Systems and methods for efficient coupling to low-loss eigenmodes of a spherical waveguide bounded by the Earth's surface and its ionosphere from multiple locations within the waveguide are disclosed. One or more eigenmodes of the Earth-ionosphere waveguide may be computed based on a mathematical model incorporating electrical properties of the terrestrial surface and plasma physics of the ionospheric layer. Multiple wave guide couplers, each including an array of electrically-connected waveguide-coupling elements, may be configured at different geographic locations for coupling to the one or more eigenmodes. By adjusting relative phases and/or amplitudes of the waveguide couplers, as well as frequencies, power may be coupled into standing waves of the Earth-ionosphere waveguide. The adjustments may also account or compensate for diurnal variations of ionosphere by steering power nodes and nulls of the standing waves away from regions of potential ohmic losses.