UC San Diego

Periodic structure has been extensively applied in many applications, such as wireless communication, sensing, and advanced applied electromagnetics such as invisible cloaking and superlens. However, as broadband systems have rapidly advanced, the bandwidth of the periodic structure has become a bottleneck. In particular, the high dispersion of the periodic structure prevents it from being applied to future wide-band practical applications. Extensive research efforts have been done on the bandwidth problem of the periodic structure, and these have concluded that there is a fundamental bandwidth limitation for passive electromagnetic structures. Nevertheless, great interest still remains in finding alternative solutions to increase the bandwidth of the periodic structures.

All conventional periodic structures are based on a repeatedly patterned passive metallic structure, which can be characterized as passive impedance that follows the Foster's theorem, resulting in a fundamental bandwidth limitation. In contrast, an active periodic structure is composed of unit cells that violate Foster's theorem. Thus, they are potentially capable of breaking the bandwidth limitation.

This thesis studies the non-Foster impedance loaded periodic structures and their broadband electromagnetic fast and slow wave propagation applications. To be more specific, a non-Foster impedance, realized by an active feedback circuit, is applied to the unit cell of the periodic structures in order to increase the bandwidth of the unit cell and eventually broaden the bandwidth of the periodic structures.

The thesis first introduces the fundamental principle of the non-Foster impedance, explains its difference to the passive impedance, and presents its possible active circuit implementation: a negative impedance converter based non-Foster circuit, which covers the derivation of the non-Foster input impedance, its stability, and a design example. Then, the thesis presents broadband fast and slow wave propagation based on the non-Foster impedance loaded periodic structures. For fast wave propagation, a periodically non-Foster impedance loaded waveguide is designed, fabricated, measured, and demonstrated for broadband dispersion-reduced fast wave propagation. The non-Foster fast wave propagation study extends to the observation of the dispersionless superluminal pulse transmission, and a fundamental limitation of a periodic non-Foster loading-based waveguide. On the other hand, this thesis also presents an extremely low-profile slow wave supported electromagnetic surface structure by loading non-Foster impedance to a conventional artificial impedance surface. All the applications presented in the thesis include the principle of loading non-Foster impedance to the conventional passive periodic electromagnetic structures, the design of the non-Foster impedance circuits, and the broadband measurement demonstration.