Wave propagation in reconfigurable broadband gain metamaterials at microwave frequencies

Abstract

The wave dispersion characteristics for loop array-based metamaterials were analyzed, based on the general transmission line model of a one-dimensional host medium interacting with a chain of coupled loops. By relating the wave propagation constant and the effective parameters of the coupled host medium, we showed that an active medium embedded with non-Foster loaded loop array can be designed to exhibit broadband negative material parameters with positive gain. Accounting for all interactions, the stability of the active medium was investigated, further yielding necessary design specifications for the non-Foster loads. Subsequently, an experimental demonstration was provided to verify the theoretical analysis, showing that stable reconfigurable broadband gain metamaterials at microwave frequencies can be obtained with proper negative impedance converter design.

Broadband matching of nuclear quadrupole resonance detector using non-Foster circuits

“…However, the practical application is limited by the amount of time it takes to make a measurement. Low measurement frequency and high environmental noise levels require high Q circuits [14], which implies a large number of relatively slow measurements. It would therefore be extremely beneficial to significantly decrease the measurement time and obtain a broadband matching with the use of the non-Foster characteristic of NIC.…”

“…The AC simulation result (see Figure 9(b)) shows some differences that NICs provide a wider matching compared with the passive matching method (see Figure 8(b)), this means the voltage is maximised across the whole band, rather than just 1 MHz. Noise characterization is important, and particularly so with active elements such as the NIC [13,14]. The SNR is about 4 dB which is similar to the use of lumped-section matching and the voltage at the output terminal is on the same order with Lumped matching (45 nV).…”

Robust stability characterizations of active metamaterials with non-Foster loads

“…As a cascaded source-load system, it can be regarded as a series connection of the loop itself Z 0 , the active load Z L , and the equivalent mutual inductance λ m . The Z L (s) = R L ||L L ||C L is a parallel negative lumped-RLC network which is used to achieve stable broadband MNG or MNZ metamaterials, [16,17] although the stability analysis proposed in the literature is only for nominal stability without considering the model uncertainty of the practical system. Particularly, considering nearest-neighbor interactions only, which is enough for the accurate prediction of the phenomenon, the m-th eigenvalue can be solved, leading to the following expression:…”

“…Comparing with the nominal stability analysis of a steady-state magneto-inductive metamaterial proposed in Refs. [16] and [17], it is clear to see that the range of stable values of non-Foster inductance will decrease if we assume GM > 1. Furthermore, it appears that as the magnitude of non-Foster capacitance decreases, the Nyquist curve will deviate from the uncertain region with respect to the PM specifications in the complex plane, thus the system robustness will be enhanced.…”

“…Particularly, in Refs. [16] and [17], the Nyquist stability criterion was used to design an actively-loaded material which is capable of modeling the practical NFC circuits, including their parasitics and nonlinearities. Based on Cauchy's argument principle, [18] the product of the source impedance and load admittance forms a closed contour in the complex plane.…”