Characterisation and Design of Novel Non-Foster Circuits for Electrically Small Antennas

Abstract

There is a demand for broadband electrically small antennas that cover large frequency bands without any requirement of reconfiguration techniques. This is particularly true at low frequencies (VHF/UHF), where wavelengths are long and antennas are physically large. The fundamental gain-bandwidth limitation was related to the electrical size of passive electrically small antennas by Wheeler and Chu; their result implied that an electrically small antenna exhibits high quality factor which limits the bandwidth. Additionally, the gain-bandwidth limitation was related to impedance matching conditions by the Bode-Fano criteria, which restricts available bandwidth using conventional reactive elements. A non-Foster circuit approach has been presented which delivers a broadband input impedance match and also overcomes the aforementioned fundamental limits. These non-Foster impedance circuits can be realised by negative impedance converters (negative inductance and/or capacitance). The thesis also explores the advantages and challenges of antenna impedance matching using negative impedance circuits based on two topologies: (1) conventional transistorbased circuits, and (2) a novel resonant tunnelling diode approach. The advantages of non-Foster circuits in the implementation of broadband small antennas include wideband performance around one-tenth of the self-resonant frequency and overcoming of the fundamental limits associated with passive antennas. Diode-based circuits are more compact, easily configurable, less sensitive to stability, have low power consumption and are less complex as compared to the transistor based designs. These features makes it a potential candidate for array and meta-material applications. However, there are few challenges for non-Foster circuit integration with an antenna due to high noise figure, which affects the system channel capacity and receiver performance in a communication system. A detailed design procedure has been developed to mitigate the effects of noise and instability and also, the system performance and measurement of the non-Foster circuit integrated antennas have been discussed.