The Observation of Dispersionless Superluminal Propagation in a Non-Foster Loaded Waveguide and Its Fundamental Limitations

Superluminal pulse propagation has been observed in past investigations. Conventional methods rely on resonance-based gain/absorption line materials, inevitably inducing significant pulse distortion. Motivated by the negative reactance slope of the non-Foster circuits, we investigated superluminal propagation based on a non-Foster loaded waveguide. Superluminal square-wave pulse transmission with negligible distortion has been observed in time-domain measurements. In addition, the propagation distance limitation for a non-Foster-based superluminal waveguide is derived by analyzing the distortion of the transmitted waveform. We conclude that a distortion-limited superluminal output pulse cannot emerge more than the original pulse duration ahead of the same pulse transmitted in vacuum, in agreement with causality. This investigation provides an alternative solution to the gain/absorption approaches, and is directed toward a more comprehensive understanding of superluminal propagation.