A rectenna is a rectifying antenna—a special type of antenna that is used to convert electromagnetic energy into direct current (DC) electricity. They are used in wireless power transmission systems that transmit power by radio waves. A simple rectenna element consists of a dipole antenna with an RF diode connected across the dipole elements. The diode rectifies the AC current induced in the antenna by the microwaves, to produce DC power, which powers a load connected across the diode. Schottky diodes are usually used because they have the lowest voltage drop and highest speed and therefore have the lowest power losses due to conduction and switching. Large rectennas consist of an array of many such dipole elements.
The invention of the rectenna in the 1960s made long distance wireless power transmission feasible. The rectenna was invented in 1964 and patented in 1969 by US electrical engineer William C. Brown, who demonstrated it with a model helicopter powered by microwaves transmitted from the ground, received by an attached rectenna. Since the 1970s, one of the major motivations for rectenna research has been to develop a receiving antenna for proposed solar power satellites, which would harvest energy from sunlight in space with solar cells and beam it down to Earth as microwaves to huge rectenna arrays. A proposed military application is to power drone reconnaissance aircraft with microwaves beamed from the ground, allowing them to stay aloft for long periods. In recent years, interest has turned to using rectennas as power sources for small wireless microelectronic devices. The largest current use of rectennas is in RFID tags, proximity cards and contactless smart cards, which contain an integrated circuit (IC) which is powered by a small rectenna element. When the device is brought near an electronic reader unit, radio waves from the reader are received by the rectenna, powering up the IC, which transmits its data back to the reader.
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.
However this example uses only one antenna having a limited capture area. A Rectenna uses multiple antennas spread over a wide area to capture more energy.
Researchers are experimenting with the use of rectennas to power sensors in remote areas.
Similar devices, scaled down to the proportions used in nanotechnology, can be used to convert light directly into electricity. This type of device is called an optical rectenna or nantenna. Theoretically, high efficiencies can be maintained as the device shrinks, but efficiency has so far been limited. The University of Missouri previously reported on work to develop low-cost, high-efficiency nantennas (optical-frequency rectennas). Other prototype devices were investigated in a collaboration between the University of Connecticut and Penn State Altoona using a grant from the National Science Foundation. With the use of atomic layer deposition it has been suggested that conversion efficiencies of solar energy to electricity higher than 70% could eventually be achieved.
Challenges to successful nantenna technology include fabricating an antenna small enough to couple optical wavelengths, and creating an ultrafast diode capable of rectifying the high frequency oscillations.
Carbon nanotube optical rectenna
In 2015, researchers at Georgia Institute of Technology fabricated an optical rectenna using arrays of 2 million multiwall carbon nanotubes (MWCNT) per cm2 coupled to nanoscale rectifying diodes. The MWCNT, which act as optical antennae due to their favorably small dimensions, are coated in aluminum oxide and capped with a metallic top layer. This combination of MWCNT, oxide, metal is claimed to be the world's fastest metal-insulator-metal (MIM) tunneling diode, capable of rectifying optical frequencies. Individual junctions were reported to have a capacitance of only 1.7 attofarads, with switching time on the order of 1 femtosecond.
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