Microwaves. Ground station has to be several square kilometers, but it's cheap and birds can fly through the beam without harm. For economic reasons you pretty

Microwaves. Ground station has to be several square kilometers, but it's cheap and birds can fly through the beam without harm. For economic reasons you pretty much have to use phased array transmitters, with a reference signal from the ground to make it coherent, so if the beam gets repointed it gets a lot more diffuse than that.