Realizing non-Foster reactances using negative-group-delay networks and applications to antennas

Non-Foster reactive elements such as negative capacitors and negative inductors hold promise for a number of important applications such as broadband small antennas (beyond the Chu limit), broadband matching networks (beyond the Bode-Fano limit) and squint-free series-fed and leaky-wave antennas. Traditionally, negative capacitors and inductors have been implemented using negative-impedance converters (NICs) and negative-impedance inverters (NIIs). However, these NIC and NII circuits are prone to instabilities and excessive noise. In this article, we highlight a new perspective in realizing non-Foster reactive elements using negative-group-delay (NGD) networks. The underlying NGD networks are completely passive and thus stable. However, NGD are associated with some kind of loss (resistive or reflective) which can be compensated using amplifiers. The stable operation of these elements can lead to interesting antenna applications discussed in this paper.