For best efficiency, ideally D = D2. This gives the maximum coupling of the magnetic field lines from the primary into the secondary. If D2 < D, some of the field lines will not get coupled, but radiate outward into space. Of course, if there are shield plates below the primary and above the secondary coil, the system is “sandwiched” such that any uncoupled field lines will be absorbed by the shielding material. The data shown here is from the WPC website for the coil geometries typically used (in terms of coil size and distance between primary/secondary). The QI specification’s max displacement allowed in the (z-axis) is 5 mm. Typical values for “Z” in real life would be 3 to 4 mm, similar to the thickness of two plastic housing layers. The coils used today are 30 to 40 mm in diameter -- so the ratio z/D is about 0.1 and there is a good coupling efficiency in the range of 90% give or take a few points.