Rectenna

A rectenna is a rectifying antenna — a special type of receiving antenna that is used for converting 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 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.[1] 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[2] 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.[3] 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.[4] 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 headphones. 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.[5]

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.[6]

RF rectennas are used for several forms of wireless power transfer. In the microwave range, experimental devices have reached a power conversion efficiency of 85-90%.[7]

Optical rectennas
Main article: Optical rectenna

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.[8][9] 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).[10] 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.[11] 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.

The creation of successful nantenna technology has two major complicating factors:

1. Fabricating an antenna small enough to couple optical wavelengths.

2. Creating an ultra-fast diode capable of rectifying the high frequency oscillations.

Below a are a few examples of potential paths to creating diodes that would be fast enough to rectify optical and near-optical radiation.

Geometric Diodes

A promising path towards creating these ultrafast diodes has been in the form of "geometric diodes".[12] Graphene geometric diodes have been reported to rectify terahertz radiation.[13] In April 2020 geometric diodes were reported in silicon nanowires.[14] The wires were shown experimentally to rectify up to 40 GHz, but that was instrument limited, and theoretically could rectify signals in the THz region as well.

Carbon Nanotube Metal-Insulator-Metal Diodes

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) tunnel 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.[8]


References
  1. Guler, Ulkuhan; Sendi, Mohammad S.E.; Ghovanloo, Maysam (2017). "A dual-mode passive rectifier for wide-range input power flow". 2017 IEEE 60th International Midwest Symposium on Circuits and Systems (MWSCAS). pp. 1376–1379. doi:10.1109/MWSCAS.2017.8053188. ISBN 978-1-5090-6389-5.
  2. US 3434678 Microwave to DC Converter William C. Brown, et al, filed May 5, 1965, granted March 25, 1969
  3. "William C. Brown". Project #07-1726: Cutting the Cord. 2007-2008 Internet Science & Technology Fair, Mainland High School. 2012. Retrieved 2012-03-30.
  4. Torrey, Lee (Jul 10, 1980). "A trap to harness the sun". New Scientist. 87 (1209): 124–127. ISSN 0262-4079. Retrieved 2012-03-30.
  5. "76.09 — Radio transmitter lights antenna bulb".
  6. "Over to you: Mythical electricity?". The Daily Telegraph. 2004-11-24. Retrieved 2009-06-25.
  7. Zhang, J (2000). Rectennas for RF wireless energy harvesting (PhD Thesis).
  8. Asha Sharma, Virendra Singh, Thomas L. Bougher, Baratunde A. Cola (9 October 2015). "A carbon nanotube optical rectenna". Nature Nanotechnology. 10 (12): 1027–1032. Bibcode:2015NatNa..10.1027S. doi:10.1038/nnano.2015.220. PMID 26414198.CS1 maint: uses authors parameter (link)
  9. Patent application WO 2014063149 relates
  10. "New solar technology could break photovoltaic limits" (Press release). University of Missouri. May 16, 2011.
  11. Colin Poitras (February 4, 2013). "UConn Professor's Patented Technique Key to New Solar Power Technology" (Press release).
  12. Zhu, Z (2013). Rectenna Solar Cells. New York: Springer. pp. 209–227.
  13. Zhu, Zixu; Joshi, Saumil; Grover, Sachit; Moddel, Garret (2013-04-15). "Graphene geometric diodes for terahertz rectennas". Journal of Physics D: Applied Physics. 46 (18): 185101. doi:10.1088/0022-3727/46/18/185101. ISSN 0022-3727.
  14. Custer, James P.; Low, Jeremy D.; Hill, David J.; Teitsworth, Taylor S.; Christesen, Joseph D.; McKinney, Collin J.; McBride, James R.; Brooke, Martin A.; Warren, Scott C.; Cahoon, James F. (2020-04-10). "Ratcheting quasi-ballistic electrons in silicon geometric diodes at room temperature". Science. 368 (6487): 177–180. doi:10.1126/science.aay8663. ISSN 0036-8075. PMID 32273466.
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