Broadband Electrically Small VLF/LF Transmitter via Time-Varying Antenna Properties

We describe a new approach to transmitting broadband signals from electrically small antennas (ESAs), with a focus on lower frequencies where ESA limitations are most pronounced. ESAs traditionally suffer from a stringent tradeoff between bandwidth and efficiency that is caused by a high reactive input impedance. Conventional resonant-matching networks can reduce this impedance to improve efficiency, but at the cost of having a narrow operational bandwidth and often a large physical structure. In this article, we propose a time-varying matching technique that removes the need for resonant matching entirely. Our concept is to rapidly vary the conductivity of the antenna in order to prevent reflections from returning to the antenna feed, effectively making the antenna nonreciprocal so that it appears infinitely long and is therefore matched to the characteristic impedance of the surrounding transmission line. An ESA with such a matching scheme would be able to radiate over a wider bandwidth since the matching occurs in the time domain. This technique would be particularly useful for generating very-low-frequency (VLF)/low-frequency (LF) waves, which are uniquely suited for many applications such as long-range and underwater communication. We present the theory of operation for such an antenna as well as results from numerical modeling, which suggests improved performance over that of a conventional ESA.