View source for Metamaterial antenna

[[File:Z Antennas at UHF Frequencies.jpg|thumb|This Z antenna tested at the [[National Institute of Standards and Technology]] is smaller than a standard antenna with comparable properties. Its high [[efficiency]] is derived from the "Z element" inside the square that acts as a [[metamaterial]], greatly boosting the radiated signal. The square is 30 millimeters on a side.]]

'''Metamaterial antennas''' are a class of [[antenna (radio)|antenna]]s which use [[metamaterials]] to increase performance of miniaturized ([[Electrically small antenna|electrically small]]) [[Antenna (radio)|antenna system]]s.<ref>Slyusar V. I. 60 Years of Electrically Small Antennas Theory.//Proceedings of the 6-th International Conference on Antenna Theory and Techniques, 17–21 September 2007, Sevastopol, Ukraine. - Pp. 116 - 118. [http://www.slyusar.kiev.ua/ICATT_2007_1.pdf]</ref> Their purpose, as with any electromagnetic antenna, is to launch [[energy]] into free space. However, this class of antenna incorporates metamaterials, which are materials engineered with novel, often [[microscopic]], structures to produce unusual [[physical properties]]. Antenna designs incorporating metamaterials can step-up the antenna's radiated [[power (physics)|power]].

Conventional antennas that are very small compared to the [[wavelength]] reflect most of the signal back to the source. A metamaterial antenna behaves as if it were much larger than its actual size, because its novel structure stores and re-radiates energy.  Established lithography techniques can be used to print metamaterial elements on a [[Printed circuit board|PC board]].<ref name=Directive-emission/><ref name=neg-group-vel-1>
{{cite journal
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| journal =[[IEEE Transactions on Antennas and Propagation]]
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{{cite journal
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* {{Cite journal
| last1 =Ziolkowski
| first1 =Richard W.
| last2 =Jin| first2 =Peng
| last3 =Nielsen| first3 =J. A.
| last4 =Tanielian| first4 =M. H.
| last5 =Holloway| first5 =Christopher L.
| author-link=Richard W. Ziolkowski
| title =Experimental Verification of Z Antennas at UHF Frequencies
| journal =[[IEEE Antennas and Wireless Propagation Letters]]
| volume =8| page =1329
| url =http://www2.engr.arizona.edu/~ziolkows/research/papers/Metamaterial_Research/Antennas/Ziolkowski_Peng_Z_antenna_measurements_AWPL_2009.pdf
| year =2009
| doi =10.1109/LAWP.2009.2038180
| bibcode = 2009IAWPL...8.1329Z| s2cid =5558201
}}</ref>

These novel antennas aid applications such as portable interaction with satellites, wide angle beam steering, emergency communications devices, [[sensor|micro-sensors]] and portable [[radar|ground-penetrating radars]] to search for geophysical features.

Some applications for metamaterial antennas are [[wireless communication]], [[Space Communications and Navigation Program|space communications]], [[GPS]], [[satellite]]s, space vehicle navigation and airplanes.
{{toclimit|3}}

== Antennas designs ==

Antenna designs incorporating metamaterials can step-up the radiated [[power (physics)|power]] of an antenna. The newest metamaterial antennas radiate as much as 95 percent of an input [[radio signal]]. Standard antennas need to be at least half the size of the signal wavelength to operate efficiently. At [[Megahertz|300 MHz]], for instance, an antenna would need to be half a meter long. In contrast, experimental metamaterial antennas are as small as one-fiftieth of a wavelength, and could have further decreases in size.

Metamaterials are a basis for further miniaturization of [[microwave antenna]]s, with efficient power and acceptable bandwidth. Antennas employing metamaterials offer the possibility of overcoming restrictive efficiency-bandwidth limitations for conventionally constructed, miniature antennas.

Metamaterials permit smaller antenna elements that cover a wider [[frequency range]], thus making better use of available space for space-constrained cases. In these instances, miniature antennas with high gain are significantly relevant because the radiating elements are combined into large antenna arrays. Furthermore, metamaterials' negative [[refractive index]] focuses [[electromagnetic radiation]] by a [[flat lens]] versus being dispersed.<ref name=Navy-prop-2007-09/><ref name=navyN07-184>{{cite web |url=http://www.creer.polymtl.ca/Halim_Boutayeb/TAPCEBG.pdf |title=Analysis and Design of a Cylindrical EBG based directive antenna, Halim Boutayeb et al |url-status=dead |archive-url=https://web.archive.org/web/20110706200456/http://www.creer.polymtl.ca/Halim_Boutayeb/TAPCEBG.pdf |archive-date=July 6, 2011 }}
* {{cite web|url=http://newswise.com/articles/view/538769|title='Metafilms' Can Shrink Radio, Radar Devices}}
* {{cite web|url=http://discovermagazine.com/2009/jan/007|title=Invisibility Becomes More than Just a Fantasy|access-date=2011-02-06|archive-url=https://web.archive.org/web/20090513101529/http://discovermagazine.com/2009/jan/007|archive-date=2009-05-13|url-status=dead}}</ref><ref name=MM2009>{{Cite conference|first=Filiberto |last=Bilotti |author2=Vegni, Lucio |title=Metamaterial-inspired electrically small radiators: it is time to draw preliminary conclusions and depict the future challenges |book-title=Proceedings of the 3rd International Congress on Advanced Electromagnetic Materials in Microwaves and Optics, London, UK, August 30th-September 4th, 2009 |publisher=METAMORPHOSE VI AISBL |url=http://proceedings.metamorphose-vi.org/2009/submission/6/ |isbn=978-0-9551179-6-1 |url-status=dead |archive-url=https://web.archive.org/web/20110825032806/http://proceedings.metamorphose-vi.org/2009/submission/6/ |archive-date=August 25, 2011 }}
* [http://proceedings.metamorphose-vi.org/2009/sessions/ Metamaterials '2009] {{webarchive|url=https://web.archive.org/web/20110626110455/http://proceedings.metamorphose-vi.org/2009/sessions/ |date=2011-06-26 }} – Sessions
* [http://proceedings.metamorphose-vi.org/ Proceedings of the Virtual Institute for Artificial Electromagnetic Materials and Metamaterials] {{webarchive|url=https://web.archive.org/web/20101231174457/http://proceedings.metamorphose-vi.org/ |date=2010-12-31 }}</ref>

=== The DNG shell ===

The earliest research in metamaterial antennas was an analytical study of a miniature dipole antenna surrounded with a metamaterial. This material is known variously as a negative index metamaterial (NIM) or double negative metamaterial (DNG) among other names.<ref name=DNG-shell-2003-10/>

This configuration analytically and numerically appears to produce an order of magnitude increase in power. At the same time, the reactance appears to offer a corresponding decrease. Furthermore, the DNG shell becomes a natural impedance matching network for this system.<ref name=DNG-shell-2003-10/>

== Ground plane applications ==

Metamaterials employed in the [[ground plane]]s surrounding antennas offer improved isolation between [[radio frequency]], or [[microwave]] channels of ([[multiple-input multiple-output]]) (MIMO) [[Phased array|antenna arrays]].<ref name=slyusarmeta>Slyusar V.I. Metamaterials on antenna solutions.// 7th International Conference on Antenna Theory and Techniques ICATT’09, Lviv, Ukraine, October 6–9, 2009. - Pp. 19 - 24 [http://www.slyusar.kiev.ua/019_024_ICATT_2009.pdf]</ref> Metamaterial, [[Tunable metamaterials|high-impedance groundplanes]] can also improve [[Radio frequency|radiation]] efficiency and axial ratio performance of low-profile antennas located close to the [[ground plane|ground plane surface]]. Metamaterials have also been used to increase [[Microwave Scanning Beam Landing System|beam scanning]] range by using both the forward and backward waves in leaky wave antennas. Various metamaterial antenna systems can be employed to support surveillance sensors, communication links, navigation systems and command and control systems.<ref name=Navy-prop-2007-09>{{cite web |last=Bukva |first=Erica |title=Metamaterial-Based Electrically Small Antenna |website=Navy SBIR / STTR |publisher=Navy Office of Small Business Programs |date=August 20 – September 19, 2007 |url=http://www.navysbir.com/n07_3/n073-184.htm |access-date=2010-03-19}}</ref>

