Analysis of Causality Issue in Near-field Superluminally Propagating   Electromagnetic and Gravitational Fields
  A simple relativistic analysis is presented which shows that near-field superluminal longitudinal electric signals generated by an electric dipole cannot be used to violate Einstein causality by using the relativistic "sync shift" effect. The analysis shows that because a signal has some time extent (i.e. after a signal is initiated one must waitat least one period for the frequency and amplitude information to be determined), an electric dipole can be used to transmit a signal backward in time, but not before the same signal was initiated, thereby prohibiting a user from changing the signal that was transmitted. The result is also shown to apply to the superluminal near-field gravitational fields generated by an oscillating mass. 
William D. Walker