Stability of active transmission-line metamaterials

The question of whether stable, active metamaterials can be created is addressed, both through a discussion of absolute instability and an analysis of a transmission-line that produces dispersion analogous to that of the familiar split-ring resonator/wire-based metamaterial. Gain is introduced using negative conductance diodes, and it is shown that the frequency bandwidth controls the window of stable gain. The diodes are located as lumped elements in the unit cell. It is demonstrated that the production of a stable, active, negative phase frequency window is possible.