  It recently has been demonstrated that signals conveyed by evanescent modes
can travel faster than light. In this report some special features of signals
are introduced and investigated, for instance the fundamental property that
signals are frequency band limited.
  Evanescent modes are characterized by extraordinary properties: Their energy
is {\it negative}, they are not directly measurable, and the evanescent region
is not causal since the modes traverse this region instantaneously. The study
demonstrates the necessity of quantum mechanical principles in order to
interprete the superluminal signal velocity of classical evanescent modes.
