Analysis of stable negative impedance loaded dipole and canonical chiral elements with application to novel active media.

摘要

It is possible to generate negative impedances at radio and microwave frequencies by means of an active two-port known as a Negative Impedance Converter (NIC). Negative impedance loading alters the current magnitude, current distribution and in some cases, the current direction of an antenna element such as a dipole exposed to an incident electromagnetic filed. Changes in element current are observable as changes in the backscatter response of the element.; However, as active impedances are capable of supplying power, the stability of networks containing active impedance loads must be assured by analysis. In this work, system stability is determined by Nyquist analysis. The Nyquist curve clearly shows if the system is stable or not and indicates in a general way the system stability margin. Furthermore, Nyquist analysis accepts system description in tabular format, a considerable advantage as antenna terminal impedance behavior does not lend itself to formulaic representation.; Because stability is the central issue in the application of negative impedances, this work experimentally confirms the results of the theoretical stability analysis by means of a frequency scaled equivalent circuit of a dipole-negative impedance converter-NIC load system. A passive LCR lumped component network that duplicates the terminal impedance of a dipole replaces the antenna in this experimental verification. High speed operational amplifiers serve as the active component of the negative impedance converter and as diagnostic voltage and current buffer amplifiers. Stable operation is achieved and experimental Nyquist curves that closely replicate those generated mathematically are produced.; Negative impedance loading enables significant alterations of the magnitude, frequency and bandwidth of dipole backscatter. Applied to artificial media comprised of randomly oriented dipole or canonical chiral inclusions, active impedance loading causes significant alteration of medium constituent parameters as determined from consideration of inclusion polarizabilities and application of the Maxwell-Garnett mixing formalism and therefore offer an additional degree of freedom in the design of such media including achievement of effective media properties not otherwise possible.; Original contributions of this work in include: (1) the investigation of the stability of dipole and chiral antenna elements with negative impedance loading, (2) the use of the Nyquist stability criteria and Nyquist curves to determine the stability of antenna elements with negative impedance loading, (3) the experimental confirmation of theoretical stability analysis using the antenna surrogate method, (4) the development of a new, broadband dipole equivalent circuit model based upon transmission fines, (5) the demonstration that dipole backscatter bandwidth broadening using negative reactance loading is possible and stable, (6) the demonstration that dipole reflection coefficient polarity reversal by means of negative impedance loading is possible and stable and (7) the development of design techniques for stable active artificial dielectric and chiral media.; The active impedance loading techniques presented in this work have a wide variety of applications including the design of active artificial media with enhanced electromagnetic response and increased bandwidth, high impedance active artificial media and broadband Sievenpiper high impedance reactive surfaces.