An intimate relation is established between non-Foster reactive elements and loss-compensated negative-group-delay (NGD) networks. It is shown that any possible network configuration containing a class of non-Foster elements operates as an NGD network. Likewise, it is demonstrated that a loss-compensated NGD network represents a reactive network with a non-Foster behavior. Consequently, these two properties can be intimately linked together and NGD networks can be utilized to implement non-Foster elements, such as negative capacitors and inductors. This result introduces another perspective in realizing non-Foster reactive elements, leading to new designs that are well behaved and more predictable in terms of stability and operation than traditional designs using negative impedance inverters and negative impedance converters. Based on this concept, loss-compensated NGD networks are proposed for realizing high-quality non-Foster reactive elements. Furthermore, entirely passive non-Foster elements with a limited quality ( ) factor are proposed for which the minimum factor and the maximum achievable bandwidth are inversely related. It is shown that the design of non-Foster reactive elements using NGD networks can lead to the realization of standalone unilateral non-Foster reactive elements in a certain bandwidth. Examples of such non-Foster reactive elements and networks are demonstrated experimentally and shown to be stable.Index Terms-Dispersion engineering, negative group delay (NGD), negative group velocity (NGV), non-Foster reactances.

Beamforming in series-fed antenna arrays can inherently suffer from beam-squinting. To overcome the beamsquinting problem, low-dispersion, fast-wave transmission lines can be employed. Such transmission lines can be designed by loading a regular transmission line with non-Foster reactive elements (e.g. negative capacitors and inductors). As a result of a recent development, these non-Foster reactive elements can be implemented using loss-compensated negative-groupdelay (NGD) networks, providing a solution to the stability issues associated with conventional non-Foster networks. In this work, transmission lines augmented by loss-compensated NGD networks, representing the non-Foster reactive-element loading, are employed for designing wideband fast-wave, lowdispersion transmission lines. This work consolidates this non-Foster reactive element loading method with earlier efforts where NGD networks were used to implement zero-degree phase shifters for beamforming at the broadside direction, and generalizes these methods for arbitrary-angle beamforming from backfire to endfire including the broadside direction. Experimental results are presented for a wideband linear four-element transmitting array feed network for beamforming at 30 o with respect to the broadside direction in the frequency range of 1 to 1.5 GHz. By connecting this feed network to four wideband taperedslot antennas, the beamforming performance is experimentally verified inside an anechoic chamber. Moreover, the antenna array is experimentally tested for transmission of a narrow pulse, where low distortion is observed at the beamforming angle over the entire operating bandwidth. The physical length of the feed network is realistic and is 0.96 wavelengths long at the center of this frequency range. In addition, switched-line phase shifters are employed for squint-free beamforming in three other angles, namely-60 o , 0 o and -30 o .Index Terms-Non-Foster reactances, linear antenna arrays, antenna feeds, negative group delay (NGD), negative group velocity (NGV), dispersion engineering. 0018-926X (c)

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Adding frequency reconfigurability to a compact metamaterial-inspired antenna is investigated. The antenna is a printed monopole with an incorporated slot and is fed by a coplanar waveguide (CPW) line. This antenna was originally inspired from the concept of negative-refractive-index metamaterial transmission lines and exhibits a dual-band behavior. By using a varactor diode, the lower band (narrowband) of the antenna, which is due to radiation from the incorporated slot, can be tuned over a broad frequency range, while the higher band (broadband) remains effectively constant. A detailed equivalent circuit model is developed that predicts the frequency-tuning behavior for the lower band of the antenna. The circuit model shows the involvement of both CPW even and odd modes in the operation of the antenna. Experimental results show that, for a varactor diode capacitance approximately ranging from 0.1-0.7 pF, a tuning range of 1.6-2.23 GHz is achieved. The size of the antenna at the maximum frequency is and the antenna is placed over a CPW ground plane ( being the wavelength in vacuum).Index Terms-Circuit models, coplanar waveguide (CPW), frequency-agile antennas, metamaterials (MTMs), microstrip antennas, multifrequency antennas, printed monopole, reconfigurable antennas, slot antennas, varactors.

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