current in that plane?

If the curl of the magnetic field is equal to the current density J plus the displacement current, I think that the curl of the magnetic field must be non zero on the surface of the floating plane and there either an actual current or a displacement current must flow there (I say this as there should be a circulating magnetic field around the signal trace).

Is it possible for current to flow in the floating plane and return to the supply? Can this work like a capacitor where there is an open circuit but current still flows?

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  • \$\begingroup\$ This question has been asked several times before, you can use a power plane for a reference. \$\endgroup\$
    – Voltage Spike
    Sep 15, 2020 at 20:38
  • \$\begingroup\$ That wasn't really my question. My question was related to current flow in a plane with no DC connection to anything. \$\endgroup\$ Sep 16, 2020 at 21:30
  • \$\begingroup\$ That question has also been asked befor \$\endgroup\$
    – Voltage Spike
    Sep 16, 2020 at 22:26
  • \$\begingroup\$ Please show me that thread. \$\endgroup\$ Sep 18, 2020 at 6:14

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Even thos the floating plane has no DC connection to the other plane, there will be parallel_plate capacitance between the two planes.

C = Eo * Er * Area/Distance

C = 9e-12 farad/meter * 4.7 * 10cm * 10cm / [1/16th inch / 3]

where we assume 4 layers of copper foil, and 3 layers of FR-4 insulation

C = 45e-12 farad/meter * 0.1 meter * 0.1 meter / 0.0005 meters

C = 45e-12 farad/meter * 0.01 meter^2 / 0.0005 meters

C = 45e-12 farad/meter * 20 meters = 900 picoFarads

and at 1GigaHertz (given 1pF has reactance of -j159 ohms at 1GHz)

our 900 pF has reactance of 0.18 ohms.

But that is a distributed reactance, and 10cm is a large fraction of the wavelength of 1GHz.

Summary: there will be useful coupling between the floating Ground and the non-floating Ground.

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