Non-foster techniques for designing broadband electrically-small antennas and biomimetic antenna arrays

Summary form only given: The input impedance of a typical electrically small antenna has a small resistive and a large frequency-varying reactive component. Therefore, conventional matching techniques based on passive lumped elements result in a very narrow impedance bandwidth. To overcome this issue, non-foster matching circuits, i.e. circuits that violate Foster´s reactance theorem, have been proposed, which cancel the reactive part of the antenna input impedance over a wide frequency band, thereby increasing the impedance bandwidth of the antenna. Non-foster matching circuits typically use negative impedance converters that often employ positive feedback. Therefore, they can potentially cause stability issues. In this presentation, we propose a new technique for the implementation of a non-foster matching network. In this technique, the current at the input terminals of the antenna is sensed and amplified using a precision low-noise instrumentation amplifier and used to implement a nonfoster element in series with the antenna. Compared to current transistor based implementations of non-foster matching networks, the proposed technique potentially offers a wider bandwidth and is less sensitive to component tolerances and temperature variation. Moreover, it is less susceptible to oscillation. To validate the proposed technique, it is first applied to the impedance matching of an electrically small dipole antenna operating in the transmitting mode at VHF frequencies. The input impedance of an electrically small dipole consists of a small resistance in series with a capacitor. The technique implements a negative capacitor of the size as seen looking into the antenna terminals, thereby cancelling the the reactive part of the input impedance seen by the source. Next, the same technique is used to implement a non-foster element for the coupling network of a two element biomimetic antenna array (BMAA) operating in the receiving mode. It has been previously demonstrated - hat BMAAs offer an enhanced angular resolution compared to regular antenna arrays at the cost of signal to noise ratio degradation (N. Behdad, M. AlJoumayly, and M. Li, IEEE Antennas Wireless Propagat. Lett., 361-364, 2011). However, this phase enhancement is obtained over a relatively narrow frequency range in general. To overcome this issue, non-foster elements have been employed in the coupling network of a BMAA to increase the bandwidth over which a considerable phase-enhancement factor may be obtained. Details of the design process and experimental measurement results of the proposed antennas will be presented and discussed at the symposium.