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Cutting the last wire

Posted: 17 Apr 2006 ?? ?Print Version ?Bookmark and Share

Keywords:reno rossetti? fairchild? wireless? power distribution? power transmission?

Rossetti: Power harvesting is a new concept from an industrial standpoint, but it is gaining acceptance in technologists' minds.

Today's much-touted wireless gadgets could be more properly called wireless "communication" devices because at some point, a wire is still needed to convey power from the wall to the appliance. On the other hand, hundreds of kilowatts of power have been successfully transmitted wirelessly across long distances for quite some time. While the focus of wireless power transmission has so far been in megawatts, such as in solar energy harvesting, or milliwatts as in RFID, the time should be ripe for porting this technology to portable wireless applications.

The idea of beaming out power is not new. In 1899, Nikola Tesla's wireless power-transmission experiments at Wardenclyffe resulted in lighting lamps filled with gas over 25 miles away without using wires.

The beaming of RF power by means of a directive antenna is a relatively lossless process that yields efficiencies of about 85 percent, rivaling that of any good switching regulator. The transmission of energy between two antennas is dictated by the size of the antennas, the wavelength of the RF wave and the distance between the two antennas. The intensity of the beam falls out of the equation if you assume no transmission losses for simplicity. Calling Dt the diameter of the transmitting antenna, Dr the diameter of the receiving antenna, the RF wavelength ( = 1/f with f as the beat of the RF), H the distance and k a proportionality constant (typically, 1.2), we have: DtDr= 2kH

Naturally, concerns over power density should also be accounted for. For example, the FDA enforces 5mW of microwave radiation per square centimeter at a distance of 2 inches from the microwave oven surface. Such safety requirements may set a limit to the miniaturization of the antennas beyond the dictates of the equation.

The need for untethered power is clear. The technology for implementation is mostly there, but it needs be ported from scales of megawatts and milliwatts into the realm of portable computing, consumer and communications devices. By eliminating the signal wire, wireless data transmission makes the power wire stand out and alone in all of its ugliness. For example, since today's flat-panel TVs can be hung on a wall, it would be nice if they could get power without having to access a plug that has to be placed high on the wall.

Beaming power is the next logical step following the harvesting of stray power available in the surrounding environment. Power harvesting itself is a new concept from an industrial standpoint, but it is gaining acceptance in technologists' minds.

One can argue that at the current state of affairs, harvesting can be better described as "scavenging," since it relies on hunting and gathering whatever stray energy is floating in the air. While current power-harvesting technology focuses on the receiving side of the power chain, beaming power, harvesting at an industrial scale starts at the source and turns the paradigm on its head?from "power is where you can find it" to "power is where you put it?wirelessly, that is."

- Reno Rossetti
Director of Integrated Circuits Group Strategy
Fairchild Semiconductor




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