Non-foster circuits and stability theory

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Abstract
Antenna engineers have realized that non-Foster circuits offer new approaches to antenna loading, broadband impedance matching, and making single- and double-negative metamaterials (SNG, DNG) having considerable bandwidth. Non-Foster circuits are examined from the perspective of active linear network theory. A definition of a non-Foster network is given. Realizability and synthesis of such networks is discussed followed by a discussion of stability assessment for linear circuits generally and non-Foster circuits in particular. An example of a non-Foster impedance matching network designed and built to match an electrically small monopole is described. The impedance match bandwidth of the non-Foster circuit was found to exceed the infinite-complexity Fano limit by a substantial margin.