Composite Metamaterial and Metasurface Integrated With Non-Foster Active Circuit Elements: A Bandwidth-Enhancement Investigation

A metamaterial is designed and integrated with non-Foster active elements to investigate wideband negative permeability characteristics. A composite structure made of periodic resonant loops terminated with capacitors is first developed, and demonstrated that the negative permeability can be achieved near the resonance where the material is highly dispersive and the response is narrowband. To overcome this limitation, a negative active inductor is designed and utilized to compensate for the frequency dispersion of the loop and to achieve wideband negative permeability. Transmission-line circuit model analogy is employed to theoretically investigate the behavior of the composite configuration and predict its physical parameters, followed by a full-wave analysis based on the CST Studio Suite to numerically validate the results. A simple periodic metasurface structure is also implemented to demonstrate the ability of negative active elements (e.g., negative C and negative L) to obtain wideband characteristics. As a result, considerable bandwidth enhancement for both composite metamaterial and metasurface integrated with negative active elements is achieved compared to traditional designs. The basic principle of non-Foster elements is discussed and Linvill's topology is used throughout the paper to obtain the negative impedance. The stability of the entire system is then investigated using the eigenvalue technique and, finally, some related issues to the stability of the non-Foster elements are discussed.