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Title: Broadband Non-Resonant Electrically-Small VLF/LF Transmitter Realized by Rapidly Time-Varying Antenna Properties
Committee:
Dr. Cohen, Advisor
Dr. Peterson, Chair
Dr. Saeedifard
Abstract: The objective of the proposed research is to numerically and experimentally validate the operation of a novel time-domain matching scheme for electrically-small antennas (ESAs) in the Very Low Frequency (VLF, 3-30 kHz) and Low Frequency (LF, 30-300 kHz) bands. VLF/LF signals have exceptionally long wavelengths, which make them well-suited to critical applications such as underwater communication, but also difficult to generate from an ESA. Conventional solutions utilize a resonant matching network to enhance ESA efficiency, but at the cost of a narrow bandwidth. Any linear time-invariant antenna is in fact fundamentally constrained by the Chu-Harrington limit, which places an upper bound on operational bandwidth. Several antenna solutions have managed to perform beyond the Chu limit by operating in a time-varying manner, but all such approaches have relied upon resonance and are thus still band-limited. Here, a new time-varying antenna is proposed in which impedance matching is performed in the time-domain. By breaking up a VLF/LF signal into narrow pulses, any reflected current can be prevented from returning to the antenna feed by rapidly varying the antenna’s conductivity over time. In doing so, the antenna becomes non-reciprocal and impedance matched to its source, but without any restriction on bandwidth since the matching occurs in the time-domain. Preliminary simulations and experiments confirm the ability to suppress reflections in this manner and suggest an improvement in radiated power. However, further modeling must be done to examine the far-field behavior of the antenna and quantify its performance. Moreover, wireless transmission measurements must be taken to prove that the antenna generates fields resembling that of an infinitesimal electric monopole.
