The US Dept. of Energy is developing 10-gigawatt-class power transmitters for nuclear fusion research. These generate microwaves from synchrotron radiation up to 280 GHz. LEO to geosynchronous (36,000 km) transmission would require an antenna only 268 meters in diameter, but only in a vacuum.

Counterintuitively, the small Skyhook which receives power beamed from the ground would need an antenna 693 meters in diameter, using 5.8 GHz frequency. Shorter distance, lower power, smaller Skyhook, but *bigger* antenna ! All because of atmospheric attenuation.

The size of the antennae is troubling because the Skyhook is supposed to be slowly tumbling as it orbits, rotating once every 1/2-hr or so. Such large antennae have to be pointed accurately, and need more support structure even under milligravity loads. It's a tough challenge, but I'm still feeling pretty good about it. Every week brings new ideas and more advances.