View source for Physics:Wireless power transfer

{{short description|Transmission of electrical energy without wires as a physical link}}
[[File:Inductive charging of LG smartphone (2).jpg|thumb|[[Inductive charging|Inductive charging pad]] for a smartphone as an example of near-field wireless transfer.  When the phone is set on the pad, a coil in the pad creates a magnetic field<ref name="Hoffman">The pad senses when a phone is on it and turns on the field.  The pad uses a small amount of energy when not in use, however in modern wireless systems this "off" power is very small compared to the power used when charging. {{cite web
  | last = Hoffman
  | first =  Chris  
  | title = How Does Wireless Charging Work?
  | website = How-To Geek
  | publisher = How-To Geek LLC
  | date = September 15, 2017
  | url = https://www.howtogeek.com/162483/no-more-cables-how-wireless-charging-works-and-how-you-can-use-it-today/
  | access-date = January 11, 2018}}</ref>  which induces a current in another coil, in the phone, charging its battery.]]

'''Wireless power transfer''' ('''WPT'''), '''wireless power transmission''', '''wireless energy transmission''' ('''WET'''), or [[Physics:Electromagnetism|electromagnetic]] power transfer is the transmission of [[Physics:Electrical energy|electrical energy]] without [[Engineering:Wire|wire]]s as a physical link. In a wireless power transmission system, an electrically powered transmitter device generates a time-varying [[Physics:Electromagnetic field|electromagnetic field]] that transmits power across space to a receiver device; the receiver device extracts power from the field and supplies it to an [[Engineering:Electrical load|electrical load]]. The technology of wireless power transmission can eliminate the use of the wires and batteries, thereby increasing the mobility, convenience, and safety of an electronic device for all users.<ref>{{cite book |doi=10.1049/cp.2016.1290 |chapter=Development of wireless electricity transmission through resonant coupling |title=4th IET Clean Energy and Technology Conference (CEAT 2016) |year=2016 |last1=Ibrahim |first1=F.N. |last2=Jamail |first2=N.A.M. |last3=Othman |first3=N.A. |pages=33 (5 .) |isbn=978-1-78561-238-1 }}</ref> Wireless power transfer is useful to power electrical devices where interconnecting wires are inconvenient, hazardous, or are not possible.

Wireless power techniques mainly fall into two categories: [[Physics:Near and far field|near field]] and [[Physics:Near and far field|far-field]].<ref>{{cite journal |last1=Kracek |first1=Jan |last2=Mazanek |first2=Milos |title=Wireless Power Transmission for Power Supply: State of Art |journal=Radioengineering |date=June 2011 |volume=20 |issue=2 |pages=457–463 |url=https://www.radioeng.cz/fulltexts/2011/11_02_457_463.pdf}}</ref>  In ''[[Physics:Near and far field|near field]]'' or ''non-radiative''  techniques, power is transferred over short distances by [[Physics:Magnetic field|magnetic field]]s using [[Engineering:Inductive coupling|inductive coupling]] between [[Engineering:Electromagnetic coil|coils of wire]], or by [[Physics:Electric field|electric field]]s using [[Engineering:Capacitive coupling|capacitive coupling]] between metal [[Physics:Electrode|electrode]]s.<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"/>  Inductive coupling is the most widely used wireless technology; its applications include charging handheld devices like phones and [[Engineering:Electric toothbrush|electric toothbrush]]es, RFID tags, [[Engineering:Induction cooking|induction cooking]], and wirelessly charging or continuous wireless power transfer in implantable medical devices like [[Artificial cardiac pacemaker|artificial cardiac pacemaker]]s, or [[Engineering:Electric vehicle|electric vehicle]]s.

In ''[[Physics:Near and far field|far-field]]'' or ''radiative'' techniques, also called ''power beaming'', power is transferred by beams of [[Physics:Electromagnetic radiation|electromagnetic radiation]], like [[Physics:Microwave|microwave]]s<ref>{{cite journal|author1=Miguel Poveda-García |author2= Jorge Oliva-Sanchez |author3=Ramon Sanchez-Iborra |author4=David Cañete-Rebenaque |author5= Jose Luis Gomez-Tornero |title=Dynamic Wireless Power Transfer for Cost-Effective Wireless Sensor Networks using Frequency-Scanned Beaming  |journal=IEEE Access |year=2019 |doi=10.1109/ACCESS.2018.2886448 |volume=7 |pages=8081–8094|bibcode= 2019IEEEA...7.8081P |doi-access=free }}</ref> or [[Physics:Laser|laser]] beams.  These techniques can transport energy longer distances but must be aimed at the receiver. Proposed applications for this type include solar power satellites and wireless powered drone aircraft.<ref name="Bush">{{cite book|last1=Bush|first1=Stephen F.|title=Smart Grid: Communication-Enabled Intelligence for the Electric Power Grid|publisher=John Wiley & Sons|date=2014|page=118|language=en|url=https://books.google.com/books?id=bUSMAgAAQBAJ&pg=PA118|isbn=978-1118820230}}</ref><ref name="PCMag">{{cite web|title=Wireless energy transfer|website=Encyclopedia of terms|publisher=PC Magazine Ziff-Davis|date=2014|url=https://www.pcmag.com/encyclopedia/term/57396/wireless-energy-transfer|access-date=15 December 2014}}</ref><ref>{{cite news |last1=Marks |first1=Paul |title=Wireless charging for electric vehicles hits the road |url=https://www.newscientist.com/article/mg22129534-900-wireless-charging-for-electric-vehicles-hits-the-road/ |work=New Scientist |date=22 January 2014 }}</ref>

An important issue associated with all wireless power systems is limiting the exposure of people and other living beings to potentially injurious [[Physics:Electromagnetic field|electromagnetic field]]s.<ref name="Lu">{{cite book
 | last1  = Lu
 | first1 = Yan
 | last2  = Ki
 | first2 = Wing-Hung
 | title  = CMOS Integrated Circuit Design for Wireless Power Transfer
 | publisher = Springer
 | date   = 2017
 | pages  = 2–3
 | url    = https://books.google.com/books?id=7fwwDwAAQBAJ&q=%22wireless+power%22+safety+exposure&pg=PA3
 | isbn   = 978-9811026157
 }}</ref><ref name="Sun3">{{cite book
 | last1  = Sun
 | first1 = Tianjia
 | last2  = Xie
 | first2 = Xiang
 | last3  = Wang
 | first3 = Zhihua
 | title  = Wireless Power Transfer for Medical Microsystems
 | publisher = Springer Science and Business Media
 | date   = 2013
 | url    = https://books.google.com/books?id=kTA_AAAAQBAJ&q=%22wireless+power%22+safety+exposure&pg=PA32
 | isbn   = 978-1461477020
 }}</ref>
{{toclimit|3}}

==Overview==
{{Further|Engineering:Coupling (electronics)}}

[[File:Wireless power system.svg|thumb|upright=1.7|Generic block diagram of a wireless power system]]
Wireless power transfer is a generic term for a number of different technologies for transmitting energy by means of [[Physics:Electromagnetic field|electromagnetic field]]s.<ref name="Shinohara1">{{cite book|last1=Shinohara|first1=Naoki|title=Wireless Power Transfer via Radiowaves|publisher=John Wiley & Sons|date=2014|pages=ix–xiii|language=en|url=https://books.google.com/books?id=TwegAgAAQBAJ&pg=PR9|isbn=978-1118862964}}</ref><ref name="Gopinath">{{cite journal|last1=Gopinath |first1=Ashwin |title=All About Transferring Power Wirelessly |journal=Electronics for You E-zine |pages=52–56 |date=August 2013 |url=http://www.efymagonline.com/pdf/52_Wireless%20Power%20Transfer_EFY%20August%202013.pdf |access-date=16 January 2015 |url-status=dead |archive-url=https://web.archive.org/web/20150119044123/http://www.efymagonline.com/pdf/52_Wireless%20Power%20Transfer_EFY%20August%202013.pdf |archive-date=19 January 2015 |df=dmy }}</ref><ref name="X. Lu">{{cite journal | last1 = Lu | first1 = X. | last2 = Wang | first2 = P. | last3 = Niyato | first3 = D. | last4 = Kim | first4 = D. I. | last5 = Han | first5 = Z. | year = 2016 | title = Wireless Charging Technologies: Fundamentals, Standards, and Network Applications | journal = IEEE Communications Surveys and Tutorials | volume = 18 | issue = 2| pages = 1413–1452 | doi=10.1109/comst.2015.2499783| arxiv = 1509.00940 | s2cid = 8639012 }}</ref>  The technologies, listed in the table below, differ in the distance over which they can transfer power efficiently, whether the transmitter must be aimed (directed) at the receiver, and in the type of electromagnetic energy they use: time varying [[Physics:Electric field|electric field]]s, [[Physics:Magnetic field|magnetic field]]s, [[Radio wave|radio wave]]s, [[Physics:Microwave|microwave]]s, [[Physics:Infrared|infrared]] or [[Physics:Visible light|visible light]] waves.<ref name="Sun">{{cite book|last1=Sun|first1=Tianjia|last2=Xie|first2=Xiang|last3=Zhihua|first3=Wang|title=Wireless Power Transfer for Medical Microsystems|publisher=Springer Science & Business Media|date=2013|pages=5–6|language=en|url=https://books.google.com/books?id=kTA_AAAAQBAJ&q=%22wireless+power%22&pg=PA6|isbn=978-1461477020}}</ref>

In general a wireless power system consists of a "transmitter" device connected to a source of power such as a mains power line, which converts the power to a time-varying electromagnetic field, and one or more "receiver" devices which receive the power and convert it back to DC or AC electric current which is used by an [[Engineering:Electrical load|electrical load]].<ref name="Shinohara1" /><ref name="Sun" />  At the transmitter the input power is converted to an oscillating [[Physics:Electromagnetic field|electromagnetic field]] by some type of "[[Engineering:Antenna (radio)|antenna]]" device.  The word "antenna" is used loosely here; it may be a coil of wire which generates a [[Physics:Magnetic field|magnetic field]], a metal plate which generates an [[Physics:Electric field|electric field]], an [[Engineering:Antenna (radio)|antenna]] which radiates radio waves, or a [[Physics:Laser|laser]] which generates light.  A similar antenna or [[Engineering:Coupling (electronics)|coupling]] device at the receiver converts the oscillating fields to an electric current.  An important parameter that determines the type of waves is the [[Physics:Frequency|frequency]], which determines the wavelength.

Wireless power uses the same fields and waves as wireless communication devices like [[Social:Radio|radio]],<ref name="Sazonov">{{cite book|last1=Sazonov|first1=Edward|last2=Neuman|first2=Michael R.|title=Wearable Sensors: Fundamentals, Implementation and Applications|publisher=Elsevier|date=2014|pages=253–255|url=https://books.google.com/books?id=wGJzAwAAQBAJ&pg=PA253|isbn=978-0124186668}}</ref><ref name="Shinohara2" >{{cite book|url=https://books.google.com/books?id=TwegAgAAQBAJ&pg=PP27|last=Shinohara|year=2014|title=Wireless Power Transfer via Radiowaves|page=27|publisher=John Wiley & Sons |isbn=9781118862964}}</ref> another familiar technology that involves electrical energy transmitted without wires by electromagnetic fields, used in cellphones, [[Engineering:Radio broadcasting|radio]] and [[Engineering:Television broadcasting|television broadcasting]], and WiFi.  In radio communication the goal is the transmission of information, so the amount of power reaching the receiver is not so important, as long as it is sufficient that the information can be received intelligibly.<ref name="Gopinath" /><ref name="Sazonov" /><ref name="Shinohara2" />  In wireless communication technologies only tiny amounts of power reach the receiver.  In contrast, with wireless power transfer the amount of energy received is the important thing, so the [[Physics:Efficiency|efficiency]] (fraction of transmitted energy that is received) is the more significant parameter.<ref name="Gopinath" />  For this reason, wireless power technologies are likely to be more limited by distance than wireless communication technologies.

{{anchor|WPC}}Wireless power transfer may be used to power up wireless information transmitters or receivers. This type of communication is known as wireless powered communication (WPC). {{anchor|SWIPT}}When the harvested power is used to supply the power of wireless information transmitters, the network is known as Simultaneous Wireless Information and Power Transfer (SWIPT);<ref>{{Cite journal |arxiv = 1409.0261|doi = 10.1109/MCOM.2014.6957150|title = Simultaneous wireless information and power transfer in modern communication systems|journal = IEEE Communications Magazine|volume = 52|issue = 11|pages = 104–110|year = 2014|last1 = Krikidis|first1 = Ioannis|last2 = Timotheou|first2 = Stelios|last3 = Nikolaou|first3 = Symeon|last4 = Zheng|first4 = Gan|last5 = Ng|first5 = Derrick Wing Kwan|last6 = Schober|first6 = Robert|bibcode = 2014arXiv1409.0261K|s2cid = 3462059}}</ref> whereas when it is used to supply the power of wireless information receivers, it is known as a Wireless Powered Communication Network (WPCN).<ref>{{Cite journal |arxiv = 1508.06366|doi = 10.1109/MWC.2016.7462480|title = Wireless powered communication networks: An overview|journal = IEEE Wireless Communications|volume = 23|issue = 2|pages = 10–18|year = 2016|last1 = Bi|first1 = Suzhi|last2 = Zeng|first2 = Yong|last3 = Zhang|first3 = Rui|author4 = Dong In Kim|last5 = Han|first5 = Zhu|s2cid = 3504276}}</ref><ref>{{Cite arXiv |eprint = 1807.05543|last1 = Lu|first1 = Xiao|title = Maximizing Ergodic Throughput in Wireless Powered Communication Networks|last2 = Wang|first2 = Ping|last3 = Niyato|first3 = Dusit|author4 = Dong In Kim|last5 = Han|first5 = Zhu|class = cs.IT|year = 2018}}</ref><ref>{{Cite journal |doi = 10.1109/MCOM.2015.7081084|title = Wireless powered communication: Opportunities and challenges|journal = IEEE Communications Magazine|volume = 53|issue = 4|pages = 117–125|year = 2015|last1 = Bi|first1 = Suzhi|last2 = Ho|first2 = Chin Keong|last3 = Zhang|first3 = Rui|arxiv = 1408.2335|s2cid = 7127575}}</ref>

These are the different wireless power technologies:<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|website=BBC website|publisher=British Broadcasting Corp.|date=20 November 2012|url=http://www.bbc.com/future/story/20121120-pulling-the-plug-on-electric-cars|access-date=10 December 2014}}</ref>

{| class="wikitable"
|-
! Technology
! Range<ref>"short", "midrange", and "long range" are defined below</ref>
! [[Engineering:Directivity|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), biomedical implants, electric vehicles, powering buses, trains, MAGLEV, RFID, smartcards.
|-
| 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, [[Engineering:Phased array|phased array]]s, [[Physics:Rectenna|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|language=en}}</ref> powering drone aircraft, powering space elevator climbers.
|}

==Field regions==
[[Physics:Electric field|Electric]] and [[Physics:Magnetic field|magnetic field]]s are created by [[Physics:Charged particle|charged particle]]s in matter such as [[Physics:Electron|electron]]s.  A stationary charge creates an electrostatic field in the space around it.  A steady [[Physics:Electric current|current]] of charges ([[Physics:Direct current|direct current]], DC) creates a static [[Physics:Magnetic field|magnetic field]] around it.  The above fields contain [[Physics:Energy|energy]], but cannot carry [[Physics: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 [[Physics:Alternating current|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|language=en|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|language=en|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|wavelength]] ('''''λ''''') of the antenna.<ref name="Shinohara1" /><ref name="Rajakaruna" /><ref name="Agbinya" /> In this region the oscillating [[Physics:Electric field|electric]] and [[Physics:Magnetic field|magnetic field]]s are separate<ref name="Sazonov" /> and power can be transferred via electric fields by [[Engineering:Capacitive coupling|capacitive coupling]] ([[Physics:Electrostatic induction|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 [[Engineering:Inductive coupling|inductive coupling]] ([[Physics:Electromagnetic induction|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}}</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.