== Novel configurations ==

Besides antenna miniaturization, the novel configurations have potential applications ranging from radio frequency devices to optical devices. Other combinations, for other devices in metamaterial antenna subsystems are being researched.<ref name=physicsengineering1/> Either [[double negative metamaterial]] slabs are used exclusively or combinations of [[Metamaterial#Double positive medium|double positive (DPS)]] with DNG slabs, or [[Metamaterial#Single negative|epsilon-negative (ENG)]] slabs with [[Metamaterial#Single negative|mu-negative (MNG)]] slabs are employed in the subsystems. Antenna subsystems that are currently being researched include [[Resonator|cavity resonators]], waveguides, scatters and antennas (radiators).<ref name=physicsengineering1>{{cite book|last = Engheta|first = Nader|author2=Richard W. Ziolkowski|title = Metamaterials: physics and engineering explorations|publisher = [[Wiley & Sons]]|date = June 2006|pages = 43–85|url = https://books.google.com/books?id=51e0UkEuBP4C|isbn = 978-0-471-76102-0}}</ref> Metamaterial antennas were commercially available by 2009.<ref name=commercial-MM-antenna>
{{Cite news| title =NETGEAR Ships 'The Ultimate Networking Machine' for Gamers, Media Enthusiasts and Small Businesses| newspaper =The New York Times| date = 2009-10-20| url =http://markets.on.nytimes.com/research/stocks/news/press_release.asp?docKey=600-200909010830PR_NEWS_USPR_____SF68348-25UDEBBTNP0TL3Q8NDL3A9RQ33&provider=PR%20Newswire&docDate=September%201%2C%202009&press_symbol=217857&scp=2&sq=metamaterial&st=cse | format= ...eight ultra-sensitive, internal, metamaterial antennas... | access-date =2009-10-20}}</ref><ref name=rspan>{{cite news|last=Hurst |first=Brian |title=RAYSPAN Ships 20 Millionth Metamaterial Antenna |work=Reuters |date=2009-09-28 |url=https://www.reuters.com/article/pressRelease/idUS93483+28-Sep-2009+PRN20090928 |access-date=2009-10-20 |url-status=dead |archive-url=https://web.archive.org/web/20091101054523/https://www.reuters.com/article/pressRelease/idUS93483%2B28-Sep-2009%2BPRN20090928 |archive-date=November 1, 2009 }}</ref><ref name=c-phone>{{cite web
| last =Das
| first =Saswato R.
| title =Metamaterials Arrive in Cellphones
| website =Metamterial antennas
| publisher =[[IEEE Spectrum]]
| date =October 2009
| url =https://spectrum.ieee.org/telecom/wireless/metamaterials-arrive-in-cellphones
| format =Online magazine article
| quote = LG Chocolate BL40 is first cellphone to use a metamaterials antenna
| access-date =2011-03-09}}</ref>

== History ==

[[Pendry]] ''et al.'' were able to show that a three-[[dimension]]al array of intersecting, thin wires could be used to create negative values of [[permittivity]] (or "'''ε'''"), and that a periodic array of copper split ring resonators could produce an effective negative [[magnetic permeability]] (or "'''μ'''").<ref name=slyusarmeta/>

In May 2000, a group of researchers, Smith ''et al.'' were the first to successfully combine the [[split-ring resonator]] (SRR), with thin wire conducting posts and produce a [[left-handed material]] that had negative values of ε, μ and [[refractive index]] for frequencies in the [[gigahertz]] or [[microwave]] range.<ref name=physicsengineering1/><ref name=AAAS2>
{{Cite journal | last1 = Shelby | first1 = R. A. | last2 = Smith | first2 = D. R. | last3 = Schultz | first3 = S. | doi = 10.1126/science.1058847 | title = Experimental Verification of a Negative Index of Refraction | journal = Science | volume = 292 | issue = 5514 | pages = 77–79 | year = 2001 | pmid =  11292865| bibcode = 2001Sci...292...77S| citeseerx = 10.1.1.119.1617 | s2cid = 9321456 }}</ref>

In 2002, a different class of negative refractive index (NRI) metamaterials was introduced that employs periodic [[Electrical reactance|reactive loading]] of a 2-D [[transmission line]] as the host [[transmission medium|medium]]. This configuration used [[index of refraction|positive index]] (DPS) material with negative index material (DNG). It employed a small, planar, [[Superlens|negative-refractive-lens]] interfaced with a positive index, parallel-plate waveguide. This was experimentally verified soon after.<ref name=first-TL-lens-proposed/><ref name=verification-of-focusing/>

Although some SRR inefficiencies were identified, they continued to be employed as of 2009 for research. SRRs have been involved in wide-ranging metamaterial research, including research on metamaterial antennas.<ref name=radiation-properties/><ref name=first-TL-lens-proposed>{{Cite book| last =Iyer| first =Ashwin K.|author2=George V. Eleftheriades | volume =2|page =1067| date =2002-06-07 | chapter-url =http://individual.utoronto.ca/iyer/index_files/IMS021248.pdf| doi =10.1109/MWSYM.2002.1011823| access-date =2009-11-08| isbn =978-0-7803-7239-9| author2-link =George V. Eleftheriades| chapter =Negative refractive index metamaterials supporting 2-D waves| title =2002 IEEE MTT-S International Microwave Symposium Digest (Cat. No.02CH37278)| s2cid =31129309}}</ref><ref name=verification-of-focusing>{{Cite journal| last1 = Iyer| first1 = Ashwin K.|title =Experimental and theoretical verification of focusing in a large, periodically loaded transmission line negative refractive index metamaterial| journal = [[Optics Express]]| volume =11| issue = 7| pages =696–708| date =2003-04-07|url =http://individual.utoronto.ca/iyer/index_files/Opex.PDF
| doi =10.1364/OE.11.000696| access-date =2009-11-08| last2 = Kremer| first2 = Peter| last3 = Eleftheriades| first3 = George| pmid=19461781|bibcode = 2003OExpr..11..696I| doi-access = free}}</ref>

A more recent view is that by using SRRs as building blocks, the electromagnetic response and associated flexibility is practical and desirable.<ref name=desirable-srr>
{{Cite journal| last1 =Chen| first1 =Hou-Tong| title =Hybrid metamaterials enable fast electrical modulation of freely propagating terahertz waves| journal =[[Applied Physics Letters]]| volume =93| pages =091117 (2008)| date =2008-09-04| url =http://www2.bc.edu/~padillaw/PDF/APL_93_091117_2008.pdf| doi =10.1063/1.2978071| access-date =2009-11-12| last2 =Palit| first2 =Sabarni| last3 =Tyler| first3 =Talmage| last4 =Bingham| first4 =Christopher M.| last5 =Zide| first5 =Joshua M. O.| last6 =O’hara| first6 =John F.| last7 =Smith| first7 =David R.| last8 =Gossard| first8 =Arthur C.| last9 =Averitt| first9 =Richard D.| bibcode =2008ApPhL..93i1117C| display-authors =1| issue =9| archive-url =https://web.archive.org/web/20110605010829/https://www2.bc.edu/~padillaw/PDF/APL_93_091117_2008.pdf| archive-date =2011-06-05| url-status =dead}}
</ref>

=== Phase compensation due to negative refraction ===

DNG can provide [[Phase (waves)|phase compensation]] due to their negative index of refraction. This is accomplished by combining a slab of conventional lossless DPS material with a slab of lossless DNG metamaterial.

