I tried previously to simulate this with LTSpice. I modeled the PCB trace as two 100 ohm transmission lines in parallel. In the simulation, it I floated one side of the transmission line, then I got lots of reflections,etc. Connecting one with a small value inductor also showed lots of signal integrity issues.
Is there a better model in Spice to simulate this? Or a different way to do it?
The rule of thumb that it follows the path of least inductance suggests that the signal is being propagated mainly by a magnetic field. This magnetic field will wrap around the signal conductor as concentric circles with the conductor in the center. Since the planes are roughly equal distance, the same number of magnetic field lines will cut across the upper plane as the lower plane. This in turn suggest that the induced current in the planes is roughly equal. That fact that one plane is attached to the same ground as the signal driving the conductor is not a big contributor. If we had to reference the same ground to magnetically couple a current then power transformers would not work. So I am back to my original thought that this will give the board signal integrity problems.