:[[Physics:Resonance|Resonance]], such as [[Engineering:Resonant inductive coupling|resonant inductive coupling]], can increase the [[Engineering: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>'''&nbsp;≤&nbsp;'''''D''<sub>ant</sub>'''.<ref name="Umenei" /><ref name="Karalis" /><ref name="Baarman">"''Typically, an inductive coupled system can transmit roughly the diameter of the transmitter.''"(p. 4) "''...mid-range is defined as somewhere between one and ten times the diameter of the transmitting coil.''"(p. 2)  {{cite journal
 |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>'''&nbsp;≤&nbsp;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 = 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|radio wave]]s, [[Physics:Microwave|microwave]]s, or light waves.<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'''&nbsp;=&nbsp;'''''c/f'''''.  At low frequencies '''''f''''' where the antenna is much smaller than the size of the waves, '''''D''<sub>ant</sub>'''&nbsp;<<&nbsp;'''λ''', very little power is radiated.  Therefore the near-field devices above, 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>'''&nbsp;≈&nbsp;'''λ'''  such as [[Engineering:Monopole antenna|monopole]] or [[Physics:Dipole antenna|dipole antenna]]s,  radiate power efficiently, but the electromagnetic waves are radiated in all directions ([[Engineering: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 [[Physics:Reflection|reflection]] or [[Physics:Refraction|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 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>'''&nbsp;>>&nbsp;'''λ'''&nbsp;=&nbsp;'''''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 below, corresponding to frequencies above 1 GHz, in the [[Physics:Microwave|microwave]] range or above.<ref name="Shinohara1" />

==Near-field (nonradiative) techniques==
At large relative distance, the near-field components of electric and magnetic fields are approximately quasi-static oscillating [[Physics:Dipole|dipole]] fields.  These fields decrease with the cube of distance: ('''''D''<sub>range</sub>'''/'''''D''<sub>ant</sub>''')<sup>−3</sup><ref name="Agbinya1" /><ref name="Shortwave">{{cite journal
  | title = Lighting Lamp by S-W Radio
  | journal = Short Wave and Television
  | volume = 8
  | issue = 4
  | page = 166
  | date = August 1937
  | url = http://www.americanradiohistory.com/Archive-Short-Wave-Television/30s/SW-TV-1937-08.pdf
  | access-date = 18 March 2015}} on http://www.americanradiohistory.com</ref>  Since power is proportional to the square of the field strength, the power transferred decreases as ('''''D''<sub>range</sub>'''/'''''D''<sub>ant</sub>''')<sup>−6</sup>.<ref name="Sazonov" /><ref name="Schantz" /><ref name="Agbinya2">{{cite journal
  | last1  = Agbinya
  | first1 = Johnson I.
  | title = Investigation of near field inductive communication system models, channels, and experiments
  | journal = Progress in Electromagnetics Research B
  | volume = 49
  | page = 130
  | date = February 2013
  | url = http://www.jpier.org/PIERB/pierb49/06.12120512.pdf
  | doi = 10.2528/pierb12120512
  | access-date = 2 January 2015}}</ref><ref name="Bolic">{{cite book
 | last1  = Bolic
 | first1 = Miodrag
 | last2  =  Simplot-Ryl
 | first2 =  David
 | last3  = Stojmenovic
 | first3 = Ivan
 | title  = RFID Systems: Research Trends and Challenges
 | publisher = John Wiley & Sons
 | date   = 2010
 | page  = 29
 | url    = https://books.google.com/books?id=VansInOpixEC&pg=PA29
 | isbn   = 978-0470975664
 }}</ref> or 60&nbsp;dB per decade.  In other words, if far apart, increasing the distance between the two antennas tenfold causes the power received to decrease by a factor of 10<sup>6</sup> = 1000000. As a result, inductive and [[Engineering:Capacitive coupling|capacitive coupling]] can only be used for short-range power transfer, within a few times the diameter of the antenna device '''''D''<sub>ant</sub>'''.  Unlike in a radiative system where the maximum radiation occurs when the dipole antennas are oriented transverse to the direction of propagation, with dipole fields the maximum coupling occurs when the dipoles are oriented longitudinally.

===Inductive coupling===
[[File:Wireless power system - inductive coupling.svg|thumb|upright=1.4|Generic block diagram of an inductive wireless power system]]
{{multiple image
| align = right
| direction = horizontal
| header   =
| image1   = 2010-12-08-Sonicare-4.jpg
| caption1 =
| width1   = 95
| image2   = Lamp powered by induction 1910.jpg
| caption2 =
| width2   = 200
| footer   = ''(left)'' Modern inductive power transfer, an electric toothbrush charger.  A coil in the stand produces a magnetic field, inducing an alternating current in a coil in the toothbrush, which is rectified to charge the batteries.<br /> ''(right)'' A light bulb powered wirelessly by induction, in 1910.
}}
In [[Engineering:Inductive coupling|inductive coupling]] (''[[Physics:Electromagnetic induction|electromagnetic induction]]''<ref name="Valtchev" /><ref name="Davis">{{cite journal
  | last1  = Davis
  | first1 = Sam
  | title = Wireless power minimizes interconnection problems
  | journal = Power Electronics Technology
  | pages = 10–14
  | date = July 2011
  | url = https://www.powerelectronics.com/technologies/power-electronics-systems/article/21861344/wireless-power-minimizes-interconnection-problems
  | access-date = 16 January 2015}}</ref> or ''inductive power transfer'', IPT), power is transferred between [[Engineering:Electromagnetic coil|coils of wire]] by a [[Physics:Magnetic field|magnetic field]].<ref name="Sazonov" />  The transmitter and receiver coils together form a ''[[Engineering:Transformer|transformer]]''<ref name="Sazonov" /><ref name="Valtchev" /> ''(see diagram)''. An [[Physics:Alternating current|alternating current]] (AC) through the transmitter coil ''(L1)'' creates an oscillating [[Physics:Magnetic field|magnetic field]] ''(B)'' by Ampere's law.  The magnetic field  passes through the receiving coil ''(L2)'', where it induces an alternating [[Physics:Electromotive force|EMF]] ([[Physics:Voltage|voltage]]) by [[Physics:Faraday's law of induction|Faraday's law of induction]], which creates an alternating current in the receiver.<ref name="Gopinath" /><ref name="Davis" />  The induced alternating current may either drive the load directly, or be [[Engineering:Rectifier|rectified]] to [[Physics:Direct current|direct current]] (DC) by a [[Engineering:Rectifier|rectifier]] in the receiver, which drives the load.  A few systems, such as electric toothbrush charging stands, work at  50/60&nbsp;Hz so AC mains current is applied directly to the transmitter coil, but in most systems an [[Engineering:Electronic oscillator|electronic oscillator]] generates a higher frequency AC current which drives the coil, because transmission efficiency improves with [[Physics:Frequency|frequency]].<ref name="Davis" />

Inductive coupling is the oldest and most widely used wireless power technology, and virtually the only one so far which is used in commercial products.  It is used in inductive charging stands for [[Engineering:Cordless|cordless]] appliances used in wet environments such as [[Engineering:Electric toothbrush|electric toothbrush]]es<ref name="Valtchev" /> and shavers, to reduce the risk of electric shock.<ref name="Wilson">{{cite web|last=Wilson|first=Tracy V.|title=How Wireless Power Works|website=How Stuff Works|publisher= InfoSpace LLC|date=2014|url=http://electronics.howstuffworks.com/everyday-tech/wireless-power.htm|access-date=15 December 2014}}</ref> Another application area is "transcutaneous" recharging of biomedical prosthetic devices [[Medicine:Implant|implanted]] in the human body, such as [[Artificial cardiac pacemaker|cardiac pacemakers]] and [[Engineering:Insulin pump|insulin pump]]s, to avoid having wires passing through the skin.<ref name="Puers" /><ref name="Sun2">{{cite book | last1  = Sun | first1 = Tianjia | last2  = Xie | first2 =  Xiang | last3  =  Zhihua | first3 = Wang | title  = Wireless Power Transfer for Medical Microsystems | publisher = Springer Science & Business Media | date   = 2013 | url    = https://books.google.com/books?id=kTA_AAAAQBAJ&q=%22wireless+power%22&pg=PA6 | isbn   = 978-1461477020 }}</ref>  It is also used to charge [[Engineering:Electric vehicle|electric vehicle]]s such as cars and to either charge or power transit vehicles like buses and trains.<ref name="Valtchev" />

However the fastest growing use is wireless charging pads to recharge mobile and handheld wireless devices such as laptop and [[Engineering:Tablet computer|tablet computer]]s, [[Computer mouse|computer mouse]], cellphones, [[Digital media player|digital media player]]s, and video game controllers.{{citation needed|date=April 2021}}   In the United States, the Federal Communications Commission (FCC) provided its first certification for a wireless transmission charging system in December 2017.<ref>{{Cite news|url=https://www.engadget.com/2017/12/26/fcc-approves-first-wireless-power-at-a-distance-charging-syste/|title=FCC approves first wireless 'power-at-a-distance' charging system|work=Engadget|access-date=2018-03-27|language=en-US}}</ref>

The power transferred increases with frequency<ref name="Davis" /> and the mutual inductance <math>M</math> between the coils,<ref name="Gopinath" /> which depends on their geometry and the distance <math>D_\text{range}</math> between them. A widely used figure of merit is the coupling coefficient <math>k\; =\; M/\sqrt{L_1 L_2}</math>.<ref name="Davis" /><ref name="Agbinya4">{{cite book|url=https://books.google.com/books?id=zDPqqBJ76ZAC&q=%22coupling+coefficient%22&pg=PA140|last=Agbinya|year=2012|title=Wireless Power Transfer|page=140|publisher=River Publishers |isbn=9788792329233}}</ref>  This dimensionless parameter is equal to the fraction of [[Physics:Magnetic flux|magnetic flux]] through the transmitter coil <math>L1</math> that passes through the receiver coil  <math>L2</math> when L2 is open circuited.  If the two coils are on the same axis and close together so all the magnetic flux from <math>L1</math> passes through <math>L2</math>, <math>k = 1</math> and the link efficiency approaches 100%.  The greater the separation between the coils, the more of the magnetic field from the first coil misses the second, and the lower <math>k</math> and the link efficiency are, approaching zero at large separations.<ref name="Davis" />  The link efficiency and power transferred is roughly proportional to <math>k^2</math>.<ref name="Davis" />  In order to achieve high efficiency, the coils must be very close together, a fraction of the coil diameter <math>D_\text{ant}</math>,<ref name="Davis" /> usually within centimeters,<ref name="Tan" /> with the coils' axes aligned.  Wide, flat coil shapes are usually used, to increase coupling.<ref name="Davis" /> [[Chemistry:Allotropes of iron|Ferrite]] "flux confinement" cores can confine the magnetic fields, improving coupling and reducing [[Engineering:Electromagnetic interference|interference]] to nearby electronics,<ref name="Davis" /><ref name="Puers" /> but they are heavy and bulky so small wireless devices often use air-core coils.

Ordinary inductive coupling can only achieve high efficiency when the coils are very close together, usually adjacent.  In most modern inductive systems [[Engineering:Resonant inductive coupling|resonant inductive coupling]] ''(described below)'' is used, in which the efficiency is increased by using resonant circuits.<ref name="Agbinya" /><ref name="Wong" /><ref name="Davis" /><ref name="Shinohara" />  This can achieve high efficiencies at greater distances than nonresonant inductive coupling. 
{{multiple image
| align = center
| direction = horizontal
| header = 
| image1 = 
| caption1 = 
| width1 = 
| image2 = Electric car wireless parking charge closeup.jpg
| caption2 = Prototype inductive electric car charging system at 2011 Tokyo Auto Show
| width2 = 230
| image3 = Power 2.0 Ecosystem illustration - Powermat charging spots on counter in a coffee shop.jpg
| caption3 = [[Engineering:Powermat Technologies|Powermat]] inductive charging spots in a coffee shop. Customers can set their phones and computers on them to recharge.
| width3 = 250
| image4 = U. S. Marine Forces Reserve scored a 100 percent on physical facility security during a Command Cyber Readiness Inspection conducted by the Defense Information Security Agency at Marine Corps Support Facility 130521-M-IU921-961.jpg
| caption4 = Wireless powered access card.
| width4 = 230
| image5 = Magne Chargers.jpg
| caption5 = GM EV1 and Toyota RAV4 EV inductively charging at a now-obsolete [[Engineering:Magne Charge|Magne Charge]] station
| width5 = 204
| footer = 
| total_width = 
| alt1 = 
}}

====Resonant inductive coupling====
{{main|Engineering:Resonant inductive coupling}}

[[Engineering:Resonant inductive coupling|Resonant inductive coupling]] (''electrodynamic coupling'',<ref name="Valtchev" /> ''strongly coupled magnetic resonance''<ref name="Karalis" />) is a form of inductive coupling in which power is transferred by magnetic fields ''(B, green)'' between two resonant circuits (tuned circuits), one in the transmitter and one in the receiver ''(see diagram, right)''.<ref name="Sazonov" /><ref name="Valtchev" /><ref name="Agbinya" /><ref name="Wilson" /><ref name="Shinohara" />  Each resonant circuit consists of a coil of wire connected to a [[Engineering:Capacitor|capacitor]], or a self-resonant coil or other [[Physics:Resonator|resonator]] with internal capacitance.  The two are tuned to resonate at the same [[Physics:Resonant frequency|resonant frequency]].  The resonance between the coils can greatly increase coupling and power transfer, analogously to the way a vibrating [[Physics:Tuning fork|tuning fork]] can induce sympathetic vibration in a distant fork tuned to the same pitch.

