Causal explanation for observed superluminal behavior of microwave propagation in free space

In this Letter we present a theoretical analysis of an experiment by Mugnai and collaborators where superluminal behavior was observed in the propagat ion of microwaves. We suggest that what was observed can be well approximat ed by the motion of a superluminal X wave. Furthermore, the experimental re sults are also explained by the so called scissor effect which occurs with the convergence of pairs of signals coming from opposite points of an annul ar region of the mirror and forming an interference peak on the intersectio n axis traveling at superluminal speed. We clarify some misunderstandings c oncerning this kind of electromagnetic wave propagation in vacuum. (C) 2001 Elsevier Science B.V. All rights reserved.