Active non-Foster Metamaterials – State of the Art

A concept of active ENZ non-Foster metamaterial, recently introduced at University of Zagreb is reviewed. This approach goes around the basic dispersion constraints of a passive material and, therefore, enables ultra-broadband operation. The problem of achieving stable operation and associated technological constrains are discussed. Several state-of-the art examples of extremely broadband (a bandwidth of more than two octaves), 1D and 2D active RF ENZ metamaterials are presented.