Editing Wireless power transfer (section)

== Field regions ==

{| class="wikitable" style="float: right; width: 50%"
|+ Wireless power technologies by range<ref name="Shinohara1" /><ref name="Sun" /><ref name="Valtchev">{{cite journal |last1=Valtchev |first1=Stanimir S. |last2=Baikova |first2=Elena N. |last3=Jorge |first3=Luis R. |title=Electromagnetic field as the wireless transporter of energy |journal=Facta Universitatis – Series: Electronics and Energetics |volume=25 |issue=3 |pages=171–181 |date=December 2012 |url=http://www.doiserbia.nb.rs/img/doi/0353-3670/2012/0353-36701203171V.pdf |doi=10.2298/FUEE1203171V |access-date=15 December 2014 |citeseerx=10.1.1.693.1938}}</ref><ref name="Ashley">{{cite web |last=Ashley |first=Steven |title=Wireless recharging: Pulling the plug on electric cars |publisher=BBC |date=20 November 2012 |url=http://www.bbc.com/future/story/20121120-pulling-the-plug-on-electric-cars |access-date=10 December 2014}}</ref>
|-
! Technology
! Range
! [[Directivity]]<ref name="Sun" />
! Frequency
! Antenna devices
! Current and/or possible future applications
|-
| Inductive coupling || Short || Low || Hz – MHz || Wire coils || Electric tooth brush and razor battery charging, induction stovetops and industrial heaters.
|-
| Resonant inductive coupling || Mid- || Low || kHz – GHz || Tuned wire coils, lumped element resonators || Charging portable devices ([[Qi (wireless power standard)|Qi]]), biomedical implants, electric vehicles, powering buses, trains, MAGLEV, [[RFID]], [[smartcard]]s.
|-
| Capacitive coupling || Short || Low || kHz – MHz || Metal plate electrodes || Charging portable devices, power routing in large-scale integrated circuits, Smartcards, biomedical implants.<ref name="Trancutaneous Capacitive Wireless Power Transfer"/><ref name="Capacitive Elements for Wireless Power Transfer to biomedical implants"/><ref name="Capacitive Wireless Power Transfer to biomedical implants"/>
|-
| Magnetodynamic coupling || Short || N.A. || Hz || Rotating magnets || Charging electric vehicles,<ref name="Ashley" /> biomedical implants.<ref name=Jiang2012>{{cite journal |title=A Low-Frequency Versatile Wireless Power Transfer Technology for Biomedical Implants |last1=Jiang |first1=Hao |last2=Zhang |first2=Junmin |last3=Lan |first3=Di |last4=Chao |first4=Kevin K. |last5=Liou |first5=Shyshenq |last6=Shahnasser |first6=Hamid |last7=Fechter |first7=Richard |last8=Hirose |first8=Shinjiro |last9=Harrison |first9=Michael| last10= Roy |first10= Shuvo |doi=10.1109/TBCAS.2012.2220763 |pmid=23893211 |journal=IEEE Transactions on Biomedical Circuits and Systems |volume=7 |issue=4 |year=2013 |pages=526–535 |s2cid=8094723}}</ref>
|-
| Microwaves || Long || High || GHz || Parabolic dishes, [[phased array]]s, [[rectenna]]s || [[Solar power satellite]], powering drone aircraft, charging wireless devices
|-
| Light waves || Long || High || ≥THz || Lasers, photocells, lenses || Charging portable devices,<ref>{{Cite news |url=http://www.eenewseurope.com/news/israeli-startup-turns-luminaires-wireless-power-chargers#new_tab |title=Israeli startup turns luminaires into wireless power chargers |date=2018-01-15 |work=eeNews Europe |access-date=2018-03-12}}</ref> powering drone aircraft.
|}

[[electric field|Electric]] and [[magnetic field]]s are created by [[charged particle]]s in matter such as [[electron]]s. A stationary charge creates an [[electrostatic field]] in the space around it. A steady [[electric current|current]] of charges ([[direct current]], DC) creates a static magnetic field around it. These fields contain [[energy]], but cannot carry [[Electric power|power]] because they are static. However time-varying fields can carry power.<ref name="Coleman">{{cite book |last1=Coleman |first1=Christopher |title=An Introduction to Radio Frequency Engineerin |publisher=Cambridge University Press |date=2004 |pages=1–3 |url=https://books.google.com/books?id=IT_mb5hXAzkC&pg=PA2 |isbn=978-1139452304}}</ref> Accelerating electric charges, such as are found in an [[alternating current]] (AC) of electrons in a wire, create time-varying electric and magnetic fields in the space around them.  These fields can exert oscillating forces on the electrons in a receiving "antenna", causing them to move back and forth.  These represent alternating current which can be used to power a load.

