A Capacitively-Coupled Single-Wire Power Tether for Aerial Platforms

Single-wire earth-return (SWER) power transfer has a long history in utility transmission applications, but recent works have emerged from the wireless power transfer community. At high frequencies, an earth return is achievable without physical contact, only capacitance to the surrounding environment is necessary. This paper applies that concept to form a SWER system for tethered aerial platforms, e.g. balloons, drones, or lighter-than-air renewables. The SWER concept is especially attractive in such applications because the power tether can be simplified to a single conducting cable. The capacitive displacement current return to earth is facilitated by a quarter-wavelength helical resonator. The theoretical model of the helical resonator, specifically tailored for system design purposes, is derived through the application of transmission line theory. Using the model, the optimal dimensions of a resonator with a large voltage gain can be selected. Analysis of the tether reveals how its impedance can be leveraged to boost the tether voltage as well. These models are combined to form an equivalent circuit for the overall system, which is confirmed with a laboratory prototype. Measurements on a horizontal (terrestrial) 70 m tether and vertical (aerial) 30 - 50 m tether demonstrate 500W power throughput with 93% and 92% efficiency, respectively.