When an eigenvalue solver loses track of mode identity, two curves can "swap" — each continuing along the other's trajectory after a crossover frequency.
Sometimes a mode is simply lost by the solver — its data contains NaN gaps where tracking failed entirely. The curve disappears and reappears unpredictably.
20 characteristic modes tracked from 9 to 11 GHz. The commercial solver (FEKO 2024) introduces both swapping and missing errors. Our method corrects them all.