The oscillating electric and magnetic fields surrounding moving electric charges in an antenna device can be divided into two regions, depending on distance ''D''<sub>range</sub> from the antenna.<ref name="Shinohara1" /><ref name="Sun" /><ref name="Sazonov" /><ref name="Valtchev" /><ref name="Rajakaruna">{{cite book |last1=Rajakaruna |first1=Sumedha |last2=Shahnia |first2=Farhad |last3=Ghosh |first3=Arindam |title=Plug in Electric Vehicles in Smart Grids: Integration Techniques |publisher=Springer |date=2014 |pages=34–36 |url=https://books.google.com/books?id=VYWhBQAAQBAJ&pg=PA35 |isbn=978-9812872999}}</ref><ref name="Agbinya">{{cite book |last1=Agbinya |first1=Johnson I. |title=Wireless Power Transfer |publisher=River Publishers |date=2012 |pages=1–2 |url=https://books.google.com/books?id=zDPqqBJ76ZAC&pg=PA1 |isbn=978-8792329233}}</ref>
<ref name="Agbinya1" >{{cite book |url=https://books.google.com/books?id=zDPqqBJ76ZAC&pg=PA126 |last=Agbinya |year=2012 |title=Wireless Power Transfer |pages=126–129 |publisher=River Publishers |isbn=9788792329233}}</ref> The boundary between the regions is somewhat vaguely defined.<ref name="Sun" /> The fields have different characteristics in these regions, and different technologies are used for transferring power:

