Editing Wireless power transfer (section)

== Near-field (nonradiative) techniques ==

At large relative distance, the near-field components of electric and magnetic fields are approximately quasi-static oscillating [[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 |archive-date=3 August 2016 |archive-url=https://web.archive.org/web/20160803232419/http://www.jpier.org/PIERB/pierb49/06.12120512.pdf |url-status=dead}}</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 [[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 ===
{{main|Inductive charging}}
{{multiple image
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| caption2 = Prototype inductive electric car charging system at 2011 Tokyo Auto Show
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| image3 = Power 2.0 Ecosystem illustration - Powermat charging spots on counter in a coffee shop.jpg
| caption3 = [[Powermat Technologies|Powermat]] inductive charging spots in a coffee shop. Customers can set their phones and computers on them to recharge.
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| 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.
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| image5 = Magne Chargers.jpg
| caption5 = GM EV1 and Toyota RAV4 EV inductively charging at a now-obsolete [[Magne Charge]] station
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}}
{{multiple image
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| image2   = Lamp powered by induction 1910.jpg
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| 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. Right: a light bulb powered wirelessly by induction, in 1910
}}

In [[inductive coupling]] (''[[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 [[electromagnetic coil|coils of wire]] by a [[magnetic field]].<ref name="Sazonov" /> The transmitter and receiver coils together form a [[transformer]].<ref name="Sazonov" /><ref name="Valtchev" /> An [[alternating current]] (AC) through the transmitter coil ''(L1)'' creates an oscillating magnetic field ''(B)'' by [[Ampere's circuital law|Ampere's law]].  The magnetic field passes through the receiving coil ''(L2)'', where it induces an alternating [[electromotive force|EMF]] ([[voltage]]) by [[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 [[rectifier|rectified]] to [[direct current]] (DC) by a 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 power|mains current]] is applied directly to the transmitter coil, but in most systems an [[electronic oscillator]] generates a higher frequency AC current which drives the coil, because transmission efficiency improves with [[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 [[cordless]] appliances used in wet environments such as [[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 |year=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]] [[implant (medicine)|implanted]] in the human body, such as [[Artificial cardiac pacemaker|cardiac pacemakers]], 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 [[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 computer|laptop]] and [[tablet computer]]s, [[computer mouse]], [[cellphones]], [[digital media player]]s, and [[video game controller]]s.{{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}}</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 [[Mutual inductance|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 [[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" /> [[Allotropes of iron|Ferrite]] "flux confinement" cores can confine the magnetic fields, improving coupling and reducing [[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 [[resonant inductive coupling]] is used, in which the efficiency is increased by using [[resonant circuit]]s.<ref name="Agbinya" /><ref name="Wong" /><ref name="Davis" /><ref name="Shinohara" /> This can achieve high efficiencies at greater distances than nonresonant inductive coupling.

=== Resonant inductive coupling ===
[[File:Wireless power system - inductive coupling.svg|thumb|upright=1.4|Generic block diagram of an inductive wireless power system]]
{{main|Resonant inductive coupling}}
{{Further|Tesla coil#Resonant transformer}}

[[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 circuit]]s (tuned circuits), one in the transmitter and one in the receiver.<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 [[capacitor]], or a [[self-resonant frequency|self-resonant]] coil or other [[resonator]] with internal capacitance. The two are tuned to resonate at the same [[resonant frequency]]. The resonance between the coils can greatly increase coupling and power transfer, analogously to the way a vibrating [[tuning fork]] can induce [[sympathetic vibration]] in a distant fork tuned to the same pitch.

[[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 [[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 [[Q factor]] [[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 [[remote control]]s is that it could drastically reduce the 6 billion [[Electric battery|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 85&nbsp;kHz 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=2 May 2021}}</ref>

=== Capacitive coupling ===
{{Main|Capacitive coupling}}

[[Capacitive coupling]] also referred to as electric coupling, makes use of electric fields for the transmission of power between two [[electrode]]s (an [[anode]] and [[cathode]]) forming a [[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=wipo-wirelesspower.com |access-date=2018-11-30}}</ref> In capacitive coupling ([[electrostatic induction]]), the conjugate of [[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 electrodes<ref name="Capacitive Elements for Wireless Power Transfer to biomedical implants"/> such as metal plates. The transmitter and receiver electrodes form a [[capacitor]], with the intervening space as the [[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 |year=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 [[electric field]] induces an alternating [[electric potential|potential]] on the receiver plate by 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 [[frequency]]<ref name="Huschens" /> the square of the voltage, and the [[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
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| 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
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}}

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 [[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}}</ref><ref name=":0">{{cite web |url=http://www.pro-physik.de/details/articlePdf/1102293/issue.html |title=Coupling games in metamaterials |year=2010 |access-date=18 January 2016 |last=Liu |first=Na |archive-date=11 October 2016 |archive-url=https://web.archive.org/web/20161011200343/http://www.pro-physik.de/details/articlePdf/1102293/issue.html |url-status=dead}}</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 [[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 [[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 [[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>

Resonance can also be used with capacitive coupling to extend the range.  At the turn of the 20th century, [[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. [[inductive coupling|electromagnetic/induction]], [[piezoelectricity|piezoelectric]], or [[capacitive coupling|capacitive]]).<ref name="Halim">{{cite book | doi=10.1109/MEMS46641.2020.9056444 | chapter=A High-Performance Electrodynamic Micro-Receiver for Low-Frequency Wireless Power Transfer | title=2020 IEEE 33rd International Conference on Micro Electro Mechanical Systems (MEMS) | date=2020 | last1=Halim | first1=Miah A. | last2=Smith | first2=Spencer E. | last3=Samman | first3=Joseph M. | last4=Arnold | first4=David P. | pages=590–593 | isbn=978-1-7281-3581-6 }}</ref><ref name="Spencer">{{cite book | doi=10.1109/MEMS51782.2021.9375416 | chapter=Dual-Transduction Electromechanical Receiver for Near-Field Wireless Power Transmission | title=2021 IEEE 34th International Conference on Micro Electro Mechanical Systems (MEMS) | date=2021 | last1=Smith | first1=Spencer E. | last2=Halim | first2=Miah A. | last3=Rendon-Hernandez | first3=Adrian A. | last4=Arnold | first4=David P. | pages=38–41 | isbn=978-1-6654-1912-3 }}</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 [[implant (medicine)|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 [[armature (electrical engineering)|armatures]], one in the transmitter and one in the receiver, which rotate synchronously, coupled together by a [[magnetic field]] generated by [[permanent magnet]]s on the armatures.<ref name="Ashley" /> The transmitter armature is turned either by or as the rotor of an [[electric motor]], and its magnetic field exerts [[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 [[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 [[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 [[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 [[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"/>