The entire tester must be placed on the ground plane.

The noise output from the tester ultimately returns to the SG inside the tester enclosure, but

impulse noise contains many high-frequency components, so the ground plane and tester enclosure (mainly the bottom surface) form a capacitor and become part of the return path.

Because the ground plane and enclosure correspond to the electrode parts of the capacitor, the capacitance differs between the case where the entire enclosure is on the ground plane and the case where only about half is on it,

which may affect the test results.