[[Biography:Nikola Tesla|Nikola Tesla]] first discovered resonant coupling during his pioneering experiments in wireless power transfer around the turn of the 20th century,<ref name="Wheeler">{{cite journal |last1=Wheeler |first1=L. P. |title=II — Tesla's contribution to high frequency |journal=Electrical Engineering |date=August 1943 |volume=62 |issue=8 |pages=355–357 |doi=10.1109/EE.1943.6435874 |s2cid=51671246 }}</ref><ref name="LeeZhongHui">{{cite conference|first1=C.K.|last1=Lee|first2=W.X.|last2=Zhong|first3=S.Y.R.|last3=Hui|title=Recent Progress in Mid-Range Wireless Power Transfer|conference=The 4th Annual IEEE Energy Conversion Congress and Exposition (ECCE 2012)|pages=3819–3821|publisher=Inst. of Electrical and Electronic Engineers|date=5 September 2012|location=Raleigh, North Carolina|url=http://hub.hku.hk/bitstream/10722/189863/1/Content.pdf|access-date=4 November 2014}}</ref><ref name="Sun1">{{cite book|url=https://books.google.com/books?id=kTA_AAAAQBAJ&q=%22resonate+inductive+coupling%22+tesla&pg=PA3|last1=Sun|last2=Xie|last3=Wang|year=2013|title=Wireless Power Transfer for Medical Microsystems|page=3|publisher=Springer |isbn=9781461477020}}</ref>  but the possibilities of using resonant coupling to increase transmission range has only recently been explored.<ref name="Beams">{{Cite book |doi = 10.1109/MWSCAS.2013.6674697 |isbn = 978-1-4799-0066-4 |chapter = Design and simulation of networks for midrange wireless power transfer |title = 2013 IEEE 56th International Midwest Symposium on Circuits and Systems (MWSCAS) |pages = 509–512 |year = 2013 |last1 = Beams |first1 = David M. |last2 = Nagoorkar |first2 = Varun |s2cid = 42092151 }}</ref>  In 2007 a team led by [[Biography:Marin Soljačić|Marin Soljačić]] at MIT used two coupled tuned circuits each made of a 25&nbsp;cm self-resonant coil of wire at 10&nbsp;MHz to achieve the transmission of 60 W of power over a distance of {{convert|2|meters|feet}} (8 times the coil diameter) at around 40% efficiency.<ref name="Valtchev" /><ref name="Karalis" /><ref name="Wilson" /><ref name="LeeZhongHui" /><ref name="Kurs">{{cite journal |last1=Kurs |first1=A. |last2=Karalis |first2=A. |last3=Moffatt |first3=R. |last4=Joannopoulos |first4=J. D. |last5=Fisher |first5=P. |last6=Soljacic |first6=M. |title=Wireless Power Transfer via Strongly Coupled Magnetic Resonances |journal=Science |date=6 July 2007 |volume=317 |issue=5834 |pages=83–86 |doi=10.1126/science.1143254 |pmid=17556549 |bibcode=2007Sci...317...83K |citeseerx=10.1.1.418.9645 |s2cid=17105396 }}</ref>

The concept behind resonant inductive coupling systems is that high [[Physics:Q factor|Q factor]] [[Physics:Resonator|resonator]]s exchange energy at a much higher rate than they lose energy due to internal [[Damping ratio|damping]].<ref name="Karalis" />  Therefore, by using resonance, the same amount of power can be transferred at greater distances, using the much weaker magnetic fields out in the peripheral regions ("tails") of the near fields.<ref name="Karalis" />  Resonant inductive coupling can achieve high efficiency at ranges of 4 to 10 times the coil diameter ('''''D''<sub>ant</sub>''').<ref name="Wong" /><ref name="Baarman" /><ref name="Agbinya3" /> This is called "mid-range" transfer,<ref name="Baarman" /> in contrast to the "short range" of nonresonant inductive transfer, which can achieve similar efficiencies only when the coils are adjacent.  Another advantage is that resonant circuits interact with each other so much more strongly than they do with nonresonant objects that power losses due to absorption in stray nearby objects are negligible.<ref name="Agbinya" /><ref name="Karalis" />

A drawback of resonant coupling theory is that at close ranges when the two resonant circuits are tightly coupled, the resonant frequency of the system is no longer constant but "splits" into two resonant peaks,<ref>{{Cite journal |doi = 10.3390/s16081229|title = Frequency Splitting Analysis and Compensation Method for Inductive Wireless Powering of Implantable Biosensors|journal = Sensors|volume = 16|issue = 8|pages = 1229|year = 2016|last1 = Schormans|first1 = Matthew|last2 = Valente|first2 = Virgilio|last3 = Demosthenous|first3 = Andreas|pmid = 27527174|pmc = 5017394|bibcode = 2016Senso..16.1229S|doi-access = free}}</ref><ref>{{Cite journal | doi=10.3390/en10040498| title=Combined Conformal Strongly-Coupled Magnetic Resonance for Efficient Wireless Power Transfer| journal=Energies| volume=10| issue=4| pages=498| year=2017| last1=Rozman| first1=Matjaz| last2=Fernando| first2=Michael| last3=Adebisi| first3=Bamidele| last4=Rabie| first4=Khaled| last5=Kharel| first5=Rupak| last6=Ikpehai| first6=Augustine| last7=Gacanin| first7=Haris| doi-access=free}}</ref><ref>{{cite web|last=smith|first=K.J.|url=http://www.lessmiths.com/~kjsmith/crystal/resonance.shtml|title=A graphical look at Resonance}}</ref> so the maximum power transfer no longer occurs at the original resonant frequency and the oscillator frequency must be tuned to the new resonance peak.<ref name="Wong" /><ref name="Neo">{{Cite web|url=http://blog.livedoor.jp/neotesla/archives/51508967.html|title=Reconsideration of Wireless Power Transfer principle which presented by MIT|website=ニコラテスラって素晴らしい|date=30 March 2017 }}</ref>

Resonant technology is currently being widely incorporated in modern inductive wireless power systems.<ref name="Davis" />  One of the possibilities envisioned for this technology is area wireless power coverage.  A coil in the wall or ceiling of a room might be able to wirelessly power lights and mobile devices anywhere in the room, with reasonable efficiency.<ref name="Wilson" />  An environmental and economic benefit of wirelessly powering small devices such as clocks, radios, music players and [[Engineering:Remote control|remote control]]s is that it could drastically reduce the 6 billion [[Physics:Battery (electricity)|batteries]] disposed of each year, a large source of toxic waste and groundwater contamination.<ref name="Tan" />

A study for the Swedish military found that 85kHz systems for dynamic wireless power transfer for vehicles can cause electromagnetic interference at a radius of up to 300 kilometers.<ref>{{citation |url=https://www.electronic.se/en/2021/05/25/interference-risks-from-wireless-power-transfer-for-electric-vehicles/ |title=Interference Risks from Wireless Power Transfer for Electric Vehicles |author=Sara Linder |publisher=Swedish Defence Research Agency (FOI) |date=May 2, 2021}}</ref>

===Capacitive coupling===
{{Main|Engineering:Capacitive coupling}}
[[Engineering:Capacitive coupling|Capacitive coupling]] also referred to as electric coupling, makes use of electric fields for the transmission of power between two [[Physics:Electrode|electrode]]s (an [[Physics:Anode|anode]] and [[Physics:Cathode|cathode]]) forming a [[Physics:Capacitance|capacitance]] for the transfer of power.<ref>{{Cite web|url=https://www.wipo-wirelesspower.com/technology/resonant-capacitive-coupling|title=Resonant Capacitive Coupling|last=Webmaster|website=www.wipo-wirelesspower.com|language=en-gb|access-date=2018-11-30}}</ref> In [[Engineering:Capacitive coupling|capacitive coupling]] ([[Physics:Electrostatic induction|electrostatic induction]]), the conjugate of [[Engineering:Inductive coupling|inductive coupling]], energy is transmitted by electric fields<ref name="ECN2011"/><ref name="Gopinath" /><ref name="Trancutaneous Capacitive Wireless Power Transfer"/><ref name="Capacitive Wireless Power Transfer to biomedical implants"/> between [[Physics:Electrode|electrode]]s<ref name="Capacitive Elements for Wireless Power Transfer to biomedical implants"/> such as metal plates. The transmitter and receiver electrodes form a [[Engineering:Capacitor|capacitor]], with the intervening space as the [[Physics:Dielectric|dielectric]].<ref name="Capacitive Elements for Wireless Power Transfer to biomedical implants"/><ref name="Gopinath" /><ref name="Sazonov" /><ref name="Valtchev" /><ref name="Puers">{{cite book | last1  = Puers | first1 = R. | title  = Omnidirectional Inductive Powering for Biomedical Implants | publisher = Springer Science & Business Media | date   = 2008 | pages  = 4–5 | url = https://books.google.com/books?id=SKW6BrWWnNgC&q=%22wireless+power%22+capacitive&pg=PA4 | isbn   = 978-1402090752 }}</ref><ref name="Huschens">{{cite journal | last1  = Huschens | first1 = Markus | title = Various techniques for wireless charging | journal = EETimes-Asia | date = 2012 | url = http://m.eetasia.com/STATIC/PDF/201206/EEOL_2012JUN01_RFD_POW_TA_01.pdf?SOURCES=DOWNLOAD | access-date = 16 January 2015}}</ref>  An alternating voltage generated by the transmitter is applied to the transmitting plate, and the oscillating [[Physics:Electric field|electric field]] induces an alternating [[Physics:Electric potential|potential]] on the receiver plate by [[Physics:Electrostatic induction|electrostatic induction]],<ref name="Gopinath" /><ref name="Huschens" /> which causes an alternating current to flow in the load circuit.  The amount of power transferred increases with the [[Physics:Frequency|frequency]]<ref name="Huschens" /> the square of the voltage, and the [[Physics:Capacitance|capacitance]] between the plates, which is proportional to the area of the smaller plate and (for short distances) inversely proportional to the separation.<ref name="Gopinath" />

{{multiple image
| align = right
| direction = vertical
| header   = Capacitive wireless power systems
| image1   = Wireless power system - capacitive bipolar.svg
| caption1 = Bipolar coupling
| image2   = Wireless power - capacitive charge sink.svg
| caption2 = Monopolar coupling
| width    = 220
| footer   =
}}

Capacitive coupling has only been used practically in a few low power applications, because the very high voltages on the electrodes required to transmit significant power can be hazardous,<ref name="Sazonov" /><ref name="Valtchev" /> and can cause unpleasant side effects such as noxious [[Chemistry:Ozone|ozone]] production. In addition, in contrast to magnetic fields,<ref name="Karalis" /> electric fields interact strongly with most materials, including the human body, due to dielectric polarization.<ref name="Puers" />  Intervening materials between or near the electrodes can absorb the energy, in the case of humans possibly causing excessive electromagnetic field exposure.<ref name="Sazonov" />  However capacitive coupling has a few advantages over inductive coupling.  The field is largely confined between the capacitor plates, reducing interference, which in inductive coupling requires heavy ferrite "flux confinement" cores.<ref name="Gopinath" /><ref name="Puers" />  Also, alignment requirements between the transmitter and receiver are less critical.<ref name="Gopinath" /><ref name="Sazonov" /><ref name="Huschens" /> Capacitive coupling has recently been applied to charging battery powered portable devices<ref name="ECN2011"/> as well as charging or continuous wireless power transfer in 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"/> and is being considered as a means of transferring power between substrate layers in integrated circuits.<ref name="Meindl">{{cite book | last1  = Meindl | first1 = James D. | title  = Integrated Interconnect Technologies for 3D Nanoelectronic Systems | publisher = Artech House | date   = 2008 | pages  = 475–477 | url    = https://books.google.com/books?id=OtY-66XCMuYC&q=%22wireless+power%22+%22capacitive+coupling%22&pg=PA475 | isbn   = 978-1596932470}}</ref>

Two types of circuit have been used:
* '''Transverse''' (bipolar) design:<ref name="Trancutaneous Capacitive Wireless Power Transfer"/><ref name="Capacitive Wireless Power Transfer to biomedical implants"/><ref name="Harakawa">{{cite web
 |last=Harakawa 
 |first=Kenichi 
 |title=Wireless power transmission at rotating and sliding elements by using the capacitive coupling technology 
 |website=2014 ANSYS Electronic Simulation Expo October 9–10, 2014, Tokyo 
 |publisher=ExH Corporation 
 |date=2014 
 |url=http://www.ansys.com/staticassets/ANSYS/staticassets/resourcelibrary/presentation/aese2014-wireless-power-transmission.pdf 
 |access-date=5 May 2015 
 |url-status=dead 
 |archive-url=https://web.archive.org/web/20150925111819/http://www.ansys.com/staticassets/ANSYS/staticassets/resourcelibrary/presentation/aese2014-wireless-power-transmission.pdf 
 |archive-date=25 September 2015 
 |df=dmy 
}}</ref><ref name=":0">{{cite web|url = http://www.pro-physik.de/details/articlePdf/1102293/issue.html|title = Coupling games in metamaterials|date = 2010|access-date = 18 January 2016|last = Liu|first = Na}}</ref> In this type of circuit, there are two transmitter plates and two receiver plates.  Each transmitter plate is coupled to a receiver plate. The transmitter [[Engineering:Electronic oscillator|oscillator]] drives the transmitter plates in opposite phase (180° phase difference) by a high alternating voltage, and the load is connected between the two receiver plates.  The alternating electric fields induce opposite phase alternating potentials in the receiver plates, and this "push-pull" action causes current to flow back and forth between the plates through the load.  A disadvantage of this configuration for wireless charging is that the two plates in the receiving device must be aligned face to face with the charger plates for the device to work.<ref name="X. Lu" /> 
* '''Longitudinal''' (unipolar) design:<ref name="Gopinath" /><ref name="Huschens" /><ref name=":0" /> In this type of circuit, the transmitter and receiver have only one active electrode, and either the [[Physics:Ground (electricity)|ground]] or a large passive electrode serves as the return path for the current.  The transmitter oscillator is connected between an active and a passive electrode. The load is also connected between an active and a passive electrode. The electric field produced by the transmitter induces alternating charge displacement in the load dipole through [[Physics:Electrostatic induction|electrostatic induction]].<ref>{{cite web
  |url = https://www.google.ch/patents/US20090206675
  |title = Device for transporting energy by partial influence through a dielectric medium 
  |date = 2006
  |access-date = 18 January 2016
  |website = Google.ch/Patents
  |publisher = TMMS Co.
  |last1 = Camurati
  |first1 = Patrick
  |last2 = Bondar
  |first2 = Henri}}</ref>

{{breakafterimages}}

====Resonant capacitive coupling====
Resonance can also be used with capacitive coupling to extend the range.  At the turn of the 20th century, [[Biography:Nikola Tesla|Nikola Tesla]] did the first experiments with both resonant inductive and capacitive coupling.