DPS has a conventional positive [[index of refraction]], while the DNG has a negative refractive index. Both slabs are [[Impedance of free space|impedance]]-matched to the outside region (e.g., free space). The desired monochromatic [[plane wave]] is radiated on this configuration. As this wave propagates through the first slab of material a [[Phase (waves)|phase difference]] emerges between the exit and entrance faces. As the wave [[wave propagation|propagates]] through the second slab the phase difference is significantly decreased and even compensated for. Therefore, as the wave exits the second slab the total phase difference is equal to zero.<ref name=DNG-MTM-overview>{{Cite journal| last = Engheta| first =Nader and| author-link =Nader Engheta|author2=Richard W. Ziolkowski| title =A Positive Future for Double-Negative Metamaterials| journal =[[IEEE Transactions on Microwave Theory and Techniques]]| volume =53| issue =4| page = 1535| date =April 2005| url =http://repository.upenn.edu/cgi/viewcontent.cgi?article=1299&context=ese_papers|doi =10.1109/TMTT.2005.845188| access-date =2009-12-27|bibcode = 2005ITMTT..53.1535E| s2cid =15293380}}</ref>

With this system a phase-compensated, [[waveguiding]] system could be produced. By stacking slabs of this configuration, the phase compensation (beam translation effects) would occur throughout the entire system. Furthermore, by changing the index of any of the DPS-DNG pairs, the speed at which the beam enters the front face, and exits the back face of the entire stack-system changes. In this manner, a volumetric, low loss, time delay [[transmission line]] could be realized for a given system.<ref name=DNG-MTM-overview/>

Furthermore, this phase compensation can lead to a set of applications, which are miniaturized, [[subwavelength]], [[cavity resonator]]s, and waveguides with applications below [[diffraction limit]]s.<ref name=DNG-MTM-overview/>

=== Transmission line dispersion compensation ===
{{Electromagnetism|cTopic=[[Classical electromagnetism|Electrodynamics]]}}

Because of DNG's [[dispersion relation|dispersive nature]] as a transmission medium, it could be useful as a dispersion compensation device for [[Fourier transform|time-domain applications]]. The dispersion produces a variance of the [[group speed]] of the signals' wave components, as they propagate in the DNG medium. Hence, stacked DNG metamaterials could be useful for modifying signal propagation along a [[microstrip|microstrip transmission line]]. At the same time, dispersion leads to distortion. However, if the dispersion could be compensated for along the microstrip line, [[Radio frequency|RF]] or microwave signals propagating along them would significantly decrease distortion. Therefore, components for attenuating distortion become less critical, and could lead to simplification of many systems. Metamaterials can eliminate dispersion along the microstrip by correcting for the frequency dependence of the effective permittivity.<ref name=compensate-sys/>

The strategy is to design a length of [[metamaterial]]-loaded transmission line that can be introduced with the original length of [[microstrip]] line to make the paired system [[Dispersion (optics)|dispersionless]] creating a dispersion-compensating segment of transmission line. This could be accomplished by introducing a metamaterial with a specific localized [[permittivity]] and a specific localized [[magnetic permeability]], which then affects the relative permittivity and permeability of the overall microstrip line. It is introduced so that the wave impedance in the metamaterial remains unchanged. The index of refraction in the medium compensates for the dispersion effects associated with the microstrip geometry itself; making the effective refractive index of the pair that of free space.<ref name=compensate-sys/>

Part of the design strategy is that the effective permittivity and permeability of such a metamaterial should be negative – requiring a DNG material.<ref name=compensate-sys>
{{Cite journal| last =Ziolkowski| first = Richard W. and|author2=Ching-Ying Cheng
| title =Tailoring double negative metamaterial responses to achieve anomalous propagation effects along microstrip transmission lines| journal =[[IEEE Transactions on Microwave Theory and Techniques]]| volume =51| issue =12| pages =203–206| date =2004-01-07| doi =10.1109/TMTT.2003.819193|bibcode = 2003ITMTT..51.2306C| s2cid = 32207634}}</ref>

=== Innovation ===

Combining left-handed segments with a conventional (right-handed) transmission line results in advantages over conventional designs. Left-handed transmission lines are essentially a high-pass filter with phase advance. Conversely, right-handed transmission lines are a low-pass filter with phase lag. This configuration is designated composite right/left-handed (CRLH) metamaterial.<ref>UCLA Technology. [http://www.research.ucla.edu/tech/ucla03-251.htm Backfire to Endfire Leaky wave antenna.] 2003. {{webarchive |url=https://web.archive.org/web/20100412152221/http://www.research.ucla.edu/tech/ucla03-251.htm |date=April 12, 2010 }}</ref><ref name=Caloz-MM-Antenna>{{Cite journal|last=Caloz |first=C. |title=Emerging Metamaterials Antennas and their advantages over conventional approaches |journal=URSI Commission B "Fields and Waves" |volume=Electromagnetic Theory Symposium 2007 (EMTS 2007) |url=http://ursi.org/B/EMTS_2007/EMTS2007.pdf |pages=01–03 |date=26–28 July 2007 |access-date=2010-04-24 |url-status=dead |archive-url=https://web.archive.org/web/20081013011622/http://ursi.org/B/EMTS_2007/EMTS2007.pdf |archive-date=October 13, 2008 }}</ref><ref name=Commission-B>
{{cite web
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The conventional Leaky Wave antenna has had limited commercial success because it lacks complete backfire-to-endfire frequency scanning capability. The CRLH allowed complete backfire-to-endfire frequency scanning, including broadside.

== Microwave lens ==

The [[metamaterial lens]], found in metamaterial antenna systems, is used as an efficient coupler to external radiation, focusing radiation along or from a [[microstrip]] transmission line into [[transmit]]ting and receiving components. Hence, it can be used as an [[input device]]. In addition, it can enhance the amplitude of [[evanescent waves]], as well as correct the phase of propagating waves.

=== Directing radiation ===

In this instance an SRR uses layers of a [[metal]]lic mesh of thin [[wire]]s – with wires in the [[Coordinate plane|three directions of space]] and slices of [[foam]]. This material's permittivity above the [[plasma frequency]] can be positive and less than one. This means that the [[refractive index]] is just above zero. The relevant parameter is often the contrast between the permittivities rather than the overall permittivity value at desired frequencies. This occurs because the equivalent (effective) permittivity has a behavior governed by a [[plasma frequency]] in the microwave domain. This low optical index material then is a good candidate for extremely convergent [[microlens]]es. Methods that have been developed theoretically using dielectric photonic crystals applied in the microwave domain to realize a directive emitter using metallic grids.<ref name=Directive-emission/>

In this instance, [[Crystal lattice#Cubic structures|arrayed wires in a cubic]], [[crystal lattice]] structure can be analyzed as an array of aerials ([[Phased array|antenna array]]). As a lattice structure it has a [[lattice constant]]. The lattice constant or lattice parameter refers to the constant distance between unit cells in a crystal lattice.<ref name=plasmafreq/>

The earlier discovery of [[plasmon]]s created the view that metal at [[plasmon frequency]] ''f<sub>p</sub>'' is a composite material. The effect of plasmons on any metal sample is to create properties in the metal such that it can behave as a [[dielectric]], independent of the wave vector of the EM excitation (radiation) field. Furthermore, a minute-fractionally small amount of plasmon energy is absorbed into the system denoted as ''γ''. For aluminium ''f<sub>p</sub>'' = 15 eV, and ''γ'' = 0.1 eV. Perhaps the most important result of the interaction of metal and the plasma frequency is that permittivity is negative below the plasma frequency, all the way to the minute value of ''γ''.<ref name=plasmafreq/><ref name=Plasma-resonance-absorption>{{Cite book|last = Bube|first =Richard H.|title =Electrons in solids: an introductory survey
| place = San Diego, CA|publisher =[[Elsevier Science]]|year =1992 |pages =155, 156
| url =https://books.google.com/books?id=u0ZJuFjPOYUC&pg=PA155|isbn =978-0-12-138553-8|access-date=2009-09-27}}</ref>

