Broadband Electrically-Small VLF/LF Transmitter Realized by Rapidly Time-Varying Antenna Properties

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Slevin, Edward Matthew
Cohen, Morris B.
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Very low frequency (VLF, 3-30 kHz) and low frequency (LF, 30-300 kHz) electromagnetic waves have exceptionally long wavelengths, which make them uniquely suited to several critical applications, but also difficult to generate. These long wavelengths require any practical VLF/LF transmitter to be an electrically small antenna (ESA), which can achieve a high efficiency through the use of a resonant matching network, but is then limited to a narrow bandwidth. This work investigates a novel impedance matching technique meant to improve ESA efficiency without restricting bandwidth, particularly at VLF/LF frequencies which are in most need of improvement. In particular, by using a high-speed switch to delay the return of signals to the input, the effective length of the antenna can be increased. This provides an impedance match by shifting the resonance of the antenna to a lower frequency, in a manner that is analogous to an external matching network, but with a much wider bandwidth. Such a method is capable of achieving a wider bandwidth because its time-varying nature allows it overcome the fundamental limits native to linear time-invariant (LTI) antennas. This work begins with an investigation into the concept of an ideal fully non-reciprocal antenna, which reveals that reflections cannot be blocked indefinitely, but can be blocked strategically. This then leads to the idea of the synthetically lengthened line (SLL), in which a signal is sampled into narrow pulses that are individually blocked by a carefully-timed switch to delay their return to the input. A full theoretical framework is established for the SLL, including a formal design procedure, an exploration of the newly formed tradeoff space, and a demonstration of its ability to surpass established limits for LTI ESAs. Finally, an experimental investigation is undertaken, in which it is shown that a signal can be delayed from returning to the input, but that the improvement in antenna performance is limited if the pulses become too distorted. The work ends by providing suggestions on how to mitigate this issue.
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