===Electrodynamic Wireless Power Transfer===
An electrodynamic wireless power transfer (EWPT) system utilizes a receiver with a mechanically resonating or rotating permanent magnet.<ref name="Garraud1">A. Garraud and D. P. Arnold, "Advancements in electrodynamic wireless power transmission", IEEE Sensors Conference, Oct. 2016, 82–84</ref><ref name=Mur-Miranda>J. O. Mur-Miranda, S. Cheng and D. P. Arnold, "Improving the efficiency of electrodynamic wireless power transmission," 2013 7th European Conference on Antennas and Propagation (EuCAP), 2013, pp. 2848-2852.</ref> When subjected to a time-varying magnetic field, the mechanical motion of the resonating magnet is converted into electricity by one or more electromechanical transduction schemes (e.g. [[Engineering:Inductive coupling|electromagnetic/induction]], [[Physics:Piezoelectricity|piezoelectric]], or [[Engineering:Capacitive coupling|capacitive]]).<ref name="Halim">M. A. Halim, S. E. Smith, J. M. Samman and D. P. Arnold, "A High-Performance Electrodynamic Micro-Receiver for Low-Frequency Wireless Power Transfer," 2020 IEEE 33rd International Conference on Micro Electro Mechanical Systems (MEMS), 2020, pp. 590-593, doi: 10.1109/MEMS46641.2020.9056444.</ref><ref name="Spencer">S. E. Smith, M. A. Halim, A. A. Rendon-Hernandez and D. P. Arnold, "Dual-Transduction Electromechanical Receiver for Near-Field Wireless Power Transmission," 2021 IEEE 34th International Conference on Micro Electro Mechanical Systems (MEMS), 2021, pp. 38-41, doi: 10.1109/MEMS51782.2021.9375416.</ref> In contrast to inductive coupling systems which usually use high frequency magnetic fields, EWPT uses low-frequency magnetic fields (<1&nbsp;kHz),<ref name="Truong">Truong, B.D.; Roundy, S. Wireless Power Transfer System with Center-Clamped Magneto-Mechano-Electric (MME) Receiver: Model Validation and Efficiency Investigation. Smart Mater. Struct. 2019, 28, 015004.</ref><ref name="Liu">Liu, G.; Ci, P.; Dong, S. Energy Harvesting from Ambient Low-Frequency Magnetic Field using Magneto-Mechano-Electric Composite Cantilever. Appl. Phys. Lett. 2014, 104, 032908.</ref><ref name="Garraud2">Garraud, N.; Alabi, D.; Varela, J.D.; Arnold, D.P.; Garraud, A. Microfabricated Electrodynamic Wireless Power Receiver for Bio-implants and Wearables. In Proceedings of the 2018 Solid-State Sensor and Actuator Workshop, Hilton Head Island, SC, USA, 3–7 June 2018; pp. 34–37.</ref> which safely pass through conductive media and have higher human field exposure limits (~2 mTrms at 1&nbsp;kHz),<ref name=IEEE1>IEEE. Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3 kHz to 300 GHz; IEEE Standard C95.1-2010; IEEE: Piscataway, NJ, USA, 2010; pp. 1–238.</ref><ref name=IEEE2>IEEE. Standard for Safety Levels with Respect to Human Exposure to Electromagnetic Fields, 0–3 kHz; IEEE Standard C95.6-2002; IEEE: Piscataway, NJ, USA, 2002; pp. 1–43.</ref> showing promise for potential use in wirelessly recharging [[Medicine:Implant|biomedical implants]]. 
For EWPT devices having identical resonant frequencies, the magnitude of power transfer is entirely dependent on critical coupling coefficient, denoted by <math>k</math>, between the transmitter and receiver devices.  For coupled resonators with same resonant frequencies, wireless power transfer between the transmitter and the receiver is spread over three regimes – under-coupled, critically coupled and over-coupled. As the critical coupling coefficient increases from an under-coupled regime (<math>k<k_{crit}</math>) to the critical coupled regime, the optimum voltage gain curve grows in magnitude (measured at the receiver) and peaks when <math>k=k_{crit}</math> and then enters into the over-coupled regime where <math>k>k_{crit}</math> and the peak splits into two.<ref>Stark, Joseph C., Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2004, http://hdl.handle.net/1721.1/18036</ref> This critical coupling coefficient is demonstrated to be a function of distance between the source and the receiver devices.<ref>A.P. Sample, D.T. Meyer and J.R.Smith, "Analysis, Experimental Results, and Range Adaptation of Magnetically Coupled Resonators for Wireless Power Transfer", in ''IEEE Transactions on Industrial Electronics'', Vol 58, No. 2, pp 544-554, Feb 2011.</ref><ref>A. A. Rendon-Hernandez, M. A. Halim, S. E. Smith and D. P. Arnold, "Magnetically Coupled Microelectromechanical Resonators for Low-Frequency Wireless Power Transfer," 2022 IEEE 35th International Conference on Micro Electro Mechanical Systems Conference (MEMS), 2022, pp. 648-651.</ref>

===Magnetodynamic coupling<!-- "Magnetodynamic coupling" doesn't appear to be fundamentally discernible from resonant inductive coupling. Section is poorly sourced. -->===
In this method, power is transmitted between two rotating armatures, one in the transmitter and one in the receiver, which rotate synchronously, coupled together by a [[Physics:Magnetic field|magnetic field]] generated by permanent magnets on the armatures.<ref name="Ashley" />  The transmitter armature is turned either by or as the rotor of an [[Physics:Electric motor|electric motor]], and its magnetic field exerts [[Physics:Torque|torque]] on the receiver armature, turning it.  The magnetic field acts like a mechanical coupling between the armatures.<ref name="Ashley" />  The receiver armature produces power to drive the load, either by turning a separate [[Engineering:Electric generator|electric generator]] or by using the receiver armature itself as the rotor in a generator.

This device has been proposed as an alternative to inductive power transfer for noncontact charging of [[Engineering:Electric vehicle|electric vehicle]]s.<ref name="Ashley" />  A rotating armature embedded in a garage floor or curb would turn a receiver armature in the underside of the vehicle to charge its batteries.<ref name="Ashley" />  It is claimed that this technique can transfer power over distances of 10 to 15&nbsp;cm (4 to 6 inches) with high efficiency, over 90%.<ref name="Ashley" /><ref name=Shahan>{{cite web|last1=Shahan|first1=Zach|title=ELIX Wireless Rolls Out A 10kW Wireless EV Charger With 92% Efficiency|url=http://evobsession.com/elix-wireless-rolls-out-a-10kw-wireless-ev-charger-with-92-efficiency/|website=EVObsession.com|access-date=20 July 2015}}</ref>  Also, the low frequency stray magnetic fields produced by the rotating magnets produce less [[Engineering:Electromagnetic interference|electromagnetic interference]] to nearby electronic devices than the high frequency magnetic fields produced by inductive coupling systems.  A prototype system charging electric vehicles has been in operation at University of British Columbia since 2012.  Other researchers, however, claim that the two energy conversions (electrical to mechanical to electrical again) make the system less efficient than electrical systems like inductive coupling.<ref name="Ashley" />

=== Zenneck Wave Transmission===
A new kind of system using the [[Physics:Zenneck wave|Zenneck type waves]] was shown  by Oruganti et al., where they demonstrated that it was possible to excite Zenneck wave type waves on flat metal-air interfaces and transmit power across metal obstacles.<ref name="auto">{{cite journal |last1=Oruganti |first1=Sai Kiran |last2=Liu |first2=Feifei |last3=Paul |first3=Dipra |last4=Liu |first4=Jun |last5=Malik |first5=Jagannath |last6=Feng |first6=Ke |last7=Kim |first7=Haksun |last8=Liang |first8=Yuming |last9=Thundat |first9=Thomas |last10=Bien |first10=Franklin |title=Experimental Realization of Zenneck Type Wave-based Non-Radiative, Non-Coupled Wireless Power Transmission |journal=Scientific Reports |date=22 January 2020 |volume=10 |issue=1 |pages=925 |doi=10.1038/s41598-020-57554-1 |pmid=31969594 |pmc=6976601 |bibcode=2020NatSR..10..925O }}</ref><ref>{{cite journal|first1=S. K.|last1=Oruganti|first2=A.|last2=Khosla|first3=T. G.|last3=Thundat|title=Wireless Power-Data Transmission for Industrial Internet of Things: Simulations and Experiments|journal=IEEE Access|volume=8|pages=187965–187974|year=2020|doi=10.1109/ACCESS.2020.3030658|bibcode=2020IEEEA...8r7965O |s2cid=225049658|doi-access=free}}
</ref><ref>{{cite journal|last1=Paul|first1=D.|last2=Oruganti|first2=S. K.|last3=Khosla|first3=A.|year=2020|title=Modelling of Zenneck Wave Transmission System in Super High Frequency spectrum|journal=SPAST Express|volume=1 |issue=1 |url=https://spast.org/ojspath/article/view/4}}
</ref>
Here the idea is to excite a localized charge oscillation at the metal-air interface, the resulting modes propagate along the metal-air interface.<ref name="auto"/>

{{anchor|radiative_far_field}}

==Far-field (radiative) techniques==
[[Physics:Near and far field|Far field]] methods achieve longer ranges, often multiple kilometer ranges, where the distance is much greater than the diameter of the device(s). High-[[Engineering:Directivity|directivity]] antennas or well-collimated laser light produce a beam of energy that can be made to match the shape of the receiving area. The maximum directivity for antennas is physically limited by [[Physics:Diffraction|diffraction]].

In general, [[Physics:Light beam|visible light]] (from lasers) and microwaves (from purpose-designed antennas) are the forms of electromagnetic radiation best suited to energy transfer.

The dimensions of the components may be dictated by the distance from [[Engineering:Transmitter|transmitter]] to receiver, the [[Wavelength|wavelength]] and the Rayleigh criterion or [[Physics:Diffraction|diffraction]] limit, used in standard [[Engineering:Radio frequency|radio frequency]] [[Engineering:Antenna (radio)|antenna]] design, which also applies to lasers. Airy's diffraction limit is also frequently used to determine an approximate spot size at an arbitrary distance from the [[Astronomy:Aperture|aperture]]. Electromagnetic radiation experiences less diffraction at shorter wavelengths (higher frequencies); so, for example, a blue laser is diffracted less than a red one.

The Rayleigh limit (also known as the [[Biography:Ernst Abbe|Abbe diffraction limit]]), although originally applied to image resolution, can be viewed in reverse, and dictates that the [[Physics:Irradiance|irradiance]] (or ''intensity'') of any electromagnetic wave (such as a microwave or laser beam) will be reduced as the beam diverges over distance at a minimum rate inversely proportional to the aperture size. The larger the ratio of a transmitting antenna's [[Engineering:Antenna aperture|aperture]] or laser's exit aperture to the [[Wavelength|wavelength]] of radiation, the more can the radiation be concentrated in a  [[Engineering:Beamwidth|compact beam]]

Microwave power beaming can be more efficient{{clarify|date=September 2020}} than lasers, and is less prone to atmospheric [[Physics:Attenuation|attenuation]] caused by dust or [[Chemistry:Aerosol|aerosol]]s such as fog.

Here, the power levels are calculated by combining the above parameters together, and adding in the [[Engineering:Antenna gain|gains]] and [[Physics:Attenuation|losses]] due to the antenna characteristics and the transparency and [[Physics:Dispersion relation|dispersion]] of the medium through which the radiation passes. That process is known as calculating a link budget.

===Microwaves===
[[File:Suntower.jpg|thumb|right|An artist's depiction of a solar satellite that could send energy by microwaves to a space vessel or planetary surface.]]
Power transmission via radio waves can be made more directional, allowing longer-distance power beaming, with shorter wavelengths of electromagnetic radiation, typically in the microwave range.<ref name="Massa2013">{{cite journal |first1= A. Massa, G. Oliveri, F. Viani, and P. Rocca| title= Array designs for long-distance wireless power transmission – State-of-the-art and innovative solutions | journal= Proceedings of the IEEE|date=June 2013 | volume=101 |issue=6 |pages= 1464–1481 |doi= 10.1109/JPROC.2013.2245491| last1= Massa | last2= Oliveri | first2= Giacomo | last3= Viani | first3= Federico | last4= Rocca | first4= Paolo | s2cid= 2990114 }}</ref> A [[Physics:Rectenna|rectenna]] may be used to convert the microwave energy back into electricity. Rectenna conversion efficiencies exceeding 95% have been realized.{{citation needed|date=September 2018}} Power beaming using microwaves has been proposed for the transmission of energy from orbiting solar power satellites to Earth and the [[Astronomy:Beam-powered propulsion|beaming of power to spacecraft]] leaving orbit has been considered.<ref name=space>{{cite book | last1 = Landis | first1 = G. A. | title = Laser Power Beaming | chapter = Applications for space power by laser transmission | editor-first1 = Jack V. | editor-first2 = Edward E. | editor-last1 = Walker | editor-last2 = Montgomery Iv | year = 1994  | volume = 2121 | pages = 252–255 | doi = 10.1117/12.174188  | bibcode = 1994SPIE.2121..252L | s2cid = 108775324 }}</ref><ref>{{cite journal|first1=G.|last1=Landis|title=Space Transfer With Ground-Based Laser/Electric Propulsion|journal=NASA Technical Memorandum|year=1992|doi=10.2514/6.1992-3213|url=https://arc.aiaa.org/doi/abs/10.2514/6.1992-3213|hdl=2060/19930011426|s2cid=109847404 |hdl-access=free}}</ref>

Power beaming by microwaves has the difficulty that, for most space applications, the required aperture sizes are very large due to [[Physics:Diffraction|diffraction]] limiting antenna directionality. For example, the 1978 [[Organization:NASA|NASA]] study of solar power satellites required a {{convert|1|km|mi|adj=mid|-diameter}} transmitting antenna and a {{convert|10|km|mi|adj=mid|-diameter}} receiving rectenna for a microwave beam at 2.45&nbsp;GHz.<ref>{{cite book |doi=10.1109/WCPEC.2006.279877 |year=2006 |last1=Landis |first1=Geoffrey |title=2006 IEEE 4th World Conference on Photovoltaic Energy Conference |chapter=RE-Evaluating Satellite Solar Power Systems for Earth |pages=1939–1942 |isbn=1-4244-0016-3 |s2cid=22181565 |hdl=2060/20070005136 |hdl-access=free }}</ref> These sizes can be somewhat decreased by using shorter wavelengths, although short wavelengths may have difficulties with atmospheric absorption and beam blockage by rain or water droplets. Because of the "[[Physics:Thinned-array curse|thinned-array curse]]", it is not possible to make a narrower beam by combining the beams of several smaller satellites.

For earthbound applications, a large-area 10&nbsp;km diameter receiving array allows large total power levels to be used while operating at the low power density suggested for human electromagnetic exposure safety. A human safe power density of 1&nbsp;mW/cm<sup>2</sup> distributed across a 10&nbsp;km diameter area corresponds to 750 megawatts total power level. This is the power level found in many modern electric power plants. For comparison, a solar PV farm of similar size might easily exceed 10,000 megawatts (rounded) at best conditions during daytime.

