Rectenna
Receiving antenna that converts radio waves into DC electricity.
A rectenna, short for rectifying antenna, is a type of receiving antenna designed to turn electromagnetic energy into direct current (DC) electricity. These devices are a key part of wireless power transmission systems, which send power via radio waves. A basic rectenna element pairs a dipole antenna with a diode connected across its two arms. The diode rectifies the alternating current (AC) that microwaves induce in the antenna, producing DC power to run a load. Schottky diodes are the standard choice here because they have the lowest voltage drop and fastest switching speed, which minimizes power losses from conduction and switching. Larger rectennas are built as arrays of many such power-receiving elements, like multiple dipole antennas.
The rectenna was invented in 1964 and patented in 1969 by American electrical engineer William C. Brown. His invention made long-distance wireless power transmission practical; he demonstrated it by powering a model helicopter with microwaves beamed from the ground, which were received by a rectenna attached to the helicopter. Since the 1970s, a major driver of rectenna research has been the concept of solar power satellites—spacecraft that would collect sunlight with solar cells and beam the energy to Earth as microwaves, where huge rectenna arrays would capture it. A proposed military use involves powering drone reconnaissance aircraft from the ground with microwaves, letting them stay airborne for extended periods. More recently, interest has shifted to using rectennas as power sources for small wireless microelectronic devices. The largest current application is in RFID tags, proximity cards, and contactless smart cards. These contain an integrated circuit (IC) powered by a small rectenna element; when brought near an electronic reader, radio waves from the reader energize the IC, which then transmits data back.
The simplest crystal radio receiver, with its antenna and demodulating diode, is essentially a rectenna, though it discards the DC component before sending the signal to headphones. People living near strong radio transmitters have sometimes found that a long receiving antenna could provide enough power to light a light bulb. That example uses only one antenna with a limited capture area, whereas a rectenna array spreads multiple antennas over a wide area to collect more energy.
- Inventor
- William C. Brown
- Year invented
- 1964
- Year patented
- 1969
- Record efficiency
- 90.6% at 2.45 GHz
- Efficiency at 5.82 ghz
- about 82%
Lore & Background
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 and received by an attached rectenna. Since the 1970s, a major motivation for rectenna research has been to develop a receiving antenna for proposed solar power satellites, which would harvest energy from sunlight in space 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 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.
Optical rectennas, scaled down to nanotechnology proportions, can in principle convert light directly into electricity. However, to date efficiency has been limited, and there has not been convincing evidence that rectification has been achieved at optical frequencies. Two major complicating factors are fabricating an antenna small enough to couple optical wavelengths and creating an ultra-fast diode capable of rectifying oscillations at about 500 THz. Promising paths include geometric diodes; graphene geometric diodes have been reported to rectify terahertz radiation, and silicon nanowire geometric diodes have been shown experimentally to rectify up to 40 GHz.
Reader's Guide
The rectenna made long-distance wireless power transmission feasible, a concept that had been impractical before its invention in the 1960s. Its significance lies in enabling both large-scale and micro-scale applications. On the large scale, it has driven research into solar power satellites that would beam microwave energy to Earth, as well as military applications for powering drones from the ground. On the small scale, rectennas have become ubiquitous in everyday technology: they power RFID tags, proximity cards, and contactless smart cards, allowing these devices to operate without batteries by harvesting energy from reader signals. The highest demonstrated conversion efficiency for a rectenna is 90.6% at 2.45 GHz, with about 82% at 5.82 GHz. Researchers continue to explore rectennas for powering sensors in remote areas and IoT networks, and optical rectennas remain a theoretical path toward high-efficiency solar energy conversion, though practical challenges at optical frequencies have not yet been overcome.
Did You Know?
- The simplest crystal radio receiver is actually a rectenna, though it discards the DC component.
- Optical rectennas have not yet convincingly demonstrated rectification at optical frequencies.
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