These facts ultimately result in the arrayed wire structure as being effectively a homogeneous medium.<ref name=plasmafreq>
{{cite journal|last =Pendry|first =J.B|author-link =John Pendry|author2 =AJ Holden|author3 =WJ Stewart|title =Extremely Low Frequency Plasmons in Metallic Mesostructures|journal =[[Phys. Rev. Lett.]]|volume =76|issue =25|year =1996|url =http://www.cmth.ph.ic.ac.uk/photonics/Newphotonics/pdf/lfplslet.pdf|doi =10.1103/PhysRevLett.76.4773|access-date =2009-09-27|pmid =10061377|bibcode =1996PhRvL..76.4773P|pages =4773–4776|archive-url =https://web.archive.org/web/20110717081513/http://www.cmth.ph.ic.ac.uk/photonics/Newphotonics/pdf/lfplslet.pdf|archive-date =2011-07-17|url-status =dead}}
</ref>

This metamaterial allows for control of the [[Polarization (waves)|direction]] of [[Emission (electromagnetic radiation)|emission]] of an electromagnetic radiation source located inside the material in order to collect all the [[energy]] in a small angular domain around the [[surface normal|normal]].<ref name=Directive-emission/> By using a slab of a metamaterial, diverging [[electromagnetic waves]] are focused into a narrow cone. Dimensions are small in comparison to the wavelength and thus the slab behaves as a homogeneous material with a low [[plasma frequency]].<ref name=Directive-emission>
{{cite journal| last1 = Enoch| first1 =Stefan
| title =A Metamaterial for Directive Emission| journal =[[Phys. Rev. Lett.]]|volume =89| issue =21| page =213902|date =2002-11-04
| url =http://people.engr.ncsu.edu/dschuri/ECE782_Fall_2009/Topics_files/e213902.pdf
| doi =10.1103/PhysRevLett.89.213902
| access-date =2009-09-16| pmid = 12443413| last2 = Tayeb| first2 = G| last3 = Sabouroux| first3 = P| last4 = Guérin| first4 = N| last5 = Vincent| first5 = P| bibcode=2002PhRvL..89u3902E}}</ref>

== Transmission line models ==

=== Conventional transmission lines ===

[[File:Transmission line symbols.svg|thumb|Variations on the [[electronic schematic|schematic]] [[electronic symbol]] for a transmission line.]]
[[File:Transmission line element.svg|thumb|Schematic representation of the elementary components of a transmission line.]]
A [[transmission line]] is the material [[Transmission medium|medium]] or structure that forms all or part of a [[Course (navigation)|path]] from one place to another for directing the [[transmission (telecommunications)|transmission]] of energy, such as [[electromagnetic wave]]s or [[electric power transmission]]. Types of transmission line include [[wire]]s, [[coaxial cable]]s, dielectric slabs, [[stripline]]s, [[optical fiber]]s, [[power line|electric power lines]] and waveguides.<ref>[[Federal Standard 1037C]]</ref>

A [[microstrip]] is a type of transmission line that can be fabricated using [[printed circuit board]] technology and is used to convey microwave-frequency signals. It consists of a conducting strip separated from a ground plane by a dielectric layer known as the '''substrate'''. Microwave components such as [[microstrip antenna|antennas]], [[directional coupler|coupler]]s, [[electronic filter|filter]]s and [[Power dividers and directional couplers#Other power dividers|power divider]]s can be formed from a microstrip.

From the simplified schematics to the right it can be seen that total impedance, conductance, reactance (capacitance and inductance) and the transmission medium (transmission line) can be represented by single components that give the overall value.

With transmission line media it is important to match the load impedance Z<sub>L</sub> to the [[characteristic impedance]] Z<sub>0</sub> as closely as possible, because it is usually desirable that the load absorbs as much power as possible.

: <math>R</math> is the [[Electrical resistance|resistance]] per unit length,
: <math>L</math> is the [[inductance]] per unit length,
: <math>G</math> is the [[Electrical conductance|conductance]] of the dielectric per unit length,
: <math>C</math> is the [[capacitance]] per unit length,
: <math>j</math> is the [[imaginary unit]], and
: <math>\omega</math> is the [[angular frequency]].

==== Lumped circuit elements ====

Often, because of the goal that moves physical metamaterial inclusions (or cells) to smaller sizes, discussion and implementation of [[Lumped-element model|lumped LC circuits]] or [[Distributed-element model|distributed LC networks]] are often examined. Lumped circuit elements are actually microscopic elements that effectively approximate their larger component counterparts. For example, circuit capacitance and inductance can be created with split rings, which are on the scale of nanometers at optical frequencies. The distributed LC model is related to the lumped LC model, however the [[distributed-element model]] is more accurate but more complex than the [[lumped-element model]].

=== Metamaterial-loaded transmission-line configurations ===

Some noted metamaterial antennas employ negative-refractive-index transmission-line metamaterials (NRI-TLM). These include [[lens (optics)|lense]]s that can overcome the [[diffraction]] limit, narrowband and broadband phase-shifting lines, small antennas, low-profile antennas, antenna feed networks, novel power architectures, and high-directivity couplers. Loading a planar metamaterial network of TLs with series capacitors and shunt inductors produces higher performance. This results in a large operating [[bandwidth (signal processing)|bandwidth]] while the refractive index is negative.<ref name=physicsengineering1/><ref name=CRLH-TL-1/>

Because [[superlens]]es can overcome the [[diffraction limit]], this allows for a more efficient coupling to external radiation and enables a broader frequency band. For example, the superlens can be applied to the TLM architecture. In conventional lenses, imaging is limited by the [[diffraction limit]]. With superlenses the details of the [[near and far field|near field]] images are not lost. Growing [[evanescent waves]] are supported in the metamaterial (''n'' < 1), which restores the decaying evanescent waves from the source. This results in a diffraction-limited resolution of λ/6, after some small losses. This compares with λ/2, the normal diffraction limit for conventional [[Microwave lens|lens]]es.<ref name=CRLH-TL-1/>

By combining right-handed (RHM) with left-handed materials (LHM) as a composite material (CRLH) construction, both a backward to forward [[Radar#Types of scan|scanning]] capability is obtained.