Following World War II, which saw the development of high-power microwave emitters known as cavity magnetrons, the idea of using microwaves to transfer power was researched. By 1964, a miniature helicopter propelled by microwave power had been demonstrated.<ref>{{cite web|url=http://stinet.dtic.mil/oai/oai?&verb=getRecord&metadataPrefix=html&identifier=AD0474925|title=Experimental Airborne Microwave Supported Platform|archive-url=https://web.archive.org/web/20100302204238/http://stinet.dtic.mil/oai/oai?&verb=getRecord&metadataPrefix=html&identifier=AD0474925 |archive-date=2 March 2010|url-status=dead|quote=Final rept. Jun 64 – Apr 65}}</ref>

Japanese researcher Hidetsugu Yagi also investigated wireless energy transmission using a directional array antenna that he designed. In February 1926, Yagi and his colleague Shintaro Uda published their first paper on the tuned high-gain directional array now known as the Yagi antenna. While it did not prove to be particularly useful for power transmission, this beam antenna has been widely adopted throughout the broadcasting and wireless telecommunications industries due to its excellent performance characteristics.<ref name="Yagi">{{cite web |url=http://ieee.cincinnati.fuse.net/reiman/05_2004.htm |title=Scanning the Past: A History of Electrical Engineering from the Past, Hidetsugu Yagi |publisher=Ieee.cincinnati.fuse.net |access-date=4 June 2009 |url-status=dead |archive-url=https://web.archive.org/web/20090611154616/http://ieee.cincinnati.fuse.net/reiman/05_2004.htm |archive-date=11 June 2009 |df=dmy-all }}</ref>

Wireless high power transmission using microwaves is well proven. Experiments in the tens of kilowatts have been performed at the [[Astronomy:Goldstone Deep Space Communications Complex|Goldstone Deep Space Communications Complex]] in California in 1975<ref name=autogenerated3>{{cite web|url=http://www.spaceislandgroup.com/solarspace.html|archive-url=https://web.archive.org/web/20220122200330/http://www.spaceislandgroup.com/solarspace.html|archive-date=2022-01-22 |title=Space Solar Energy Initiative |publisher=Space Island Group |access-date=4 June 2009}}</ref><ref name=autogenerated1>{{cite journal|url=http://www.sspi.gatech.edu/wptshinohara.pdf|title=Wireless Power Transmission for Solar Power Satellite (SPS)|edition=Second Draft|first=N.|last=Shinohara|journal=Space Solar Power Workshop|publisher=Georgia Institute of Technology}}</ref><ref name="Brown1984">{{cite journal |last1=Brown |first1=W.C. |title=The History of Power Transmission by Radio Waves |journal=IEEE Transactions on Microwave Theory and Techniques |date=September 1984 |volume=32 |issue=9 |pages=1230–1242 |doi=10.1109/TMTT.1984.1132833 |bibcode=1984ITMTT..32.1230B |s2cid=73648082 }}</ref> and more recently (1997) at Grand Bassin on Reunion Island.<ref>{{cite web|url=http://www2.univ-reunion.fr/~lcks/Old_Version/PubIAF97.htm|archive-url=https://web.archive.org/web/20051023080942/http://www2.univ-reunion.fr/~lcks/Old_Version/PubIAF97.htm|archive-date=2005-10-23|title=POINT-TO-POINT WIRELESS POWER TRANSPORTATION IN REUNION ISLAND|work=48th International Astronautical Congress|location=Turin, Italy|date=6–10 October 1997|first1=J. D.|last1=Lan Sun Luk|first2=A.|last2=Celeste|first3=P.|last3=Romanacce|first4=L.|last4=Chane Kuang Sang|first5=J. C.|last5=Gatina|publisher=University of La Réunion – Faculty of Science and Technology}}</ref> These methods achieve distances on the order of a kilometer.

Under experimental conditions, microwave conversion efficiency was measured to be around 54% across one meter.<ref>{{cite journal |last1=Brown |first1=W.C. |last2=Eves |first2=E.E. |title=Beamed microwave power transmission and its application to space |journal=IEEE Transactions on Microwave Theory and Techniques |date=June 1992 |volume=40 |issue=6 |pages=1239–1250 |doi=10.1109/22.141357 |bibcode=1992ITMTT..40.1239B }}</ref>

A change to 24&nbsp;GHz has been suggested as microwave emitters similar to LEDs have been made with very high quantum efficiencies using [[Physics:Negative resistance|negative resistance]], i.e., Gunn or IMPATT diodes, and this would be viable for short range links.

In 2013, inventor Hatem Zeine demonstrated how wireless power transmission using phased array antennas can deliver electrical power up to 30 feet. It uses the same radio frequencies as WiFi.<ref>{{Cite web|url=https://newatlas.com/cota-ossia-wireless-charging-microwave-phased-array/29217/|title=Cota system transmits power wirelessly at up to 30 feet|website=newatlas.com|language=en|access-date=2018-01-05|date=30 September 2013}}</ref><ref>{{Cite news|url=https://techcrunch.com/2013/09/09/cota-by-ossia-wireless-power/|title=Cota By Ossia Aims To Drive A Wireless Power Revolution And Change How We Think About Charging|last=Etherington|first=Darrell|work=TechCrunch|access-date=2018-01-05|language=en}}</ref>

In 2015, researchers at the University of Washington introduced power over Wi-Fi, which trickle-charges batteries and powered battery-free cameras and temperature sensors using transmissions from Wi-Fi routers.<ref name=powifi>{{cite arXiv|eprint=1505.06815|title=Powering the Next Billion Devices with Wi-Fi  |last1=Talla  |first1=Vamsi  |last2=Kellogg  |first2=Bryce  |last3=Ransford  |first3=Benjamin  |last4=Naderiparizi  |first4=Saman  |last5=Gollakota  |first5=Shyamnath  |last6= Smith  |first6=Joshua R.  |class=cs.NI  |year=2015  }}</ref><ref>{{Cite web|url=https://www.technologyreview.com/s/538031/first-demonstration-of-a-surveillance-camera-powered-by-ordinary-wi-fi-broadcasts/|title=First Demonstration of a Surveillance Camera Powered by Ordinary Wi-Fi Broadcasts|last=arXiv|first=Emerging Technology from the|access-date=2016-09-28}}</ref> Wi-Fi signals were shown to power battery-free temperature and camera sensors at ranges of up to 20 feet. It was also shown that Wi-Fi can be used to wirelessly trickle-charge nickel–metal hydride and lithium-ion coin-cell batteries at distances of up to 28 feet.

In 2017, the Federal Communications Commission (FCC) certified the first mid-field radio frequency (RF) transmitter of wireless power.<ref>{{Cite web|url=https://ir.energous.com/press-releases/detail/596/energous-receives-industry-first-fcc-certification-for|title=Energous Receives Industry-First FCC Certification for Over-the-Air, Power-at-a-Distance Wireless Charging :: Energous Corporation (WATT)|website=Energous Corporation|language=en-US|access-date=2018-01-05}}</ref>  In 2021 the FCC granted a license to a over-the-air (OTA) wireless charging system that combines near-field and far-field methods by using a frequency of about 900&nbsp;MHz. Due to the radiated power of about 1 W this system is intended for small [[Internet of things|IoT]] devices as various sensors, trackers, detectors and monitors.<ref>{{Cite web|last=Emilio|first=Maurizio Di Paolo|date=2021-11-08|title=Energous Enables Wireless Power Transfer Solutions at any Distance for U.S. and Europe|url=https://www.eetimes.eu/energous-enables-wireless-power-transfer-solutions-at-any-distance-for-u-s-and-europe/|access-date=2021-11-11|website=EE Times Europe|language=en-US}}</ref>

===Lasers===
[[File:Laser-powered model airplane.jpg|thumb|A laser beam centered on a panel of photovoltaic cells provides enough power to a lightweight model airplane for it to fly.]]

In the case of electromagnetic radiation closer to the visible region of the spectrum (.2 to 2 micrometers), power can be transmitted by converting electricity into a [[Physics:Laser|laser]] beam that is received and concentrated onto photovoltaic cells (solar cells).<ref name="FraunhoferISE">{{cite web |url=https://www.ise.fraunhofer.de/en/business-areas/iii-v-and-concentrator-photovoltaics/research-topics/power-by-light|title=Power-by-Light|website=Fraunhofer ISE}}</ref><ref name="OpticalWPT">{{Cite book |doi= 10.1109/ICSOS.2011.5783662 |isbn= 978-1-4244-9686-0|chapter= Optical wireless power transmission at long wavelengths|title= 2011 International Conference on Space Optical Systems and Applications (ICSOS)|pages= 164–170|year= 2011|last1= Sahai|first1= Aakash|last2= Graham|first2= David|s2cid= 18985866}}</ref> This mechanism is generally known as 'power beaming' because the power is beamed at a receiver that can convert it to electrical energy. At the receiver, special photovoltaic laser power converters which are optimized for monochromatic light conversion are applied.<ref name="Bett2008">{{Cite book |doi= 10.1109/pvsc.2008.4922910  | isbn= 978-1-4244-1640-0| chapter= III–V solar cells under monochromatic illumination| title= 2008 33rd IEEE Photovolatic Specialists Conference| pages= 1–5| year= 2008| last1= Bett| first1= Andreas W.| last2= Dimroth| first2= Frank| last3= Lockenhoff| first3= Rudiger| last4= Oliva| first4= Eduard| last5= Schubert| first5= Johannes| s2cid= 21042923}}</ref>

Advantages compared to other wireless methods are:<ref>{{cite news|url=https://www.theguardian.com/science/2009/jan/04/wireless-power-technology-witricity|title=Wireless power spells end for cables|work=The Observer |location=London |date=4 January 2009  | first=David | last=Smith}}</ref>
* Collimated monochromatic [[Wavefront|wavefront]] propagation allows narrow beam cross-section area for transmission over large distances. As a result, there is little or no reduction in power when increasing the distance from the transmitter to the receiver.
* Compact size: solid state lasers fit into small products.
* No radio-frequency interference to existing radio communication such as [[Wi-Fi]] and cell phones.
* Access control: only receivers hit by the laser receive power.

Drawbacks include:
* Laser radiation is hazardous. Without a proper safety mechanism, low power levels can blind humans and other animals. High power levels can kill through localized spot heating.
* Conversion between electricity and light is limited. Photovoltaic cells achieve a maximum of 40%–50% efficiency.<ref>{{cite web|url=http://www.laserfocusworld.com/display_article/245124/12/ARCHI/none/Feat/PHOTONIC-FRONTIERS:-Photonic-power-delivery:-Photonic-power-conversion-delivers-power-via-laser-beam |title=power transmission via lasers |publisher=Laserfocusworld.com |access-date=4 June 2009}}</ref> 
* Atmospheric absorption, and absorption and scattering by clouds, fog, rain, etc., causes up to 100% losses.
* Requires a direct line of sight with the target. (Instead of being beamed directly onto the receiver, the laser light can also be guided by an optical fiber. Then one speaks of power-over-fiber technology.)

Laser 'powerbeaming' technology was explored in [[Physics:Directed-energy weapon|military weapons]]<ref>{{cite web|last=Skillings |first=Jonathan |url=http://news.cnet.com/8301-11386_3-10024153-76.html |title=Laser weapons: A distant target, CNET news August 23, 2008 1:41&nbsp;pm PDT |publisher=News.cnet.com |date=23 August 2008 |access-date=4 June 2009}}</ref><ref>{{cite web|url=http://www.defensetech.org/2006/01/12/laser-weapons-almost-ready-not/ |title=Laser Weapons "Almost Ready?" Not! |publisher=Defensetech.org |access-date=4 June 2009|date=2006-01-12 }}</ref><ref>{{cite web|url=http://www.army.mil/-news/2009/01/30/16279-white-sands-testing-new-laser-weapon-system/ |title=White Sands testing new laser weapon system, US Army.mil, 30 Jan 2009 |publisher=Army.mil |date=30 January 2009 |access-date=4 June 2009}}</ref> and [[Engineering:Laser propulsion|aerospace]]<ref>{{cite web|url=http://www.defensetech.org/2003/11/06/lasers-power-planes-drones/ |title=Lasers Power Planes, Drones |publisher=Defensetech.org |access-date=4 June 2009|date=2003-11-06 }}</ref><ref>{{cite news|url=http://www.space.com/businesstechnology/051024_spaceelevator_challenge.html |title=Riding a Beam of Light: NASA's First Space Elevator Competition Proves Highly Challenging |first=Roger G. |last=Gilbertson |work=Space.com |date=24 October 2005 |access-date=4 June 2009}}</ref> applications. Also, it is applied for the powering of various kinds of sensors in industrial environments. Lately, it is developed for powering commercial and [[Engineering:Consumer electronics|consumer electronics]]. Wireless energy transfer systems using lasers for consumer space have to satisfy laser safety requirements standardized under IEC 60825.{{citation needed|date=September 2013}}

The first wireless power system using lasers for consumer applications was demonstrated in 2018, capable of delivering power to stationary and moving devices across a room. This wireless power system complies with safety regulations according to IEC 60825 standard. It is also approved by the US Food and Drugs Administration (FDA).<ref>{{Cite news|url=https://www.businesswire.com/news/home/20180110006324/en/Wi-Charge-Wins-CES-2018-Innovation-Award#new_tab|title=Wi-Charge Wins CES 2018 Best of Innovation Award|access-date=2018-03-12|language=en}}</ref>

Other details include [[Physics:Diffraction#Propagation of a laser beam|propagation]],<ref>{{cite web|url=http://www.ieee.org/organizations/pubs/newsletters/leos/oct05/free_space.html |title=Free-Space Laser Propagation: Atmospheric Effects |publisher=Ieee.org |access-date=4 June 2009}}<br />[https://web.archive.org/web/20060317124222/http://www.mellesgriot.com/pdf/CatalogX/X_36_6-9.pdf Propagation Characteristics of Laser Beams – Melles Griot catalog]<br />{{cite book|url={{Google books|id=4NXHYg70qqIC|plainurl=yes}} |title=L. C. Andrews and R. L. Phillips, Laser Beam Propagation through Random Media, 2nd ed. (SPIE Press, 2005) |access-date=4 June 2009 | isbn=978-0-8194-5948-0|year=2005|last1=Andrews |first1=Larry C |last2=Phillips |first2=Ronald L |publisher=SPIE Press }}</ref> and the coherence and the range limitation problem.<ref>{{cite web|author=Dr. Rüdiger Paschotta |url=https://www.rp-photonics.com/coherence.html |title=An explanation of Coherence |publisher=Rp-photonics.com |access-date=4 June 2009}}</ref>

[[Biography:Geoffrey A. Landis|Geoffrey Landis]]<ref>{{cite web|url=http://www.islandone.org/Settlements/EvolutionaryPathSPS.html |title=An Evolutionary Path to SPS |publisher=Islandone.org |access-date=4 June 2009}}</ref><ref>{{cite web|url=http://www.geoffreylandis.com/supersynch.html |title=A Supersynchronous SPS |publisher=Geoffreylandis.com |date=28 August 1997 |access-date=4 June 2009}}</ref><ref>{{cite journal|url=http://www.sff.net/people/Geoffrey.Landis/papers.html |title=Papers Relating to Space Photovoltaic Power, Power beaming, and Solar Power Satellites |journal=Astrobiology |volume=1 |issue=2 |pages=161–4 |doi=10.1089/153110701753198927 |pmid=12467119 |access-date=4 June 2009|year=2001 |last1=Landis |first1=Geoffrey A. |bibcode=2001AsBio...1..161L }}</ref> is one of the pioneers of solar power satellites<ref>{{cite web|url=http://www.nss.org/settlement/ssp/ |title=Limitless clean energy from space |publisher=Nss.org |access-date=4 June 2009}}</ref> and laser-based transfer of energy, especially for space and lunar missions. The demand for safe and frequent space missions has resulted in proposals for a laser-powered [[Astronomy:Space elevator|space elevator]].<ref>{{cite web |url=http://www.spaceward.org/elevator2010-pb |title=Power Beaming (Climber) Competition |publisher=Spaceward.org |access-date=4 June 2009 |url-status=dead |archive-url=https://web.archive.org/web/20090724060700/http://www.spaceward.org/elevator2010-pb |archive-date=24 July 2009 |df=dmy }}</ref><ref>{{cite web|url=http://www.spaceelevator.com/ |title=From Concept to Reality |publisher=The Space Elevator |access-date=4 June 2009}}<br />{{cite web |date=31 January 2009 |url=http://crnano.typepad.com/crnblog/2009/01/space-elevator-tethers-coming-closer.html |title=Space Elevator Tethers Coming Closer |publisher=Crnano.typepad.com |access-date=4 June 2009}}</ref>

NASA's Dryden Flight Research Center has demonstrated a lightweight unmanned model plane powered by a laser beam.<ref>{{cite web|url=http://www.nasa.gov/centers/dryden/news/FactSheets/FS-087-DFRC.html |title=Dryden Flight Research Center, Beamed Laser Power For UAVs |publisher=Nasa.gov |date=7 May 2008 |access-date=4 June 2009}}</ref> This proof-of-concept demonstrates the feasibility of periodic recharging using a laser beam system.

