Unexpected poor performance of presumed high-quality power line filter, and how it improved

A 20 A, 440 V three phase power line filter for a radar transmitter, designed using the best components available and following the correct rules for filter design, showed unexpected poor common mode attenuation levels at frequencies above, already, a few megahertz. Component self-parasitic, as well as mutual component parasitic effects were not the main cause of the poor performance. A redesign using additional capacitors at the input, as well as improving the ground plane, resulted in improvement, but the dominant cause appeared to be the impedance between the ground plane and the reference plane.