Metamaterials were first used for antenna technology around 2005. This type of antenna used the established capability of SNGs to couple with external [[radiation]]. Resonant [[coupling]] allowed for a wavelength larger than the antenna. At microwave frequencies this allowed for a smaller antenna.<ref name=radiation-properties>{{cite journal|last =Kamil| first =Boratay Alici|author2=Ekmel Özbay| title =Radiation properties of a split ring resonator and monopole composite| journal =Physica Status Solidi B| volume =244| issue =4| pages =1192–1196| date =2007-03-22| url = http://www.fen.bilkent.edu.tr/~ozbay/Papers/154-07-bora-pssb.pdf| doi = 10.1002/pssb.200674505| access-date =2009-09-17|bibcode = 2007PSSBR.244.1192A| hdl =11693/49278| s2cid =5348103| hdl-access =free}}</ref><ref name=CRLH-TL-1/>

A metamaterial-loaded transmission line has significant advantages over conventional or standard delay transmission lines. It is more compact in size, it can achieve positive or negative [[phase shift]] while occupying the same short physical length and it exhibits a linear, flatter [[phase response]] with [[frequency]], leading to shorter group delays. It can work in lower frequency because of high series distributed-capacitors and has smaller plane dimensions than its equivalent coplanar structure.<ref name=CRLH-TL-1>
{{Cite journal| last1 = Sanada| first1 = Atsushi| title = Characteristics of the Composite Right/Left-Handed Transmission Lines| journal = [[IEEE Microwave and Wireless Components Letters]]| volume = 14| issue = 2| pages = 68–70| date = 2004-02-26| url = http://www-ap.apsci.yamaguchi-u.ac.jp/01268100.pdf| doi = 10.1109/LMWC.2003.822563| access-date = 2009-12-28| last2 = Caloz| first2 = C.| last3 = Itoh| first3 = T.| s2cid = 22121283| url-status = dead| archive-url = https://web.archive.org/web/20070722032240/http://www-ap.apsci.yamaguchi-u.ac.jp/01268100.pdf| archive-date = 2007-07-22}}</ref>

==== Negative refractive index metamaterials supporting 2-D waves ====

In 2002, rather than using SRR-wire configuration, or other 3-D media, researchers looked at planar configurations that supported backward wave propagation, thus demonstrating negative refractive index and focusing as a consequence.<ref name=first-TL-lens-proposed/>

It has long been known that transmission lines [[Periodic function|periodically]] loaded with capacitive and inductive elements in a high-pass configuration support certain types of backward waves. In addition, planar transmission lines are a natural match for 2-D wave propagation. With lumped circuit elements they retain a compact configuration and can still support the lower RF range. With this in mind, high pass and cutoff, periodically loaded, two-dimensional LC transmission line networks were proposed. The LC networks can be designed to support backward waves, without bulky SRR/wire structure. This was the first such proposal which veered away from bulk media for a negative refractive effect. A notable property of this type of network is that there is no reliance on resonance, Instead the ability to support backward waves defines negative refraction.<ref name=first-TL-lens-proposed/>

The principles behind focusing are derived from Veselago and Pendry. Combining a conventional, flat, (planar) DPS slab, M-1, with a left-handed medium, M-2, a propagating electromagnetic wave with a [[wave vector]] k1 in M-1, results in a refracted wave with a wave vector k2 in M-2. Since, M-2 supports backward wave propagation k2 is refracted to the opposite side of the normal, while the [[Poynting vector]] of M-2 is anti-parallel with k2. Under such conditions, power is refracted through an effectively negative angle, which implies an effectively negative index of refraction.<ref name=first-TL-lens-proposed/>

Electromagnetic waves from a point source located inside a conventional DPS can be focused inside an LHM using a planar interface of the two media. These conditions can be modeled by exciting a single node inside the DPS and observing the magnitude and phase of the voltages to ground at all points in the LHM. A focusing effect should manifest itself as a “spot” distribution of voltage at a predictable location in the LHM.<ref name=first-TL-lens-proposed/>

Negative refraction and focusing can be accomplished without employing resonances or directly synthesizing the permittivity and permeability. In addition, this media can be practically fabricated by appropriately loading a host transmission line medium. Furthermore, the resulting planar topology permits LHM structures to be readily integrated with conventional planar microwave circuits and devices.<ref name=first-TL-lens-proposed/>

When transverse electromagnetic propagation occurs with a transmission line medium, the analogy for permittivity and permeability is ε = L, and μ = C. This analogy was developed with positive values for these parameters. The next logic step was realizing that negative values could be achieved. In order to synthesize a left-handed medium (ε < 0 and μ < 0) the series reactance and shunt susceptibility should become negative, because the material parameters are directly proportional to these circuit quantities.<ref name=BW-2002-11-15/>

A transmission line that has lumped circuit elements that synthesize a left-handed medium is referred to as a "dual transmission line" as compared to "conventional transmission line". The dual transmission line structure can be implemented in practice by loading a host transmission line with lumped series capacitors (C) and shunt inductors (L). In this periodic structure, the loading is strong such that the lumped elements dominate the propagation characteristics.<ref name=BW-2002-11-15>

{{Cite journal| last =Grbic| first =Anthony|author2=George V. Eleftheriades| title =Experimental verification of backward-wave radiation from a negative refractive index metamaterial| journal =Journal of Applied Physics| volume =92| issue =10| page =5930| date =2002-11-15| url =http://www.waves.utoronto.ca/prof/gelefth/Backup_Old/jpub/GrbicJAP.pdf
| doi =10.1063/1.1513194| access-date =2009-11-30|bibcode = 2002JAP....92.5930G}}</ref>

==== Left-handed behavior in LC loaded transmission lines ====

Using SRRs at [[Radio frequency|RF frequencies]], as with wireless devices, requires the resonators to be scaled to larger dimensions. This worked against making the devices more compact. In contrast, [[tank circuit|LC network]] configurations could be scaled to both microwave and RF frequencies.<ref name=Lhm-Lc/>

LC-loaded [[transmission lines]] enabled a new class of metamaterials to produce a [[negative refractive index]]. Relying on LC networks to emulate electrical [[permittivity]] and [[magnetic permeability]] resulted in a substantial increase in operating bandwidths.<ref name=Lhm-Lc/>

Moreover, their unit cells are connected through a transmission-line network and may be equipped with [[lumped circuit]] elements, which permit them to be compact at frequencies where an SRR cannot be compact. The flexibility gained through the use of either discrete or printed elements enables planar metamaterials to be scalable from the [[megahertz]] to the [[gigahertz|tens of gigahertz]] range. In addition, replacing capacitors with [[varactor]]s allowed the material properties to be dynamically tuned. The proposed media are planar and inherently support two-dimensional (2-D) wave propagation, making them well-suited for RF/microwave device and circuit applications.<ref name=Lhm-Lc>
{{Cite journal| last1 = Eleftheriades| first1 =George V.| title =Planar Negative Refractive Index Media Using Periodically L–C Loaded Transmission Lines| journal = IEEE Transactions on Microwave Theory and Techniques| volume =50| issue =12| page =2702| date =December 2002| url =http://www.waves.utoronto.ca/prof/gelefth/metamaterials_file/GVEAsh_MTT2002_Enhanced.pdf| doi =10.1109/TMTT.2002.805197| access-date =2009-11-26| last2 = Iyer| first2 = A.K.| last3 = Kremer| first3 = P.C.|bibcode = 2002ITMTT..50.2702E}}</ref>

==== Growing evanescent waves in negative-refractive-index transmission-line media ====

The periodic 2-D LC loaded transmission-line (''TL'') was shown to exhibit NRI properties over a broad frequency range. This network will be referred to as a dual TL structure since it is of a high-pass configuration, as opposed to the low-pass representation of a conventional TL structure.<ref name=EW-2003-03-24/> Dual TL structures have been used to experimentally demonstrate backward-wave radiation and focusing at microwave frequencies.<ref name=first-TL-lens-proposed/><ref name=EW-2003-03-24/>

As a negative refractive index medium, a dual TL structure is not simply a phase compensator. It can enhance the amplitude of evanescent waves, as well as correct the phase of propagating waves. Evanescent waves actually grow within the dual TL structure.<ref name=EW-2003-03-24>{{Cite journal|last=Grbic |first=Anthony |author2=George V. Eleftheriades |title=Growing evanescent waves in negative-refractive-index transmission-line media |journal=Applied Physics Letters |volume=82 |issue=12 |page=1815 |date=2003-03-24 |url=http://www.waves.toronto.edu/prof/gelefth/Copy%20of%20publications_files/pdf/APL1_Grbic_Eleftheriades.pdf |doi=10.1063/1.1561167 |access-date=2009-11-30 |bibcode=2003ApPhL..82.1815G |url-status=dead |archive-url=https://web.archive.org/web/20110720060049/http://www.waves.toronto.edu/prof/gelefth/Copy%20of%20publications_files/pdf/APL1_Grbic_Eleftheriades.pdf |archive-date=July 20, 2011 }}</ref>

