The Earth-Sun L2 point is 1.5 million kilometers away. To achieve 88% transmission efficiency at 2.45 GHz would require two equal-sized antennae 18,454 meters in diameter. If transmission were at 60 GHz the antenna would only have to be 3,729 meters in diameter. However, since the power is being transmitted through the atmosphere, he chose 5.8 GHz to minimize attenuation through the atmosphere. At this frequency the antennae would have to be 11,994 meters in diameter. Researchers at JPL have previously suggested the lunar distance as the upper limit for microwave SPS, due to the huge antenna size. I think the fundamental limitation may be atmospheric attenuation at higher frequencies. They may have a good idea, but their target market is all wrong IMO.

I can choose a higher microwave frequency for power transmission at much greater distances for my ARMXER Orbital Skyhook Infrastructure. Since the energy is only transmitted through space, not through the atmosphere, a much higher frequency can be used, resulting in a much smaller antenna. In which case I think his Supersynchronous SPS is a good idea. I had previously estimated an antenna diameter of just under 3 km for a SCBPS in geosynchronous orbit. Bumping it up to 3.7 km diameter seems reasonable to guarantee exposure to sunlight 24-7, year-round, without exception, at the Earth-Sun L2 point. AFAIK, only Mark Prado of Permanent.com and myself have suggested beaming power to other spacecraft. As yet no spacecraft have been power hungry enough to justify the effort. Not until the ARMXER Orbital Skyhook anyway.

Besides, energy for transportation is the biggest impending problem on Earth. Liquid biofuels are the best solution I've found. For human space transportation, not surface transportation, the SPS might find its niche.