Scientists from the Chinese Academy of Sciences have developed a proof-of-concept of utilizing a dual-wavelength laser to wirelessly charge portable devices or UAVs.<ref>{{cite journal|title=A coupled model on energy conversion in laser power beaming|journal=Journal of Power Sources|volume=393|pages=211–216|date=15 May 2018 |doi=10.1016/j.jpowsour.2018.05.010|last1=Wu|first1=Chen-Wu|last2=Wang|first2=Jyhwen|last3=Huang|first3=Chen-Guang|url=http://dspace.imech.ac.cn/handle/311007/77622|bibcode=2018JPS...393..211W|s2cid=104165547 }}</ref>

==Atmospheric plasma channel coupling==

In atmospheric plasma channel coupling, energy is transferred between two electrodes by electrical conduction through ionized air.<ref name="Nawaz">{{Cite journal|last1=Nawaz|first1=Suddiyas|title=Wireless Power Transmission|url=https://www.academia.edu/9539890|website=Academia 2015|access-date=31 December 2015}}</ref> When an electric field gradient exists between the two electrodes, exceeding 34 kilovolts per centimeter at sea level atmospheric pressure, an electric arc occurs.<ref name="Ray_2009">{{Cite book|title = An Introduction to High Voltage Engineering|last = Ray|first = Subir|publisher = PHI Learning|year = 2009|isbn = 978-8120324176|pages = 19–21|url = https://books.google.com/books?isbn=812032417X}}</ref> This atmospheric dielectric breakdown results in the flow of electric current along a random trajectory through an ionized [[Physics:Plasma channel|plasma channel]] between the two electrodes. An example of this is natural lightning, where one electrode is a virtual point in a cloud and the other is a point on Earth. Laser Induced Plasma Channel (LIPC) research is presently underway using ultrafast lasers to artificially promote development of the plasma channel through the air, directing the electric arc, and guiding the current across a specific path in a controllable manner.<ref>{{cite web|title=Electrolaser|website=WiseGeek website|publisher=Conjecture Corp.|date=2015|url=http://www.wisegeek.com/what-is-an-electrolaser.htm|access-date=25 October 2015}}</ref> The laser energy reduces the atmospheric dielectric breakdown voltage and the air is made less insulating by superheating, which lowers the density (<math>p</math>) of the filament of air.<ref name="Scheller">{{cite journal | last1 = Scheller | first1 = Maik | last2 = Born | first2 = Norman | last3 = Cheng | first3 = Weibo | last4 = Polynkin | first4 = Pavel | year = 2014| title = Channeling the electrical breakdown of air by optically heated plasma filaments | journal = Optica | volume = 1 | issue = 2| pages = 125–128 | doi = 10.1364/OPTICA.1.000125 | bibcode = 2014Optic...1..125S | doi-access = free }}</ref>

This new process is being explored for use as a laser lightning rod and as a means to trigger lightning bolts from clouds for natural lightning channel studies,<ref name="Rakov">{{cite book|last1=Rakov|first1=Vladimir A.|last2=Uman|first2=Martin A.|title=Lightning: Physics and Effects|publisher=Cambridge Univ. Press|date=2003|pages=296–298|url=https://books.google.com/books?id=TuMa5lAa3RAC&q=laser&pg=PA296|isbn=978-0521035415}}</ref> for artificial atmospheric propagation studies, as a substitute for conventional radio antennas,<ref name="Stahman_1964">{{cite web|last1=Stahmann|first1=J. R.|title=LASER TYPE ULTRA-VIOLET RADIATION FEASIBILITY FOR LIGHTNING AND ATMOSPHERIC PROPAGATION STUDIES|url=http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=AD0609217|archive-url=https://web.archive.org/web/20160126215236/http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=AD0609217|url-status=dead|archive-date=26 January 2016|website=DEFENSE TECHNICAL INFORMATION CENTER OAI|publisher=LIGHTNING AND TRANSIENTS RESEARCH INST ST PAUL MN|access-date=16 January 2016|date=Oct 1964}}</ref> for applications associated with electric welding and machining,<ref name="Lawrence">{{cite book|last1=Lawrence|first1=Jonathan R.|last2=Waugh|first2=D.|title=Laser Surface Engineering: Processes and Applications|publisher=Elsevier|date=2014|pages=456–460|url=https://books.google.com/books?id=n297AwAAQBAJ&q=%22laser+guided+discharge%22&pg=PA456|isbn=978-1782420798}}</ref><ref name="Forestier">{{cite journal |last1=Forestier |first1=B. |last2=Houard |first2=A. |last3=Revel |first3=I. |last4=Durand |first4=M. |last5=André |first5=Y. B. |last6=Prade |first6=B. |last7=Jarnac |first7=A. |last8=Carbonnel |first8=J. |last9=Le Nevé |first9=M. |last10=de Miscault |first10=J. C. |last11=Esmiller |first11=B. |last12=Chapuis |first12=D. |last13=Mysyrowicz |first13=A. |title=Triggering, guiding and deviation of long air spark discharges with femtosecond laser filament |journal=AIP Advances |date=March 2012 |volume=2 |issue=1 |pages=012151 |doi=10.1063/1.3690961 |bibcode=2012AIPA....2a2151F |doi-access=free }}</ref> for diverting power from high-voltage capacitor discharges, for [[Physics:Directed-energy weapon|directed-energy weapon]] applications employing electrical conduction through a ground return path,<ref name="Giulietti">{{cite book |doi=10.1007/978-3-642-03825-9_6 |chapter=On Lightning Control Using Lasers |title=Progress in Ultrafast Intense Laser Science |series=Springer Series in Chemical Physics |year=2010 |last1=Kasparian |first1=Jérôme |last2=Wolf |first2=Jean-Pierre |volume=98 |pages=109–122 |isbn=978-3-642-03824-2 }}</ref><ref name="Franklin">{{cite book|last1=Franklin|first1=Steve|title=Non-Lethal Weapon Handbook|publisher=Digital Services|date=2015|pages=161–162|url=http://www.rottys.eu/download-pdf-non-lethal-weapon-handbook-book-by-digital-services.pdf}}</ref><ref name="Quick_2012">{{cite web|last1=Quick|first1=Darren|title=U.S. Army weapon shoots lightning bolts down laser beams|url=http://www.gizmag.com/laser-induced-plasma-channel/23117/|website=Gizmag|publisher=Gizmag Limited|access-date=16 January 2016|date=28 June 2012}}</ref><ref name="Kaneshiro">{{cite web|last=Kaneshiro|first=Jason|title=Picatinny engineers set phasers to 'fry'|website=news Archives|publisher=US Army official website www.mil.gov|date=21 June 2012|url=https://www.army.mil/article/82262/Picatinny_engineers_set_phasers_to__fry_/|access-date=25 October 2015}}</ref> and [[Radio jamming|electronic jamming]].<ref name="Clerici">{{cite journal |last1=Clerici |first1=Matteo |last2=Hu |first2=Yi |last3=Lassonde |first3=Philippe |last4=Milián |first4=Carles |last5=Couairon |first5=Arnaud |last6=Christodoulides |first6=Demetrios N. |last7=Chen |first7=Zhigang |last8=Razzari |first8=Luca |last9=Vidal |first9=François |last10=Légaré |first10=François |last11=Faccio |first11=Daniele |last12=Morandotti |first12=Roberto |title=Laser-assisted guiding of electric discharges around objects |journal=Science Advances |date=June 2015 |volume=1 |issue=5 |pages=e1400111 |doi=10.1126/sciadv.1400111 |pmid=26601188 |pmc=4640611 |bibcode=2015SciA....1E0111C }}</ref>

==Energy harvesting==
{{main|Engineering:Energy harvesting}}
In the context of wireless power, ''[[Engineering:Energy harvesting|energy harvesting]]'', also called ''power harvesting'' or ''energy scavenging'', is the conversion of ambient energy from the environment to electric power, mainly to power small autonomous wireless electronic devices.<ref name="Beeby">{{cite book
 | last1  = Beeby
 | first1 = Stephen
 | last2  = White
 | first2 = Neil
 | title  = Energy Harvesting for Autonomous Systems
 | publisher = Artech House
 | date   = 2010
 | pages  = 1–2
 | url    = https://books.google.com/books?id=7H9xdFd4sikC&q=%22energy+harvesting%22&pg=PA1
 | isbn   = 978-1596937192
 }}</ref>  The ambient energy may come from stray electric or magnetic fields or radio waves  from nearby electrical equipment, light, [[Physics:Thermal energy|thermal energy]] (heat), or [[Physics:Kinetic energy|kinetic energy]] such as vibration or motion of the device.<ref name="Beeby" />  Although the efficiency of conversion is usually low and the power gathered often minuscule (milliwatts or microwatts),<ref name="Beeby" /> it can be adequate to run or recharge small micropower wireless devices such as remote sensors, which are proliferating in many fields.<ref name="Beeby" />  This new technology is being developed to eliminate the need for battery replacement or charging of such wireless devices, allowing them to operate completely autonomously.<ref>{{cite book |doi=10.1145/2486001.2486015 |chapter=Ambient backscatter |title=Proceedings of the ACM SIGCOMM 2013 conference on SIGCOMM |year=2013 |last1=Liu |first1=Vincent |last2=Parks |first2=Aaron |last3=Talla |first3=Vamsi |last4=Gollakota |first4=Shyamnath |last5=Wetherall |first5=David |last6=Smith |first6=Joshua R. |pages=39–50 |isbn=9781450320566 |s2cid=6730636 }}</ref><ref>{{cite journal |last1=Kang |first1=X. |last2=Ho |first2=C. K. |last3=Sun |first3=S. |title=Full-Duplex Wireless-Powered Communication Network With Energy Causality |journal=IEEE Transactions on Wireless Communications |date=October 2015 |volume=14 |issue=10 |pages=5539–5551 |doi=10.1109/TWC.2015.2439673 |arxiv=1404.0471 |s2cid=2445116 }}</ref>

==History==
===19th century developments and dead ends===
The 19th century saw many developments of theories, and counter-theories on how electrical energy might be transmitted. In 1826, [[Biography:André-Marie Ampère|André-Marie Ampère]] discovered a connection between current and magnets.  [[Biography:Michael Faraday|Michael Faraday]] described in 1831 with his [[Physics:Faraday's law of induction|law of induction]] the [[Physics:Electromotive force|electromotive force]] driving a current in a conductor loop by a time-varying magnetic flux. Transmission of electrical energy without wires was  observed by many inventors and experimenters,<ref>[[Biography:Luigi Galvani|Luigi Galvani]] (1791), Peter Samuel Munk (1835), [[Biography:Joseph Henry|Joseph Henry]] (1842), Samuel Alfred Varley (1852), Edwin Houston, Elihu Thomson, [[Biography:Thomas Edison|Thomas Edison]] (1875) and [[Biography:David Edward Hughes|David Edward Hughes]] (1878)</ref><ref>{{cite book|first1=T. K.|last1=Sarkar|first2=Robert|last2=Mailloux|first3=Arthur A.|last3=Oliner|first4=M.|last4=Salazar-Palma|first5=Dipak L.|last5=Sengupta|title=History of Wireless|publisher=John Wiley & Sons|year=2006|pages=258–261|isbn=9780471718147|author-link3=Arthur A. Oliner}}</ref><ref>Christopher H. Sterling, Encyclopedia of Radio 3-Volume, Routledge - 2004, page 831</ref> but lack of a coherent theory attributed these phenomena vaguely to [[Physics:Electromagnetic induction|electromagnetic induction]].<ref>{{cite book|first=W. Bernard|last=Carlson|title=Innovation as a Social Process: Elihu Thomson and the Rise of General Electric|publisher=Cambridge University Press|year=2003|pages=57–58}}</ref> A concise explanation of these phenomena would come from the 1860s [[Physics:Maxwell's equations|Maxwell's equations]]<ref name="Shinohara">{{cite book|url=https://books.google.com/books?id=TwegAgAAQBAJ&pg=PP11|last=Shinohara|year=2014|title=Wireless Power Transfer via Radiowaves|page=11|publisher=John Wiley & Sons |isbn=9781118862964}}</ref> by [[Biography:James Clerk Maxwell|James Clerk Maxwell]], establishing a theory that unified electricity and magnetism to [[Physics:Electromagnetism|electromagnetism]], predicting the existence of electromagnetic waves as the "wireless" carrier of electromagnetic energy. Around 1884 John Henry Poynting defined the [[Poynting vector]] and gave [[Physics:Poynting's theorem|Poynting's theorem]], which describe the flow of power across an area within [[Physics:Electromagnetic radiation|electromagnetic radiation]] and allow for a correct analysis of wireless power transfer systems.<ref name="Shinohara" /><ref name="Angelo">{{cite book | last1  = Angelo | first1 = Joseph A. | title  = Encyclopedia of Space and Astronomy | publisher = Infobase Publishing | date   = 2009 | pages  = 292–293 | url    = https://books.google.com/books?id=VUWno1sOwnUC&pg=PA293 | isbn   = 978-1438110189 }}</ref> This was followed on by Heinrich Rudolf Hertz' 1888 validation of the theory, which included the evidence for [[Radio wave|radio wave]]s.<ref name="Angelo" />

During the same period two schemes of wireless signaling were put forward by William Henry Ward (1871) and Mahlon Loomis (1872) that were based on the erroneous belief that there was an electrified atmospheric stratum accessible at low altitude.<ref>{{cite book|first=Christopher|last=Cooper|title=The Truth About Tesla: The Myth of the Lone Genius in the History of Innovation|publisher=Race Point Publishing|year=2015|pages=154, 165}}</ref><ref>{{cite book|first1=Theodore S.|last1=Rappaport|first2=Brian D.|last2=Woerner|first3=Jeffrey H.|last3=Reed|title=Wireless Personal Communications: Trends and Challenges|publisher=Springer Science & Business Media|year=2012|pages=211–215}}</ref> Both inventors' patents noted this layer connected with a return path using "Earth currents"' would allow for wireless telegraphy as well as supply power for the telegraph, doing away with artificial batteries, and could also be used for lighting, heat, and motive power.<ref>{{cite book|first=Christopher|last=Cooper|title=The Truth About Tesla: The Myth of the Lone Genius in the History of Innovation|publisher=Race Point Publishing|year=2015|pages=154}}</ref><ref>{{Cite web|url=https://earlyradiohistory.us/sec021.htm|title=21. Fakes, Frauds, and Cranks (1866-1922)|website=earlyradiohistory.us}}</ref> A more practical demonstration of wireless transmission via conduction came in [[Biography:Amos Dolbear|Amos Dolbear]]'s 1879 magneto electric telephone that used ground conduction to transmit over a distance of a quarter of a mile.<ref name="Christopher Cooper 2015, page 165">{{cite book|first=Christopher|last=Cooper|title=The Truth About Tesla: The Myth of the Lone Genius in the History of Innovation|publisher=Race Point Publishing|year=2015|pages=165}}</ref>

===Tesla===
[[File:TeslaWirelessPower1891.png|thumb|upright=1.5|Tesla demonstrating wireless transmission by "electrostatic induction" during an 1891 lecture at Columbia College.&nbsp; The two metal sheets are connected to a Tesla coil oscillator, which applies high-voltage radio frequency alternating current.&nbsp; An oscillating electric field between the sheets ionizes the low-pressure gas in the two long Geissler tubes in his hands, causing them to glow in a manner similar to neon tubes.]]