==== Backward wave antenna using an NRI loaded transmission line ====

Grbic ''et al.'' used one-dimensional LC loaded transmission line network, which supports fast backward-wave propagation to demonstrate characteristics analogous to "reversed Cherenkov radiation". Their proposed backward-wave radiating structure was inspired by negative refractive index LC materials. The simulated E-plane pattern at 15&nbsp;GHz showed radiation towards the backfire direction in the far-field pattern, clearly indicating the excitation of a backward wave. Since the transverse dimension of the array is electrically short, the structure is backed by a long metallic trough. The trough acts as a waveguide below cut-off and recovers the back radiation, resulting in unidirectional far-field patterns.<ref name=Backward-wave-materials>{{Cite book|last=Grbic |first=Anthony |author2=George V. Eleftheriades |volume=4 |pages=340–343 |date=2002-08-07 |chapter-url=http://www.waves.utoronto.ca/prof/gelefth/publications_files/pdf/Grbic_APS_TX_01016992.pdf |doi=10.1109/APS.2002.1016992 |isbn=978-0-7803-7330-3 |url-status=dead |archive-url=https://web.archive.org/web/20110706233212/http://www.waves.utoronto.ca/prof/gelefth/publications_files/pdf/Grbic_APS_TX_01016992.pdf |archive-date=July 6, 2011 |chapter=A backward-wave antenna based on negative refractive index L-C networks |title=IEEE Antennas and Propagation Society International Symposium (IEEE Cat. No.02CH37313) |s2cid=118881068 }}</ref>

==== Planar NIMs with periodic loaded transmission lines ====

Planar media can be implemented with an effective negative refractive index. The underlying concept is based on appropriately loading a printed network of transmission lines periodically with inductors and capacitors. This technique results in effective permittivity and permeability material parameters that are both inherently and simultaneously negative, obviating the need to employ separate means. The proposed media possess other desirable features including very wide bandwidth over which the refractive index remains negative, the ability to guide 2-D TM waves, scalability from RF to millimeter-wave frequencies and low transmission losses, as well as the potential for tunability by inserting varactors and/or switches in the unit cell. The concept has been verified with circuit and full-wave simulations. A prototype focusing device has been tested experimentally. The experimental results demonstrated focusing of an incident cylindrical wave within an octave bandwidth and over an electrically short area; suggestive of near-field focusing.<ref name=Planar-NIM-one/>

RF/microwave devices can be implemented based on these proposed media for applications in wireless communications, surveillance and radars.<ref name=Planar-NIM-one>
{{cite journal| last =Eleftheriades| first = G.V. |author2=Iyer, A.K. |author3=Kremer, P.C. | title =Planar negative refractive index media using periodically L-C loaded transmission lines| journal =IEEE Transactions on Microwave Theory and Techniques
| volume =50| issue =12| pages =2702–2712| date =2002-12-16
| url =http://individual.utoronto.ca/iyer/index_files/MTTrans_iyer01097986.pdf|doi =10.1109/TMTT.2002.805197|bibcode = 2002ITMTT..50.2702E}}</ref>

=== Larger transmission lines ===

According to some researchers SRR/wire-configured metamaterials are bulky 3-D constructions that are difficult to adapt for RF/microwave device and circuit applications. These structures can achieve a negative index of refraction only within a narrow bandwidth. When applied to wireless devices at RF frequencies the split ring-resonators have to be scaled to larger dimensions, which, in turn forces a larger device size.<ref name=Planar-NIM-one/>

The proposed structures go beyond the wire/SRR composites in that they do not rely on SRRs to synthesize the material parameters, thus leading to dramatically increased operating bandwidths. Moreover, their unit cells are connected through a transmission-line network and they may, therefore, be equipped with lumped elements, which permit them to be compact at frequencies where the SRR cannot be compact. The flexibility gained through the use of either discrete or printed elements enables planar metamaterials to be scalable from the megahertz to the tens of gigahertz range. In addition, by utilizing varactors instead of capacitors, the effective material properties can be dynamically tuned. Furthermore, the proposed media are planar and inherently support two-dimensional (2-D) wave propagation. Therefore, these new metamaterials are well suited for RF/microwave device and circuit applications.<ref name=Planar-NIM-one/>

In the long-wavelength regime, the permittivity and permeability of conventional materials can be artificially synthesized using periodic LC networks arranged in a low-pass configuration. In the dual (high-pass) configuration, these equivalent material parameters assume simultaneously negative values, and may therefore be used to synthesize a negative refractive index.<ref name=verify-focus>{{cite journal| last = Iyer| first =Ashwin|author2=Peter Kremer |author3=George Eleftheriades  |title =Experimental and theoretical verification of focusing in a large, periodically loaded transmission line negative refractive index metamaterial|journal =Optics Express| volume =11| issue =7|pages =696–708 |year =2003|doi =10.1364/OE.11.000696| pmid = 19461781|bibcode = 2003OExpr..11..696I|doi-access =free}}</ref>

== Configurations ==
{{Main|Antenna (radio)}}
Antenna theory is based on [[Classical electromagnetism|classical electromagnetic theory]] as described by [[Maxwell's equations]].<ref name=antenna-theory-practice/> Physically, an antenna is an arrangement of one or more [[Electrical conductor|conductor]]s, usually called elements. An [[alternating current]] is created in the elements by applying a voltage at the antenna terminals, causing the elements to radiate an electromagnetic field. In reception, the reverse occurs: an electromagnetic field from another source induces an alternating current in the elements and a corresponding voltage at the antenna's terminals. Some receiving antennas (such as parabolic and horn types) incorporate shaped reflective surfaces to collect EM waves from free space and direct or focus them onto the actual conductive elements.

An antenna creates sufficiently strong electromagnetic fields at large distances. Reciprocally, it is sensitive to the electromagnetic fields impressed upon it externally. The actual coupling between a transmitting and receiving antenna is so small that amplifier circuits are required at both the transmitting and receiving stations. Antennas are usually created by modifying ordinary circuitry into transmission line configurations.<ref name=antenna-theory-practice/>

The required antenna for any given application is dependent on the bandwidth employed, and range (power) requirements. In the microwave to millimeter-wave range – wavelengths from a few meters to millimeters – the following antennas are usually employed:<ref name=antenna-theory-practice/>

Dipole antennas, short antennas, parabolic and other reflector antennas, horn antennas, periscope antennas, helical antennas, spiral antennas, surface-wave and leaky wave antennas. Leaky wave antennas include dielectric and dielectric loaded antennas, and the variety of microstrip antennas.<ref name=antenna-theory-practice>
{{Cite book| last =Chatterjee| first =Rajeswari| title =Antenna theory and practice| publisher = New Age International| year =1996| location =New Delhi| pages =1, 2| url =https://books.google.com/books?id=J4YcUA-rxJoC&q=microwave%20antenna%20systems| isbn =978-0-470-20957-8}}</ref>

