Basic concepts of active dispersionless metamaterial based on non-foster elements

Recently introduced concept of active non-Foster metamaterial is reviewed with an emphasis on the counter-intuitive background physics. A basic building block of this active metamaterial is an active `tank circuit´ that contains both conventional reactance and non-Foster negative reactance. It is shown that active metamaterial is stable if equivalent permittivity or permeability is arbitrarily small but positive number (ENZ or MNZ metamaterial). It is also shown that this metamaterial may be extremely broadband (several octaves) with almost no dispersion. The propagation within the operating band may support counter-intuitive superluminal phase and group velocities. However, causality is always preserved due to finite bandwidth of any realistic non-Foster element. Several illustrative examples of practical 1D and 2D active ENZ metamaterials developed at University of Zagreb are presented.