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Non-Foster Circuit Design and Stability Analysis for Wideband Antenna Applications
Elfrgani, Aseim M. N

2015, Doctor of Philosophy, Ohio State University, Electrical and Computer Engineering.
In recent years, there has been a great interest in wide-band small antennas for wireless communication in both ground and airborne applications. Electrically small antennas; however, are narrow bandwidth since they exhibit high a quality factor (Q). Therefore, matching networks are required to improve their input impedance and radiation characteristics. Unfortunately, due to gain-bandwidth restrictions, wideband matching cannot be achieved using passive networks unless a high order matching network is used. Fortunately, the so-called non-Foster circuits (NFCs) employ active networks to bypass the well-known gain-bandwidth restrictions derived by Bode-Fano. Although NFCs can be very useful in numerous microwave and antenna applications, they are difficult to design because they are potentially instable. Consequently, an accurate and efficient systematic stability assessment is necessary during the design process to predict any undesired behavior.
In this dissertation, the design, stability, and measurement of two non-Foster matching networks for two different small monopole antennas, a non-Foster circuit embedded within half loop antenna, a combination of Foster and non-Foster matching network for small monopole antenna are presented. A third circuit; namely, a non-Foster coupling network for a two-element monopole array is also presented for phase enhancement applications. It all examples, the NFCs substantially improve the antenna’s performance over a wide frequency band.
First, the stability properties of NFCs are discussed with a time-domain technique that computes the largest Lyapunov exponent for time series signals. In case of instability, it is shown how the circuit can be stabilized with different controllers. The proposed stability approach has been successfully applied to a negative capacitor to match a 3" electrically small monopole receiver antenna. Measured results verified that the system is stable and the non-Foster matching networks improve both the antenna gain and the signal to noise ratio (SNR).
Second, an efficient way to improve the performance of an electrically small antenna by embedding a non-Foster circuit (NFC) within the antenna structure is presented. This technique is based on the theory of Characteristic Modes (CMs). The antenna, loaded with a non-Foster circuit, shows remarkable improvement in gain and SNR. Furthermore, by loading the antenna the omnidirectional pattern is maintained up to a higher frequency by delaying the onset of higher order current modes.
Third, to the best of our knowledge, this is the first time a complete systematic stability analysis of non-Foster systems is introduced. The method was applied to two non-Foster matching networks; namely, a series connection of a negative capacitor-inductor and a combination of Foster and non-Foster circuits for use with a small monopole antenna. Measured results show that the non-Foster matching networks improve both the antenna gain and the SNR for the largest frequency band reported in the literature.
Lastly, a stable system consisting of a non-Foster coupling circuit is introduced to amplify the output phase difference of an array of two coupled monopole antennas over a wide frequency band. Measured data of the output phase response of the NFC-two-element biomimetic antenna array (BMAA) shows that it has much wider 3-dB bandwidth in comparison with the passive case. The NFC improves the normalized available output power of the BMAA compared to the case when passive coupling networks are used.
Roberto Rojas, Prof. (Advisor)
Patrick Roblin, Prof. (Committee Member)
Fernando Teixeira, Prof. (Committee Member)

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Elfrgani, A. (2015). Non-Foster Circuit Design and Stability Analysis for Wideband Antenna Applications. (Electronic Thesis or Dissertation). Retrieved from https://etd.ohiolink.edu/

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Elfrgani, Aseim. "Non-Foster Circuit Design and Stability Analysis for Wideband Antenna Applications." Electronic Thesis or Dissertation. Ohio State University, 2015. OhioLINK Electronic Theses and Dissertations Center. 23 Sep 2017.

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Elfrgani, Aseim "Non-Foster Circuit Design and Stability Analysis for Wideband Antenna Applications." Electronic Thesis or Dissertation. Ohio State University, 2015. https://etd.ohiolink.edu/

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Full text release has been delayed at the author's request until March 31, 2019