=== Radiation properties with SRRs ===

The SRR was introduced by Pendry in 1999, and is one of the most common elements of [[metamaterial]]s.<ref name=Pendry-1999>{{cite journal|author=Pendry, J.B.|journal= IEEE Transactions on Microwave Theory and Techniques|volume=47|page=2075|year=1999|doi=10.1109/22.798002|title=Magnetism from conductors and enhanced nonlinear phenomena|bibcode = 1999ITMTT..47.2075P|display-authors=1|last2=Holden|first2=A.J.|last3=Robbins|first3=D.J.|last4=Stewart|first4=W.J.|issue=11|citeseerx=10.1.1.564.7060 }}</ref> As a nonmagnetic conducting unit, it comprises an array of units that yield an enhanced negative effective magnetic permeability, when the frequency of the incident electromagnetic field is close to the SRR resonance frequency. The resonant frequency of the SRR depends on its shape and physical design. In addition, resonance can occur at wavelengths much larger than its size.<ref name=Hsu-2004>{{cite journal|doi=10.1063/1.1767290|title=Electromagnetic resonance in deformed split ring resonators of left-handed meta-materials|year=2004|last1=Hsu|first1=Yi-Jang|last2=Huang|first2=Yen-Chun|last3=Lih|first3=Jiann-Shing|last4=Chern|first4=Jyh-Long|journal=Journal of Applied Physics|volume=96|page=1979|bibcode = 2004JAP....96.1979H|issue=4}}</ref><ref name=K-Aydin-2005>{{cite journal|doi=10.1088/1367-2630/7/1/168|title=Investigation of magnetic resonances for different split-ring resonator parameters and designs|year=2005|last1=Aydin|first1=Koray|last2=Bulu|first2=Irfan|last3=Guven|first3=Kaan|last4=Kafesaki|first4=Maria|last5=Soukoulis|first5=Costas M|last6=Ozbay|first6=Ekmel|journal=New Journal of Physics|volume=7|issue=1|page=168|bibcode = 2005NJPh....7..168A|doi-access=free}}</ref> For the further shape optimization of the elements it is expedient to use genetic and other optimization algorithms. In multi-frequency designs one may apply fractal designs such as those of Sierpensky, Koch or other fractals  instead of SRRs.<ref name=slyusarmeta/>

==== Double negative metamaterials ====

Through the application of [[double negative metamaterial]]s (DNG), the [[Power (physics)|power radiated]] by [[Electromagnetism|electrically]] small [[dipole]] antennas can be notably increased. This could be accomplished by surrounding an antenna with a shell of double negative (DNG) material. When the electric dipole is embedded in a [[homogeneity (physics)|homogeneous]] DNG medium, the antenna acts inductively rather than capacitively, as it would in [[free space]] without the interaction of the DNG material. In addition, the dipole-DNG shell combination increases the real power radiated by more than an [[order of magnitude]] over a free space antenna. A notable decrease in the reactance of the dipole antenna corresponds to the increase in radiated power.<ref name=DNG-shell-2003-10/>

The reactive power indicates that the DNG shell acts as a natural matching network for the dipole. The DNG material matches the intrinsic reactance of this antenna system to free space, hence the impedance of DNG material matches free space. It provides a natural matching circuit to the antenna.<ref name=DNG-shell-2003-10>{{Cite journal|last=Ziolkowski |first=Richard Wly |author2=Allison D. Kipple |title=Application of Double Negative Materials to Increase the Power Radiated by Electrically Small Antennas |journal=IEEE Transactions on Antennas and Propagation |volume=51 |issue=10 |page=2626 |date=2003-10-14 |url=http://cc.ee.ntu.edu.tw/~yclin/EM_Theory/Readings/10meta-antenna%20application.pdf |doi=10.1109/TAP.2003.817561 |access-date=2009-11-30 |bibcode=2003ITAP...51.2626Z |citeseerx=10.1.1.205.5571 }}{{dead link|date=May 2016|bot=medic}}{{cbignore|bot=medic}}</ref>

==== Single negative SRR and monopole composite ====

The addition of an [[Split-ring resonator|SRR-DNG metamaterial]] increased the [[Power (physics)|radiated power]] by more than an [[order of magnitude]] over a comparable free space antenna. Electrically small antennas, high [[directivity]] and tunable operational frequency are produced with negative magnetic permeability. When combining a right-handed material (RHM) with a Veselago-left-handed material (LHM) other novel properties are obtained. A single negative material resonator, obtained with an SRR, can produce an electrically small antenna when operating at microwave frequencies, as follows:<ref name=radiation-properties/>

The configuration of an SRR assessed was two concentric [[annulus (mathematics)|annular]] rings with relative opposite gaps in the inner and outer ring. Its [[geometrical]] parameters were R = 3.6&nbsp;mm, r = 2.5&nbsp;mm, w = 0.2&nbsp;mm, t = 0.9&nbsp;mm. R and r are used in annular parameters, w is the spacing between the rings and t = the width of the outer ring. The material had a thickness of 1.6&nbsp;mm. Permittivity was 3.85 at 4&nbsp;GHz. The SRR was fabricated with an etching technique onto a 30 [[μm]] thick [[copper]] substrate. The SRR was excited by using a [[monopole antenna]]. The monopole antenna was composed of a [[coaxial cable]], ground plane and radiating components. The ground plane material was [[aluminium]]. The operation frequency of the antenna was 3.52&nbsp;GHz, which was determined by considering the geometrical parameters of SRR. An 8.32&nbsp;mm length of wire was placed above the ground plane, connected to the antenna, which was one quarter of the operation wavelength. The antenna worked with a feed wavelength of 3.28&nbsp;mm and feed frequency of 7.8&nbsp;GHz. The SRR's resonant frequency was smaller than the monopole operation frequency.<ref name=radiation-properties/>

The monopole-SRR antenna operated efficiently at (λ/10) using the SRR-wire configuration. It demonstrated good coupling efficiency and sufficient radiation efficiency. Its operation was comparable to a conventional antenna at λ/2, which is a conventional antenna size for efficient coupling and radiation. Therefore, the monopole-SRR antenna becomes an acceptable electrically small antenna at the SRR's resonance frequency.<ref name=radiation-properties/><ref name=slyusarmeta/>

When the SRR is made part of this configuration, characteristics such as the antenna's radiation pattern are entirely changed in comparison to a conventional monopole antenna. With modifications to the SRR structure the antenna size could reach ('''λ/40'''). Coupling 2, 3, and 4 SRRs side by side slightly shifts radiation patterns.<ref name=radiation-properties/>

=== Patch antennas ===

In 2005 a [[patch antenna]] with a [[metamaterial cover]] was proposed that enhanced [[directivity]]. According to the numerical results, the antenna showed significant improvement in directivity, compared to conventional patch antennae. This was cited in 2007 for an efficient design of directive patch antennas in mobile communications using metamaterials.<ref name=slyusarmeta/>  This design was based on the left-handed material (LHM) transmission line model, with the circuit elements L and C of the LHM [[equivalent circuit]] model. This study developed [[formula]]e to determine the L and C values of the LHM equivalent circuit model for desirable characteristics of directive patch antennas. Design examples derived from actual [[frequency]] bands in [[mobile communications]] were performed, which illustrates the efficiency of this approach.<ref name=IEEE-V3-2005>
{{Cite book| last1 =Fangming Zhu| last2 =Qingchun Lin| last3 =Jun Hu| title =2005 Asia-Pacific Microwave Conference Proceedings| volume =3| page =1| year =2005| doi =10.1109/APMC.2005.1606717| chapter =A Directive Patch Antenna with a Metamaterial Cover| isbn =978-0-7803-9433-9| s2cid =27288814}}</ref><ref name=IJIMW-V28-2007>
{{Cite journal|last1 =Wang|first1 =Rui|last2 =Yuan|first2 =Bo|last3 =Wang|first3 =Gaofeng|last4 =Yi|first4 =Fan|doi =10.1007/s10762-007-9249-1|title =Efficient Design of Directive Patch Antennas in Mobile Communications Using Metamaterials|year =2007|page =639|volume =28|journal =International Journal of Infrared and Millimeter Waves|bibcode = 2007IJIMW..28..639W|issue =8|s2cid =108959389}}</ref><ref name=ieee-Alu-v55-1>{{Cite journal| last1 =Alu| first1 =Andrea| last2 =Bilotti| first2 =Filiberto| last3 =Engheta| first3 =Nader| last4 =Vegni| first4 =Lucio| title =Subwavelength, Compact, Resonant Patch Antennas Loaded With Metamaterials| journal =IEEE Transactions on Antennas and Propagation| volume =55| issue =1| page =13| year =2007| doi =10.1109/TAP.2006.888401|bibcode = 2007ITAP...55...13A| s2cid =6091311| url =https://repository.upenn.edu/cgi/viewcontent.cgi?article=1244&context=ese_papers}}</ref>

