According to one embodiment, a rotor head includes a yoke, a torque-splitter assembly, and a joint assembly. The semiconductor devices are stacked in an active surface-to-backside configuration. The radiation conductor has a bottom portion and a top portion that is distant from the ground surface and is wider than the bottom portion. Example embodiments include cog arrangements, which include a plurality of projecting teeth. By changing speed change gear set, the rotary speed of the driving shaft is changeable.