Hassan Mirzaei

Realizing Non-Foster Reactive Elements Using Negative-Group-Delay Networks

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 ( Q) factor are proposed for which the minimum Q 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.

A Compact Frequency-Reconfigurable Metamaterial-Inspired Antenna

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 0.056 λ0 × 0.047 λ0 and the antenna is placed over a 0.237 λ0 × 0.111 λ0 CPW ground plane (λ0 being the wavelength in vacuum).

A Resonant Printed Monopole Antenna With an Embedded Non-Foster Matching Network

Non-Foster reactive elements are embedded inside compact resonant antennas, rather than being employed in matching networks located at the antenna terminals. These embedded non-Foster elements interact with the inherent Foster reactances of the antenna, resulting in a broadband antenna with a high radiation resistance. The class of suitable antennas for this application is discussed, a design procedure is presented and a sensitivity and a stability analysis are performed. Finally, experimental results for a representative non-Foster antenna are presented and discussed.

A wideband metamaterial-inspired compact antenna using embedded non-Foster matching

Passive electrically small antennas have a small radiation resistance and a narrow bandwidth. In this paper, a new type of active antenna is reported in which an active circuit generating a non-Foster impedance is embedded in a metamaterial-inspired small antenna with an inherent sizable radiation resistance. This circuit interacts with the reactive elements of the antenna. The end result is a compact, broadband antenna which is well matched and has the potential for a high efficiency. In this process, the need for an active (or passive) step-up transformer for the radiation resistance is eliminated. This work opens up the possibility of utilizing non-Foster components embedded in metamaterial-based antenna structures to obtain efficient antennas by manipulating the current and field distribution in and around the antenna.

Arbitrary-Angle Squint-Free Beamforming in Series-Fed Antenna Arrays Using Non-Foster Elements Synthesized by Negative-Group-Delay Networks

Beamforming in series-fed antenna arrays can inherently suffer from beam-squinting. To overcome the beam-squinting 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-group-delay (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, low-dispersion 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° with respect to the broadside direction in the frequency range 1-1.5 GHz. By connecting this feed network to four wideband tapered-slot 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: 60°, 0°, and

Squint-free beamforming in series-fed antenna arrays using synthesized non-foster elements

-30^{\circ}

Anomalous Negative Group Velocity in Coupled Positive-Index/Negative-Index Guides Supporting Complex Modes

.

Negative and zero group velocity in microstrip/negative-refractive-index transmission-line couplers

A method for squint-free arbitrary-angle broadband beamforming in series-fed antenna arrays is introduced. This method originates from the idea of loading a feedline with non-Foster elements. This type of loading reduces the per unit length inductance and capacitance of the transmission line and a fast-wave non-dispersive propagation, required for squint-free beamforming in series-fed antenna arrays, is obtained. Importantly, the challenges associated with traditional implementations of non-Foster reactive elements (e.g. stability) are circumvented by introducing a new methodology using negative-group-delay (NGD) networks. This method is established by showing that non-Foster reactive elements and NGD networks influence propagating waves in a similar manner. Subsequently, a series feeding network for linear antenna arrays is designed by loading a host transmission line with loss-compensated NGD networks. In summary, this paper introduces a new method for synthesizing stable non-Foster reactances, using NGD networks, which is utilized to present a solution to the beam squinting problem in series-fed antenna arrays.

Unilateral non-Foster elements using loss-compensated negative-group-delay networks for guided-wave applications

Anomalous negative group velocity (NGV; group velocity antiparallel to the power flow) is reported in a guided-wave structure supporting complex modes. This structure consists of a coupled-line system comprising a positive-index microstrip line edge coupled to a negative-index line. The NGV can be observed on the positive-index line under suitable excitation and termination conditions. What is remarkable about this structure is that the anomalous NGV neither requires any material losses nor any strong reflections on the observed line. This work verifies that absorption or reflection are not necessary conditions for observing NGV, rather NGV can be observed in specific lossless coupled-line structures as well. The general conditions for obtaining NGV in coupled positive-index/negative-index guides are analytically derived and corresponding NGV observations are experimentally reported at microwave frequencies. Specifically, we report the propagation of modulated pulses exhibiting a negative group delay as well as phase shifters maintaining a constant phase over a broad bandwidth. This opens up the possibility of utilizing such coupled-line guides for applications in pulse shaping, delay control, constant phase shifters (vs. frequency) and reducing beam squinting in series-fed antenna arrays.

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

The existence of negative group velocity in the complex-mode region of an infinite length MS/NRI transmission-line coupler is shown. The negative group velocity is associated with only one of the complex modes (c-mode) and exists only when that mode is dominantly excited. The possibility of independently exciting either of the complex modes in an infinite coupler is discussed. A method is proposed for terminating a finite length coupler at one end with a two-port network which provides a matched boundary condition for the complex modes and extends the applicability of theorems and methods of infinite couplers to finite length MS/NRI-TL couplers. A circuit containing a 360° MS/NRI-TL coupler is designed and the presence of negative (and zero) group velocity is shown in design and experiment. Applications include constant with frequency phase shifters, dispersion compensation and broadside series-fed antennas with reduced-beam squinting.