It was with much interest that I read Nancy Friedrich's editorial on wireless power transmission ("Energy-Harvesting Solution Powers Wireless Devices," Breaking News, January 2007 Microwaves & RF Notepad.) This time they did it at 900 MHz! This is a problem that Mr. Tesla already tried to solve. It comes back again and again.
The fact that Ms. Friedrich wrote, that you can get ~1 mW over one meter from a 10-W transmitter, has been known since 1900; possibly even Mr. Hertz was able to prove that electromagnetic waves in free space propagate with inverse-square attenuation. Later, we learned that close to the antenna, field intensity drops even faster, with inverse-cube proportionality.
So, this is all a very unexpected discovery. Now, with a 10-W transmitter at 900 MHz, you get 1 mW DC over that one meter (if you find a very good RF rectifier). Such power can charge a 100µF capacitor over a minute, and blink an LED several times.
I do not see how to feed electronic gadgets which need up to 1 W. Yes, there is an option: connect them directly to the power supply feeding the transmitter. That would not be wireless, I suppose, but efficient enough.
And we failed to address the health effects of such multiple generators of tens of watts at 900 MHz. But everybody is very afraid of cell-phone radiation and brain cancer. A considerable amount of money has been and is being spent to do more research. Possibly, this will encourage more money to pour in for research purposes. (Wasn't that the main purpose?)
Thank you for an interesting read.
Jiri Polivka
Spacek Labs, Inc.
Nancy Friedrich responds: Thank you, Jiri. I'm glad you enjoyed the article. You made some great points.