* ''Near-field'' or ''nonradiative'' region: This means the area within about 1 [[wavelength]] (''λ'') of the antenna.<ref name="Shinohara1" /><ref name="Rajakaruna" /><ref name="Agbinya" /> In this region the oscillating [[electric field|electric]] and [[magnetic field]]s are separate<ref name="Sazonov" /> and power can be transferred via electric fields by [[capacitive coupling]] ([[electrostatic induction]]) between metal electrodes,<ref name="ECN2011"/><ref name="Trancutaneous Capacitive Wireless Power Transfer"/><ref name="Capacitive Elements for Wireless Power Transfer to biomedical implants"/><ref name="Capacitive Wireless Power Transfer to biomedical implants"/> or via magnetic fields by [[inductive coupling]] ([[electromagnetic induction]]) between coils of wire.<ref name="Gopinath" /><ref name="Sun" /><ref name="Sazonov" /><ref name="Valtchev" /> These fields are not ''radiative'',<ref name="Agbinya" /> meaning the energy stays within a short distance of the transmitter.<ref name="Umenei">{{cite web |last1=Umenei |first1=A. E. |title=Understanding Low Frequency Non-radiative Power Transfer |publisher=Fulton Innovation, Inc. |date=June 2011 |url=http://www.wirelesspowerconsortium.com/data/downloadables/6/8/9/understanding-low-frequency-non-radiative-power-transfer-8_8_11.pdf |access-date=3 January 2015}}</ref> If there is no receiving device or absorbing material within their limited range to "couple" to, no power leaves the transmitter.<ref name="Umenei" /> The range of these fields is short, and depends on the size and shape of the "antenna" devices, which are usually coils of wire. The fields, and thus the power transmitted, decrease [[exponential decay|exponentially]] with distance,<ref name="Rajakaruna" /><ref name="Agbinya1" /><ref name="Schantz">{{cite book |doi=10.1109/APS.2007.4396365 |pages=3792–3795 |year=2007 |last1=Schantz |first1=Hans G. |title=2007 IEEE Antennas and Propagation Society International Symposium |chapter=A real-time location system using near-field electromagnetic ranging |isbn=978-1-4244-0877-1 |s2cid=36079234 |url=http://q-track.com/wp-content/uploads/phocadownload/IEEE-APS-2007-.pdf |access-date=2 January 2015 |archive-date=3 June 2016 |archive-url=https://web.archive.org/web/20160603125651/http://q-track.com/wp-content/uploads/phocadownload/IEEE-APS-2007-.pdf |url-status=dead}}</ref> so if the distance between the two "antennas" ''D''<sub>range</sub> is much larger than the diameter of the "antennas" ''D''<sub>ant</sub> very little power will be received. Therefore, these techniques cannot be used for long range power transmission. [[Resonance]], such as [[resonant inductive coupling]], can increase the [[Coupling (electronics)|coupling]] between the antennas greatly, allowing efficient transmission at somewhat greater distances,<ref name="Shinohara1" /><ref name="Sazonov" /><ref name="Valtchev" /><ref name="Rajakaruna" /><ref name="Karalis">{{cite journal |last1=Karalis |first1=Aristeidis |last2=Joannopoulos |first2=J. D. |last3=Soljačić |first3=Marin |title=Efficient wireless non-radiative mid-range energy transfer |journal=Annals of Physics |volume=323 |issue=1 |pages=34–48 |date=January 2008 |doi=10.1016/j.aop.2007.04.017 |arxiv=physics/0611063 |bibcode=2008AnPhy.323...34K |s2cid=1887505}}</ref><ref name="Wong" /> although the fields still decrease exponentially. Therefore the range of near-field devices is conventionally divided into two categories: 
** ''Short range'': up to about one antenna diameter: ''D''<sub>range</sub> ≤ ''D''<sub>ant</sub>.<ref name="Umenei" /><ref name="Karalis" /><ref name="Baarman">{{cite web |last1=Baarman |first1=David W. |last2=Schwannecke |first2=Joshua |title=White paper: Understanding Wireless Power |publisher=Fulton Innovation |date=December 2009 |url=http://ecoupled.com/system/files/pdf/eCoupled_UnderstandingWirelessPower_WhitePaper.pdf |archive-url=https://web.archive.org/web/20110409115933/http://ecoupled.com/system/files/pdf/eCoupled_UnderstandingWirelessPower_WhitePaper.pdf |url-status=dead |archive-date=2011-04-09 |access-date=3 January 2015}}</ref> This is the range over which ordinary nonresonant capacitive or inductive coupling can transfer practical amounts of power.
** ''Mid-range'': up to 10 times the antenna diameter:  ''D''<sub>range</sub> ≤ 10 ''D''<sub>ant</sub>.<ref name="Karalis" /><ref name="Wong">{{cite web |last=Wong |first=Elvin |title=Seminar: A Review on Technologies for Wireless Electricity |website=HKPC |publisher=[[Hong Kong Electronic Industries Association|The Hong Kong Electronic Industries Association Ltd.]] |date=2013 |url=http://www.hkeia.org/Chi/hkeia_activities/2013/20130327_seminar_c.html |access-date=3 January 2015}}</ref><ref name="Baarman" /><ref name="Agbinya3" >"''...strongly coupled magnetic resonance can work over the mid-range distance, defined as several times the resonator size.''"  [https://books.google.com/books?id=zDPqqBJ76ZAC&pg=PA126  Agbinya (2012) ''Wireless Power Transfer'', p. 40]</ref> This is the range over which resonant capacitive or inductive coupling can transfer practical amounts of power.
* ''Far-field'' or ''radiative'' region: Beyond about 1 wavelength (''λ'') of the antenna, the electric and magnetic fields are perpendicular to each other and propagate as an [[electromagnetic wave]]; examples are [[radio wave]]s, [[microwave]]s, or [[light wave]]s.<ref name="Shinohara1" /><ref name="Valtchev" /><ref name="Rajakaruna" /> This part of the energy is ''radiative'',<ref name="Agbinya" /> meaning it leaves the antenna whether or not there is a receiver to absorb it. The portion of energy which does not strike the receiving antenna is dissipated and lost to the system. The amount of power emitted as electromagnetic waves by an antenna depends on the ratio of the antenna's size ''D''<sub>ant</sub> to the wavelength of the waves ''λ'',<ref name="Smith">{{cite book |last1=Smith |first1=Glenn S. |title=An Introduction to Classical Electromagnetic Radiation |publisher=Cambridge University Press |date=1997 |page=474 |url=https://books.google.com/books?id=m8RzbqS772IC&pg=PA474 |isbn=978-0521586986}}</ref> which is determined by the frequency: ''λ'' = ''c/f''.  At low frequencies ''f'' where the antenna is much smaller than the size of the waves, ''D''<sub>ant</sub> << ''λ'', very little power is radiated. Therefore near-field devices, which use lower frequencies, radiate almost none of their energy as electromagnetic radiation. Antennas about the same size as the wavelength ''D''<sub>ant</sub> ≈ ''λ'' such as [[monopole antenna|monopole]] or [[dipole antenna]]s, radiate power efficiently, but the electromagnetic waves are radiated in all directions ([[Omnidirectional antenna|omnidirectionally]]), so if the receiving antenna is far away, only a small amount of the radiation will hit it.<ref name="Agbinya" /><ref name="Karalis" /> Therefore, these can be used for short range, inefficient power transmission but not for long range transmission.<ref name="Tan">{{cite book |last1=Tan |first1=Yen Kheng |title=Energy Harvesting Autonomous Sensor Systems: Design, Analysis, and Practical Implementation |publisher=CRC Press |date=2013 |pages=181–182 |url=https://books.google.com/books?id=UCBnHOg8Je0C&pg=PA181 |isbn=978-1439892732}}</ref> However, unlike fields, electromagnetic radiation can be focused by [[Reflection (physics)|reflection]] or [[refraction]] into beams. By using a [[high-gain antenna]] or [[optical system]] which concentrates the radiation into a narrow beam aimed at the receiver, it can be used for long range power transmission.<ref name="Karalis" /><ref name="Tan" /> From the [[Angular resolution#The Rayleigh criterion|Rayleigh criterion]], to produce the narrow beams necessary to focus a significant amount of the energy on a distant receiver, an antenna must be much larger than the wavelength of the waves used: ''D''<sub>ant</sub> >> ''λ'' = ''c/f''.<ref name="Feynman">{{cite book |last1=Feynman |first1=Richard Phillips |last2=Leighton |first2=Robert B. |last3=Sands |first3=Matthew |title=The Feynman Lectures on Physics Vol. 1: Mainly Mechanics, Radiation, and Heat |publisher=California Institute of Technology |date=1963 |pages=30.6–30.7 |url=https://books.google.com/books?id=bDF-uoUmttUC&pg=SA30-PA7 |isbn=978-0465024933}}</ref> Practical ''beam power'' devices require wavelengths in the centimeter region or lower, corresponding to frequencies above 1&nbsp;GHz, in the [[microwave]] range or above.<ref name="Shinohara1" />