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Active metamaterials incorporating with non-Foster elements have been considered as one of the means of overcoming inherent limitations of the passive counterparts,thus achieving broadband or gain metamaterials.However,realistic active metamaterials,especially non-Foster loaded medium,would face the challenge of the possibility of instability.Moreover,they normally appear to be time-variant and in unsteady states,which leads to the necessity of a stability method to cope with the stability issue considering the system model uncertainty.In this paper,we propose an immittance-based stability method to design a non-Foster loaded metamaterial ensuring robust stability.First,the principle of this stability method is introduced after comparing different stability criteria.Based on the equivalent system model,the stability characterization is used to give the design specifications to achieve an active metamaterial with robust stability.Finally,it is applied to the practical design of active metamaterial with non-Foster loaded loop arrays.By introducing the disturbance into the non-Foster circuit (NFC),the worst-case model uncertainty is considered during the design,and the reliability of our proposed method is verified.This method can also be applied to other realistic design of active metamaterials.