After 1890, inventor [[Biography:Nikola Tesla|Nikola Tesla]] experimented with transmitting power by inductive and capacitive coupling using spark-excited [[Engineering:Radio frequency|radio frequency]] resonant transformers, now called [[Organization:Tesla coil|Tesla coil]]s, which generated high AC voltages.<ref name="Shinohara" /><ref name="LeeZhongHui" /><ref name="Tesla1891" >Tesla, Nikola (May 20, 1891) [http://www.tfcbooks.com/tesla/1891-05-20.htm   ''Experiments with Alternate Currents of Very High Frequency and Their Application to Methods of Artificial Illumination''], lecture before the American Inst. of Electrical Engineers, Columbia College, New York.  Reprinted as a {{cite book
 | title = book of the same name by
 | publisher = Wildside Press
 | date   = 2006
 | url    = https://books.google.com/books?id=94eH3rULPy4C
 | isbn   = 978-0809501625
 }}</ref>  Early on he attempted to develop a wireless lighting system based on [[Physics:Near and far field|near-field]] inductive and capacitive coupling<ref name="LeeZhongHui" /> and conducted a series of public demonstrations where he lit [[Physics:Geissler tube|Geissler tube]]s and even incandescent light bulbs from across a stage.<ref name="LeeZhongHui" /><ref name="Tesla1891" /><ref>{{cite book|first1=W. Bernard|last1=Carlson|title=Tesla: Inventor of the Electrical Age|publisher=Princeton University Press|year=2013|page=132|isbn=978-1400846559|url=https://books.google.com/books?id=5I5c9j8BEn4C}}</ref> He found he could increase the distance at which he could light a lamp by using a receiving [[Engineering:LC circuit|LC circuit]] tuned to [[Physics:Resonance|resonance]] with the transmitter's LC circuit.<ref name="Wheeler" /> using [[Engineering:Resonant inductive coupling|resonant inductive coupling]].<ref name="LeeZhongHui" /><ref name="Sun1" /> Tesla failed to make a commercial product out of his findings<ref>{{cite book|first=Christopher|last=Cooper|title=The Truth About Tesla: The Myth of the Lone Genius in the History of Innovation|publisher=Race Point Publishing|year=2015|pages=143–144}}</ref> but his resonant inductive coupling method is now widely used in electronics and is currently being applied to short-range wireless power systems.<ref name="LeeZhongHui" /><ref name="Leyh">{{cite book |doi=10.1109/NAPS.2008.5307364 |year=2008 |last1=Leyh |first1=G. E. |last2=Kennan |first2=M. D. |title=2008 40th North American Power Symposium |chapter=Efficient wireless transmission of power using resonators with coupled electric fields |pages=1–4 |isbn=978-1-4244-4283-6 |s2cid=3685850 }}</ref>

{{multiple image
| align = left
| direction = horizontal
| header   =
| image1   =  TeslaWirelessIllustration.png
| caption1 =
| width1   = 120
| image2   = Tesla Broadcast Tower 1904.jpeg
| caption2 =
| width2   = 217
| footer   = ''(left)'' Experiment in resonant inductive transfer by Tesla at Colorado Springs 1899. The coil is in resonance with Tesla's magnifying transmitter nearby, powering the light bulb at bottom. ''(right)'' Tesla's unsuccessful Wardenclyffe power station.
}}

Tesla went on to develop a wireless power distribution system that he hoped would be capable of transmitting power long distance directly into homes and factories. Early on he seemed to borrow from the ideas of Mahlon Loomis,<ref>{{cite book|first1=W. Bernard|last1=Carlson|title=Tesla: Inventor of the Electrical Age|publisher=Princeton University Press|year=2013|page=H-45|isbn=978-1400846559|url=https://books.google.com/books?id=5I5c9j8BEn4C}}</ref><ref>{{cite book|first=Marc J.|last=Seifer|title=Wizard: The Life and Times of Nikola Tesla : Biography of a Genius|publisher=Citadel Press|year=1996|page=107}}</ref> proposing a system composed of balloons to suspend transmitting and receiving electrodes in the air above {{convert|30,000|feet}} in altitude, where he thought the pressure would allow him to send high voltages (millions of volts) long distances. To further study the conductive nature of low pressure air he set up a test facility at high altitude in Colorado Springs during 1899.<ref>{{cite book|first=Robert|last=Uth|title=Tesla, Master of Lightning|publisher=Barnes & Noble Publishing|year=1999|page=92}}</ref><ref>{{Cite web|url=https://www.pbs.org/tesla/ll/ll_colspr.html|title= Tesla - Master of Lightning: Colorado Springs|website=PBS}}</ref><ref>{{cite book|first1=W. Bernard|last1=Carlson|title=Tesla: Inventor of the Electrical Age|publisher=Princeton University Press|year=2013|page=264|isbn=978-1400846559|url=https://books.google.com/books?id=5I5c9j8BEn4C}}</ref> Experiments he conducted there with a large coil operating in the megavolts range, as well as observations he made of the electronic noise of lightning strikes, led him to conclude incorrectly<ref>{{cite book|first1=W. Bernard|last1=Carlson|title=Tesla: Inventor of the Electrical Age|publisher=Princeton University Press|year=2013|page=301|isbn=978-1400846559|url=https://books.google.com/books?id=5I5c9j8BEn4C}}</ref><ref name="Christopher Cooper 2015, page 165"/> that he could use the entire globe of the Earth to conduct electrical energy. The theory included driving alternating current pulses into the Earth at its resonant frequency from a grounded Tesla coil working against an elevated capacitance to make the potential of the Earth oscillate. Tesla thought this would allow alternating current to be received with a similar capacitive antenna tuned to resonance with it at any point on Earth with very little power loss.<ref>{{cite book|first1=W. Bernard|last1=Carlson|title=Tesla: Inventor of the Electrical Age|publisher=Princeton University Press|year=2013|pages=209–211|isbn=978-1400846559|url=https://books.google.com/books?id=5I5c9j8BEn4C}}</ref><ref name="Tesla1904">{{cite journal|last=Tesla|first=Nikola|title=The Transmission of Electric Energy Without Wires|journal=Electrical World and Engineer|volume=43|pages=23760–23761|date=March 5, 1904|url=http://www.tfcbooks.com/tesla/1904-03-05.htm}}, reprinted in ''[https://books.google.com/books?id=qPI5AQAAMAAJ&pg=PA30 Scientific American Supplement, Munn and Co., Vol. 57, No. 1483, 4 June 1904, p. 23760–23761]''</ref><ref name="Sewall">{{cite book
 | last1  = Sewall
 | first1 = Charles Henry 
 | title  = Wireless telegraphy: its origins, development, inventions, and apparatus
 | publisher = D. Van Nostrand Co. 
 | date   = 1903
 | pages  = 38–42
 | url    = https://books.google.com/books?id=q_J3qYI5erkC&q=Tesla&pg=PA38
 }}</ref> His observations also led him to believe a high voltage used in a coil at an elevation of a few hundred feet would "break the air stratum down", eliminating the need for miles of cable hanging on balloons to create his atmospheric return circuit.<ref>{{cite book|first1=W. Bernard|last1=Carlson|title=Tesla: Inventor of the Electrical Age|publisher=Princeton University Press|year=2013|page=252|isbn=978-1400846559|url=https://books.google.com/books?id=5I5c9j8BEn4C}}</ref><ref name="Cooper_1916_110">Cooper, Drury W., internal document of the law firm Kerr, Page & Cooper, New York City, 1916. (Cited in {{cite book|last1=Anderson|first1=Leland|title=Nikola Tesla on His Work with Alternating Currents and Their Application to Wireless Telegraphy, Telephony, and Transmission of Power: An Extended Interview|publisher=Sun Publishing Company|date=1992|page=110|url=https://books.google.com/books?id=KRg9HWakBmQC|isbn=978-1893817012|quote=At that time I was absolutely sure that I could put up a commercial plant, if I could do nothing else but what I had done in my laboratory on Houston Street; but I had already calculated and found that I did not need great heights to apply this method. My patent says that I break down the atmosphere "at or near" the terminal. If my conducting atmosphere is 2 or 3 miles above the plant, I consider this very near the terminal as compared to the distance of my receiving terminal, which may be across the Pacific. That is simply an expression. I saw that I would be able to transmit power provided I could construct a certain apparatus – and I have, as I will show you later. I have constructed and patented a form of apparatus which, with a moderate elevation of a few hundred feet, can break the air stratum down.}}</ref> Tesla would go on the next year to propose a "[[Engineering:World Wireless System|World Wireless System]]" that was to broadcast both information and power worldwide.<ref name="Carlson2" >{{cite book|first1=W. Bernard|last1=Carlson|title=Tesla: Inventor of the Electrical Age|publisher=Princeton University Press|year=2013|pages=302–367|isbn=978-1400846559|url=https://books.google.com/books?id=5I5c9j8BEn4C}}</ref><ref name="CenturyMag">{{cite journal
  | last = Tesla
  | first = Nikola
  | title = The Problem of Increasing Human Energy
  | journal = Century Magazine
  | date = June 1900
  | url = http://www.tfcbooks.com/tesla/1900-06-00.htm
  | access-date = November 20, 2014}}</ref> In 1901, at Shoreham, New York he attempted to construct a large high-voltage wireless power station, now called Wardenclyffe Tower, but by 1904 investment dried up and the facility was never completed.

===Near-field and non-radiative technologies===
Inductive power transfer between nearby wire coils was the earliest wireless power technology to be developed, existing since the [[Engineering:Transformer|transformer]] was developed in the 1800s.  [[Physics:Induction heating|Induction heating]] has been used since the early 1900s and is used for [[Engineering:Induction cooking|induction cooking]].<ref>{{Cite book|title=Handbook of Induction Heating|edition=Second |isbn=978-1351643764 |last1=Rudnev |first1=Valery |last2=Loveless |first2=Don |last3=Cook |first3=Raymond L |date=2017-07-14 |publisher=CRC Press }}</ref>

With the advent of [[Engineering:Cordless|cordless]] devices, induction charging stands have been developed for appliances used in wet environments, like [[Engineering:Electric toothbrush|electric toothbrush]]es and [[Engineering:Electric razor|electric razor]]s, to eliminate the hazard of electric shock.  One of the earliest proposed applications of inductive transfer was to power electric locomotives.  In 1892 Maurice Hutin and Maurice Leblanc patented a wireless method of powering railroad trains using resonant coils inductively coupled to a track wire at 3&nbsp;kHz.<ref name="Patent527857A">{{cite patent |country=United States |number=527857A|status= |title=Transformer system for electric railways|pubdate= |gdate=23 October 1894 |fdate=16 November 1892|invent1=Maurice Hutin |invent2=Maurice Leblanc |url=https://www.google.com/patents/US527857}}</ref>

In the early 1960s resonant inductive wireless energy transfer was used successfully in implantable medical devices<ref>{{cite journal | last1 = Schuder | first1 = J. C. | year = 2002| title = Powering an artificial heart: Birth of the inductively coupled-radio frequency system in 1960 | journal = Artificial Organs | volume = 26 | issue = 11| pages = 909–915 | doi = 10.1046/j.1525-1594.2002.07130.x | pmid = 12406141 }}</ref> including such devices as pacemakers and artificial hearts.  While the early systems used a resonant receiver coil, later systems<ref>{{cite book|last1=SCHWAN|first1=M. A.|first2=P.R.|last2=Troyk|title=Images of the Twenty-First Century. Proceedings of the Annual International Engineering in Medicine and Biology Society |chapter=High efficiency driver for transcutaneously coupled coils |journal=IEEE Engineering in Medicine & Biology Society 11th Annual International Conference|date=November 1989|pages=1403–1404|doi=10.1109/IEMBS.1989.96262|s2cid=61695765}}</ref> implemented resonant transmitter coils as well. These medical devices are designed for high efficiency using low power electronics while efficiently accommodating some misalignment and dynamic twisting of the coils. The separation between the coils in implantable applications is commonly less than 20&nbsp;cm. Today resonant inductive energy transfer is regularly used for providing electric power in many commercially available medical implantable devices.<ref>{{cite web |url=http://www.cochlearamericas.com/Products/11.asp |title=What is a cochlear implant? |publisher=Cochlearamericas.com |date=2009-01-30 |access-date=2009-06-04 |url-status=dead |archive-url=https://web.archive.org/web/20081224181329/http://www.cochlearamericas.com/Products/11.asp |archive-date=2008-12-24 }}</ref>

The first passive RFID (Radio Frequency Identification) technologies were invented by Mario Cardullo<ref name="Patent">{{cite patent |country=United States |number=3713148A|title=Transponder apparatus and system |gdate=23 January 1973 |fdate=21 May 1970 |invent1=Mario W. Cardullo |invent2=William L. Parks|url=http://www.google.com/patents/US3713148}}</ref> (1973) and Koelle et al.<ref name="Koelle">{{cite journal |last1=Koelle |first1=A. R. |last2=Depp | first2 = S. W.  | last3  = Freyman  | first3 = R. W. |title= Short-range radio-telemetry for electronic identification, using modulated RF backscatter| journal = Proceedings of the IEEE| volume = 63 | issue = 8 | pages = 1260–1261 | date = 1975 | doi = 10.1109/proc.1975.9928 }}</ref> (1975) and by the 1990s were being used in proximity cards and contactless smartcards.

