Unwired energy questions asked, answered

The “Unwired Energy” Update item (Physics Today January 2007, page 26 ) reports a wireless energy-transmission system working over a few meters, proposed by Marin Soljačićc, Aristeidis Karalis, and John Joannopoulos of MIT.

The item concludes with the statement that “Soljačić and his MIT colleagues are now working on demonstrating the technology in practice.” I strongly recommend that they consult with the MIT Radio Society and the Federal Communications Commission (FCC) before testing anything, since simple calculations show that their system would be a terrible and illegal electromagnetic polluter.

The system proposed by the researchers uses conducting wire rings that are series-resonated with capacitors and that operate at a frequency of 6.4 MHz. Practical examples of these resonated rings for 7.0 MHz are sold by various vendors.

The proposed system provides 4 watts of output from the receiving ring, at a cost of 11 W dissipated as heat in the transmitting ring and 1.5 W of power radiated from the system. Simple calculation shows that the 1.5 W exceeds the FCC’s radiated-emission power limit by a factor of 800 000.

As most amateur radio operators know, 1.5 W of radiated power at 7.0 MHz is enough for worldwide radio communications under good conditions, and indeed some of the resonated rings available for purchase are marketed as radio antennas.

The 3- to 30-MHz frequency range is intensively used for long-distance radio communications and broadcasting even today, because it is the only channel that provides long-distance communications with no transmission infrastructure at all.

I consider it deeply unwise to pollute this unique channel for the sake of short-range systems.