Rectenna

A rectenna is also a scanning and tracking array in the Star Wars universe.

A rectenna is a rectifying antenna, a special type of antenna that is used to directly convert microwave energy into DC electricity. Its elements are usually arranged in a multi element phased array with a mesh pattern reflector element to make it directional. Rectennas are being developed as the receiving antennas in proposed microwave power transmission schemes, which transmit electric power to distant locations using microwaves. Rectennas are used in RFID tags; the energy to power the computer chip in the tag is received from the querying radio signal by a small rectenna. One possible future application is a receiving antenna for solar power satellites.

A simple rectenna element consists of a dipole antenna with a Schottky diode placed across the dipole elements. The diode rectifies the AC current induced in the antenna by the microwaves, to produce DC power. Schottky diodes are used because they have the lowest voltage drop and highest speed and therefore waste the least amount of power due to conduction and switching. Large rectennas consist of an array of many such dipole elements.

Rectennas are highly efficient at converting microwave energy to electricity.^[1] In laboratory environments, efficiencies of over 85%^[2] have been observed. Some experimentation has been done with inverse rectennas, converting electricity into microwave energy, but efficiencies are much lower—only in the area of 1%.^[3]

Radio frequency rectennas

The simplest crystal radio receiver, employing an antenna and a demodulating diode (rectifier), is actually a rectenna - although it discards the DC component before sending the signal to the earphones. People living near strong radio transmitters would occasionally discover that with a long receiving antenna, they could get enough electric power to light a light bulb. Researchers are experimenting the use of rectennae to power sensors in remote areas.^[4]

RF rectennas are used for several forms of wireless energy transfer.

Optical rectennas

Main article: nantenna

It has been theorized that similar devices, scaled down to the proportions used in nanotechnology, could be used to convert light into electricity at much greater efficiencies than what is currently possible with solar cells. This type of device is called an optical rectenna. Theoretically, high efficiencies can be maintained as the device shrinks, but experiments funded by the United States National Renewable Energy Laboratory have so far only obtained roughly 1% efficiency while using infrared light. Nevertheless, Missouri University recently reported on work to develop low-cost, high-efficiency nantennas (optical-frequency rectennas).

External links

• William C. Brown's Distinguished Career

References

1. ^ SPS Technical Issues
2. ^ NREL: Photovoltaic Technologies Beyond the Horizon: Optical Rectenna Solar CellPDF (981 KB)
3. ^ Inverse Rectennas for Two-Way Wireless Power Transmission
4. ^ "Over to you: Mythical electricity?". The Daily Telegraph. 2004-11-24. http://www.telegraph.co.uk/scienceandtechnology/technology/3336114/Over-to-you-Mythical-electricity.html. Retrieved 2009-06-25.