The proliferation of portable wireless communication devices such as [[Engineering:Mobile phone|mobile phone]]s, [[Engineering:Tablet computer|tablet]], and laptop computers in recent decades is currently driving the development of mid-range wireless powering and charging technology to eliminate the need for these devices to be tethered to wall plugs during charging.<ref name="Sayer">{{cite journal |last1=Sayer |first1=Peter |title=Wireless Power Consortium to Unleash Electronic Gadgets |journal=PCWorld |date=19 December 2008 |url=http://www.pcworld.com/article/155766/article.html |access-date=8 December 2014}}</ref> The Wireless Power Consortium was established in 2008 to develop interoperable standards across manufacturers.<ref name="Sayer" />  Its Qi inductive power standard published in August 2009 enables high efficiency charging and powering of portable devices of up to 5 watts over distances of 4&nbsp;cm (1.6 inches).<ref name="Qi">{{Cite news  | title = Global Qi Standard Powers Up Wireless Charging  | newspaper = PRNewswire  | publisher = UBM plc | date = 2 September 2009 | url = http://www.prnewswire.com/news-releases/global-qi-standard-powers-up-wireless-charging-102043348.html | access-date = 8 December 2014}}</ref> The wireless device is placed on a flat charger plate (which can be embedded in table tops at cafes, for example) and power is transferred from a flat coil in the charger to a similar one in the device. In 2007, a team led by Marin Soljačić at MIT used a dual resonance transmitter with a 25&nbsp;cm diameter secondary tuned to 10&nbsp;MHz to transfer 60 W of power to a similar dual resonance receiver over a distance of {{convert|2|meters|feet}} (eight times the transmitter coil diameter) at around 40% efficiency.<ref name="LeeZhongHui" /><ref name="Kurs" />

In 2008 the team of Greg Leyh and Mike Kennan of Nevada Lightning Lab used a grounded dual resonance transmitter with a 57&nbsp;cm diameter secondary tuned to 60&nbsp;kHz and a similar grounded dual resonance receiver to transfer power through coupled electric fields with an earth current return circuit over a distance of {{convert|12|meters|feet}}.<ref name="Leyh-Kennan_2008'''">{{cite conference|first1=G. E.|last1=Leyh|first2=M. D.|last2=Kennan|title=2008 40th North American Power Symposium |chapter=Efficient wireless transmission of power using resonators with coupled electric fields|conference=NAPS 2008 40th North American Power Symposium, Calgary, 28–30 September 2008|pages=1–4|publisher=IEEE |date=28 September 2008|chapter-url=http://www.lod.org/uploads/8/2/1/1/82111982/naps2008final.pdf|doi=10.1109/NAPS.2008.5307364|isbn=978-1-4244-4283-6|access-date=20 November 2014}}</ref> In 2011, Dr. Christopher A. Tucker and Professor [[Biography:Kevin Warwick|Kevin Warwick]] of the [[Organization:University of Reading|University of Reading]], recreated Tesla's 1900 patent 0,645,576 in miniature and demonstrated power transmission over {{convert|4|meters|feet}} with a coil diameter of {{convert|10|cm|in}} at a resonant frequency of 27.50&nbsp;MHz, with an effective efficiency of 60%.<ref name="Tucker">{{cite journal | last1 = Tucker | first1 = Christopher A. | last2 = Warwick | first2 = Kevin | last3 = Holderbaum | first3 = William | title = A contribution to the wireless transmission of power | journal = International Journal of Electrical Power & Energy Systems | volume = 47 | pages = 235–242 | date = 2013 | url = https://www.academia.edu/3800273 | doi = 10.1016/j.ijepes.2012.10.066 }}</ref>

===Microwaves and lasers===
Before World War II, little progress was made in wireless power transmission.<ref name="Brown1984"/> [[Social:Radio|Radio]] was developed for communication uses, but could not be used for power transmission since the relatively low-[[Physics:Frequency|frequency]] [[Radio wave|radio wave]]s spread out in all directions and little energy reached the receiver.<ref name="Shinohara" /><ref name="Brown1984"/> In radio communication, at the receiver, an [[Engineering:Amplifier|amplifier]] intensifies a weak signal using energy from another source. For power transmission, efficient transmission required [[Engineering:Transmitter|transmitter]]s that could generate higher-frequency [[Physics:Microwave|microwave]]s, which can be focused in narrow beams towards a receiver.<ref name="Shinohara" /><ref name="Brown1984"/><ref name="Curty">{{cite book | last1 = Curty | first1 = Jari-Pascal | last2 = Declercq | first2 = Michel | last3  = Dehollain | first3 = Catherine | last4 = Joehl | first4 = Norbert | title  = Design and Optimization of Passive UHF RFID Systems | publisher = Springer | date = 2006 | page=4 | url = https://books.google.com/books?id=uFjpH3Cl7z8C&pg=PA4 | isbn = 978-0387447100 }}</ref>

The development of microwave technology during World War II, such as the [[Physics:Klystron|klystron]] and magnetron tubes and [[Astronomy:Parabolic antenna|parabolic antenna]]s,<ref name="Brown1984"/>  made radiative ([[Physics:Near and far field|far-field]]) methods practical for the first time, and the first long-distance wireless power transmission was achieved in the 1960s by [[Biography:William C. Brown|William C. Brown]].<ref name="Shinohara" />  In 1964, Brown invented the [[Physics:Rectenna|rectenna]] which could efficiently convert microwaves to DC power, and in 1964 demonstrated it with the first wireless-powered aircraft, a model helicopter powered by microwaves beamed from the ground.<ref name="Brown1984"/>  A major motivation for microwave research in the 1970s and 1980s was to develop a solar power satellite.<ref name="Shinohara" /><ref name="Brown1984"/>  Conceived in 1968 by [[Biography:Peter Glaser|Peter Glaser]], this would harvest energy from sunlight using [[Physics:Solar cell|solar cell]]s and beam it down to Earth as [[Physics:Microwave|microwave]]s to huge rectennas, which would convert it to electrical energy on the electric power grid.<ref name="Glaser">{{cite journal
  | last = Glaser
  | first = Peter E.
  | title = Power from the Sun: Its future
  | journal = Science
  | volume = 162
  | issue = 3856
  | pages = 857–861
  | date = November 22, 1968
  | url = http://www.science-sainte-rose.net/GrandBassin/documents/downloads/GlaserSPS68.pdf
  | doi = 10.1126/science.162.3856.857
  | pmid = 17769070
  | access-date = 4 November 2014|bibcode = 1968Sci...162..857G }}</ref> In landmark 1975 experiments as technical director of a JPL/Raytheon program, Brown demonstrated long-range transmission by beaming 475 W of microwave power to a rectenna a mile away, with a microwave to DC conversion efficiency of 54%.<ref name="Brown2">{{cite web|last1=Friend|first1=Michael|last2=Parise|first2=Ronald J.|title=Cutting the Cord: ISTF 07-1726|url=http://mainland.cctt.org/istf2008/Brown.asp|publisher=Mainland High School, Daytona Beach, Florida|access-date=7 October 2016}}</ref> At NASA's Jet Propulsion Laboratory, he and Robert Dickinson transmitted 30&nbsp;kW DC output power across 1.5&nbsp;km with 2.38&nbsp;GHz microwaves from a 26 m dish to a 7.3 x 3.5 m rectenna array. The incident-RF to DC conversion efficiency of the rectenna was 80%.<ref name="Dickinson">{{cite journal |doi=10.1109/mwsym.1976.1123672 |title=Performance of a High-Power, 2.388-GHZ Receiving Array in Wireless Power Transmission over 1.54 km |journal=MTT-S International Microwave Symposium Digest |year=1976 |last1=Dickinson |first1=R.M. |volume=76 |pages=139–141 }}</ref> In 1983 Japan launched [[Engineering:Microwave Ionosphere Nonlinear Interaction Experiment|Microwave Ionosphere Nonlinear Interaction Experiment]] (MINIX),  a rocket experiment to test transmission of high power microwaves through the ionosphere.{{citation needed|date=April 2021}}

In recent years a focus of research has been the development of wireless-powered drone aircraft, which began in 1959 with the Dept. of Defense's RAMP (Raytheon Airborne Microwave Platform) project<ref name="Brown1984"/> which sponsored Brown's research.  In 1987 Canada's Communications Research Center developed a small prototype airplane called Stationary High Altitude Relay Platform (SHARP) to relay telecommunication data between points on earth similar to a [[Astronomy:Communications satellite|communications satellite]].  Powered by a rectenna, it could fly at 13 miles (21&nbsp;km) altitude and stay aloft for months. In 1992 a team at Kyoto University built a more advanced craft called MILAX (MIcrowave Lifted Airplane eXperiment).

In 2003 NASA flew the first laser powered aircraft.  The small model plane's motor was powered by electricity generated by photocells from a beam of infrared light from a ground-based laser, while a control system kept the laser pointed at the plane.

==See also==
{{div col|colwidth=23em}}
* [[Astronomy:Beam-powered propulsion|Beam-powered propulsion]]
* Beam Power Challenge – one of the [[Organization:NASA|NASA]] [[Organization:Centennial Challenges|Centennial Challenges]]
* Electricity distribution
* [[Finance:Electric power transmission|Electric power transmission]]
* [[Engineering:Electromagnetic compatibility|Electromagnetic compatibility]]
* [[Biology:Electromagnetic radiation and health|Electromagnetic radiation and health]]
* [[Engineering:Energy harvesting|Energy harvesting]]
* [[Physics:Friis transmission equation|Friis transmission equation]]
* Microwave power transmission
* [[Engineering:Qi (standard)|Qi (standard)]]
* [[Physics:Space-based solar power|Space-based solar power]]
* [[Engineering:Resonant inductive coupling|Resonant inductive coupling]]
* Thinned array curse
* [[Company:UBeam|uBeam]] – acoustic energy transfer system
* Wardenclyffe Tower
* Wi-Charge – far-field infrared wireless power
* [[Engineering:World Wireless System|World Wireless System]]
{{div col end}}

==References==
{{Reflist|refs=
<ref name="ECN2011">{{cite journal | title = World's first!! Production starts for Capacitive Coupling Wireless Power Transmission Module | journal = ECN Magazine | date = 27 October 2011 | url = http://www.ecnmag.com/news/2011/10/worlds-first-production-starts-capacitive-coupling-wireless-power-transmission-module | access-date = 16 January 2015}}</ref>
<ref name="Trancutaneous Capacitive Wireless Power Transfer">{{Cite book |doi=10.1109/ISCAS.2017.8050940 |isbn=978-1-4673-6853-7|chapter=Transcutaneous capacitive wireless power transfer (C-WPT) for biomedical implants|title=2017 IEEE International Symposium on Circuits and Systems (ISCAS)|pages=1–4|year=2017|last1=Erfani|first1=Reza|last2=Marefat|first2=Fatemeh|last3=Sodagar|first3=Amir M.|last4=Mohseni|first4=Pedram|s2cid=23159251}}</ref>
<ref name="Capacitive Elements for Wireless Power Transfer to biomedical implants">{{cite journal |last1=Erfani |first1=Reza |last2=Marefat |first2=Fatemeh |last3=Sodagar |first3=Amir M. |last4=Mohseni |first4=Pedram |title=Modeling and Characterization of Capacitive Elements With Tissue as Dielectric Material for Wireless Powering of Neural Implants |journal=IEEE Transactions on Neural Systems and Rehabilitation Engineering |date=May 2018 |volume=26 |issue=5 |pages=1093–1099 |doi=10.1109/TNSRE.2018.2824281 |pmid=29752245 |s2cid=13716374 }}</ref>
<ref name="Capacitive Wireless Power Transfer to biomedical implants">{{cite journal |last1=Erfani |first1=Reza |last2=Marefat |first2=Fatemeh |last3=Sodagar |first3=Amir M. |last4=Mohseni |first4=Pedram |title=Modeling and Experimental Validation of a Capacitive Link for Wireless Power Transfer to Biomedical Implants |journal=IEEE Transactions on Circuits and Systems II: Express Briefs |date=July 2018 |volume=65 |issue=7 |pages=923–927 |doi=10.1109/TCSII.2017.2737140 |s2cid=49541743 }}</ref>
}}

==Further reading==
===Books and articles===
{{refbegin}}
* {{cite book | last  = de Rooij | first = Michael A. | title  = Wireless Power Handbook | publisher = Power Conversion Publications | date   = 2015 | url    = http://epc-co.com/epc/Products/Publications/WirelessPowerHandbook.aspx | isbn   = 978-0996649216 }}  Latest work on AirFuel Alliance class 2 and class 3 transmitters, adaptive tuning, radiated EMI, multi-mode wireless power systems, and control strategies.
* {{cite book | last  = Agbinya | first = Johnson I., Ed. | title  = Wireless Power Transfer | publisher = River Publishers | date   = 2012 | url    = https://books.google.com/books?id=zDPqqBJ76ZAC&pg=PA1 | isbn   = 978-8792329233 }}  Comprehensive, theoretical engineering text
* {{cite book | last1  = Shinohara | first1 = Naoki | title  = Wireless Power Transfer via Radiowaves | publisher = John Wiley & Sons | date   = 2014 | url    = https://books.google.com/books?id=TwegAgAAQBAJ&pg=PP6 | isbn   = 978-1118862964}}  Engineering text
* {{cite journal |last1=Kurs |first1=A. |last2=Karalis |first2=A. |last3=Moffatt |first3=R. |last4=Joannopoulos |first4=J. D. |last5=Fisher |first5=P. |last6=Soljacic |first6=M. |title=Wireless Power Transfer via Strongly Coupled Magnetic Resonances |journal=Science |date=6 July 2007 |volume=317 |issue=5834 |pages=83–86 |doi=10.1126/science.1143254 |pmid=17556549 |bibcode=2007Sci...317...83K |citeseerx=10.1.1.418.9645 |s2cid=17105396 }}
* Thibault, G. (2014). [https://www.academia.edu/11307646/Wireless_Pasts_and_Wired_Futures Wireless Pasts and Wired Futures]. In J. Hadlaw, A. Herman, & T. Swiss (Eds.), Theories of the Mobile Internet. Materialities and Imaginaries. (pp.&nbsp;126–154). London: Routledge.  A short cultural history of wireless power
{{refend}}

===Patents===
{{refbegin}}
* {{US patent|4955562}}, Microwave powered aircraft, John E. Martin, et al. (1990).
* {{US patent|3933323}}, Solid state solar to microwave energy converter system and apparatus, Kenneth W. Dudley, et al. (1976).
* {{US patent|3535543}}, Microwave power receiving antenna, Carroll C. Dailey (1970).
{{refend}}

==External links==
* [http://electronics.howstuffworks.com/wireless-power.htm Howstuffworks "How Wireless Power Works"] – describes near-range and mid-range wireless power transmission using induction and radiation techniques.
* [https://web.archive.org/web/20060217051802/http://www.kurasc.kyoto-u.ac.jp/space-group/sps/history1-e.html Microwave Power Transmission], – its history before 1980.
* [http://www.friendsofcrc.ca/Projects/SHARP/sharp.html The Stationary High Altitude Relay Platform (SHARP)], – microwave beam powered.
* [https://www.mit.edu/~soljacic/wireless_power.html Marin Soljačić's ''MIT WiTricity''] – wireless power transmission pages.
* [http://www.rezence.com Rezence] – official site of a wireless power standard promoted by the Alliance for Wireless Power
* [http://www.wirelesspowerconsortium.com Qi] – official site of a wireless power standard promoted by the Wireless Power Consortium
* [http://www.powermatters.org PMA] – official site of a wireless power standard promoted by the Power Matters Alliance
* [http://www.wipow.org WiPow] – official site of the WiPow Coalition, promoting standardized wireless power for medical, mobility and wheeled devices
{{Photonics}}
{{emerging technologies|energy=yes}}

{{DEFAULTSORT:Wireless Energy Transfer}}
[[Category:Electric power distribution]]
[[Category:Electromagnetic compatibility]]
[[Category:Microwave transmission]]

{{Sourceattribution|Wireless power transfer}}