Wideband VLF/LF Transmission from an Electrically-Small Antenna by Means of Time-Varying Non-Reciprocity via High-Speed Switches

We present a novel approach to transmitting wideband signals from electrically-small antennas (ESAs), with a focus on low frequencies where ESA limitations are most severe. Very low frequency (VLF) and low frequency (LF) signals have exceptionally long wavelengths, which make them well-suited to several critical applications but also difficult to generate. Conventional VLF/LF transmitters employ a resonant matching network that can achieve high efficiency, but only within a fundamentally narrow bandwidth. Here, we instead offer a time-varying matching technique that removes the need for resonance entirely. By actively manipulating reflections from the end of the antenna via highspeed switches, the antenna becomes non-reciprocal and is therefore matched to its generator. Moreover, because the matching occurs in the time-domain, this solution works over a very broad (orders of magnitude) frequency band, and therefore does not limit the system bandwidth. Numerical modeling of the transmitter confirms the theory of operation and suggests an improved performance over conventional antennas. In addition, experimental results demonstrate the ability to achieve non-reciprocity with high-speed switches, and indicate higher radiated power from a time-varying electrically-small VLF/LF transmitter prototype.