Negative-group-delay and Non-foster Electromagnetic Structures
Keywords: Antennas
Dispersion Engineering
Negative Group Delay (NGD)
Negative-Group Velocity (NGV)
Non-Foster Reactances
Phased Arrays

This PhD dissertation explores novel electromagnetic applications using negative-group-delay (NGD) and non-Foster structures and establishes an intimate connection between these two phenomena. As a result, novel implementations of non-Foster reactive elements using NGD networks are proposed. To this end, NGD and non-Foster structures are investigated separately in Chapters 1 and 2 and related corresponding applications are explored. Then, a two-way link between these two phenomena is proposed, analytically demonstrated and experimentally verified in Chapters 3. This link provides a novel perspective in the realization of non-Foster reactive elements, leading to new designs that are well-behaved and more predictable in terms of stability and operation than traditional designs. Particularly, a class of non-Foster elements, called unilateral reactive non-Foster elements, is presented where losses are compensated using regular unilateral amplifiers. This type of non-Foster element is implemented by cascading an amplifier and a passive load, operating as a NGD network, and provides a stable operation for new applications. In Chapters 4, in preparation for the following chapter where a novel application of reactive non-Foster elements in embedded-matching-network applications is introduced, a frequency-reconfigurable antenna is presented which provides an excellent compromise among size, frequency tuning range and count of tuning elements. In Chapters 5, a design procedure for replacing the tuning element with a circuit, generating a non-Foster reactance, is presented. This embedded circuit creates a wide matching bandwidth at the antenna terminals. In Chapters 6, passive and active approaches for eliminating beam-squinting in linear series-fed arrays are investigated and experimentally verified. The passive approaches are designed based on the parameters of the branches connecting the feedline to the antenna elements, whereas the active approach works on the parameters of the feedline. The active approach employs unilateral reactive non-Foster elements, synthesized by NGD networks, in the feedline. Chapters 7 presents the properties of positive-index/negative-index coupled-line structures, where simplified relations describing these structures in the complex-mode band are derived. Based on these relations, the properties of these structures are discussed. By employing these properties, two applications, namely a NGD structure and a novel traveling-wave resonator, are demonstrated. Although this dissertation focuses on non-Foster and negative-group-delay structures, extra work is described in parts of the thesis to explore relevant structures and to propose novel applications. In this regard, in Chapters 4, before presenting the antenna with an embedded matching network and in a relevant application, a frequency-reconfigurable antenna is presented. Moreover, in Chapters 5, passive approaches for squint-free beamforming in series-fed arrays are explored and implemented. Further, in Chapters 6, the applications of positive-index/negative-index guides in constructing a novel type of resonator with multiple resonant frequencies, in a certain bandwidth, are investigated.