=== Flat lens horn antenna ===

This configuration uses a flat aperture constructed of zero-index metamaterial. This has advantages over ordinary (conventional) curved lenses, which results in a much improved directivity.<ref name=slyusarmeta/> These investigations have provided capabilities for the miniaturization of microwave source and non-source devices, circuits, antennas and the improvement of electromagnetic performance.<ref name=flat-lens-antenna>{{Cite journal|last1 =WU|first1 =Q.|title =A novel flat lens horn antenna designed based on zero refraction principle of metamaterials|journal =Appl. Phys. A|volume =87|pages =151–156
| date =2007-01-31|doi =10.1007/s00339-006-3820-9|last2 =Pan|first2 =P.|last3 =Meng|first3 =F.-Y.|last4 =Li|first4 =L.-W.|last5 =Wu|first5 =J.|bibcode = 2007ApPhA..87..151W|issue =2|s2cid =122690235}}</ref>

===Metamaterials surface antenna technology===
'''Metamaterials surface antenna technology''' (M-SAT) is an invention that uses metamaterials to direct and maintain a consistent broadband [[radio frequency]] beam locked on to a [[satellite]] whether the platform is in motion or stationary. [[Gimbal]]s and motors are replaced by arrays of metamaterials in a planar configuration. Also, with this new technology [[phase shifter]]s are not required as with [[phased array]] equipment. The desired affect is accomplished by varying the pattern of activated metamaterial elements as needed. The technology is a practical application of [[metamaterial cloaking]] theory. The antenna is approximately the size of a laptop computer.<ref name=savitz>{{cite magazine | last = Eric Savitz | title =Bill Gates Invests In Intellectual Ventures' Spin-Out Kymeta | magazine = Forbes | date = August 21, 2012 | url = https://www.forbes.com/sites/ericsavitz/2012/08/21/bill-gates-invests-in-intellectual-ventures-spin-out-kymeta/ }}</ref><ref name=palmer>{{cite journal| last = Katie M. Palmer | title =Intellectual Ventures Invents Beam-Steering Metamaterials Antenna | journal = IEEE Spectrum | date = January 2012 | url = https://spectrum.ieee.org/telecom/wireless/intellectual-ventures-invents-beamsteering-metamaterials-antenna/0 }}</ref><ref name=sbee>{{cite news| title =Kymeta spins out from Intellectual Ventures after closing $12 million funding| newspaper =The Sacramento Bee| date =Aug 21, 2012| url =http://www.sacbee.com/2012/08/21/4744424/kymeta-spins-out-from-intellectual.html| quote =Company to commercialize IV's metamaterials-based satellite antenna technology| url-status =dead| archive-url =https://web.archive.org/web/20120901161756/http://www.sacbee.com/2012/08/21/4744424/kymeta-spins-out-from-intellectual.html| archive-date =2012-09-01}}</ref>

Research and applications of metamaterial based antennas. Related components are also researched.<ref>[http://www2.engr.arizona.edu/~ziolkows/research/Metamaterial-Engineered%20Antennas.html Metamaterial-Engineered Antennas]. University of Arizona. Accessed 2011-03-12.</ref><ref>[http://www.wpafb.af.mil/news/story_print.asp?id=123120782 AFRL-Demonstrated Metamaterials Technology Transforms Antenna Radiation Pattern] {{webarchive|url=https://web.archive.org/web/20110605114601/http://www.wpafb.af.mil/news/story_print.asp?id=123120782 |date=2011-06-05 }}. U.S. Air Force research.Accessed 2011-03-12</ref>

== Subwavelength cavities and waveguides ==

When the interface between a pair of materials that function as optical [[transmission media]] interact as a result of opposing permittivity and / or permeability values that are either ordinary (positive) or extraordinary (negative), notable anomalous behaviors may occur. The pair would be a DNG metamaterial (layer), paired with a DPS, ENG or MNG layer. Wave propagation behavior and properties may occur that would otherwise not happen if only DNG layers are paired together.<ref name=tang-field/>

At the interface between two media, the concept of the continuity of the tangential electric and magnetic field components can be applied. If either the permeability or permittivity of two media has opposite signs then the normal components of the tangential field, on both sides of the interface, will be discontinuous at the boundary. This implies a concentrated resonant phenomenon at the interface. This appears to be similar to the current and voltage distributions at the junction between an inductor and capacitor, at the resonance of an L-C circuit. This "''interface resonance''" is essentially independent of the total thickness of the paired layers, because it occurs along the discontinuity between two such conjugate materials.<ref name=tang-field>{{cite journal|doi=10.1109/TAP.2003.817553|title=Pairing an epsilon-negative slab with a mu-negative slab: Resonance, tunneling and transparency|year=2003|last1=Alu|first1=A.|last2=Engheta|first2=N.|journal=IEEE Transactions on Antennas and Propagation|url=http://repository.upenn.edu/cgi/viewcontent.cgi?article=1002&context=ese_papers|volume=51|page=2558|bibcode = 2003ITAP...51.2558A|issue=10}}</ref><ref name=cavity-resonator/>

=== Parallel-plate waveguiding structures ===

The geometry consists of two parallel plates as perfect conductors (PEC), an idealized structure, filled by two stacked planar slabs of homogeneous and isotropic materials with their respective constitutive parameters ε<sub>1</sub>, ε<sub>2</sub>, u<sub>1</sub>, u<sub>2</sub>. Each slab has thickness = d, slab 1 = d<sub>1</sub>, and slab 2 = d<sub>2</sub>. Choosing which combination of parameters to employ involves pairing DPS and DNG or ENG and MNG materials. As mentioned previously, this is one pair of oppositely-signed constitutive parameters, combined.<ref name=sng-dng>
{{Cite journal
| last = Alù| first =Andrea and|author2=Nader Engheta |author-link = Andrea Alù
| title =Guided Modes in a Waveguide Filled With a Pair of Single-Negative (SNG), Double-Negative (DNG), and/or Double-Positive (DPS) Layers
| journal =IEEE Transactions on Microwave Theory and Techniques| volume =52| issue =1| page =199| date =January 2004
| url =http://repository.upenn.edu/cgi/viewcontent.cgi?article=1001&context=ese_papers
| doi =10.1109/TMTT.2003.821274|access-date =2010-01-03|bibcode = 2004ITMTT..52..199A| s2cid =234001| author2-link =Nader Engheta}}</ref>

=== Thin subwavelength cavity resonators ===

====Phase compensation====

The real component values for negative permittivity and permeability results in real component values for negative refraction n. In a lossless medium, all that would exist are real values. This concept can be used to map out phase compensation when a conventional lossless material, DPS, is matched with a lossless NIM (DNG).<ref name=cavity-resonator/>

In phase compensation, the DPS of thickness d<sub>1</sub> has ε > 0 and µ > 0. Conversely, the NIM of thickness d<sub>2</sub> has ε < 0 and µ < 0. Assume that the intrinsic impedance of the DPS dielectric material (d<sub>1</sub>) is the same as that of the outside region and responding to a normally incident planar wave. The wave travels through the medium without any reflection because the DPS impedance and the outside impedance are equal. However, the plane wave at the end of DPS slab is out of phase with the plane wave at the beginning of the material.<ref name=cavity-resonator/>

The plane wave then enters the lossless NIM (d<sub>2</sub>). At certain frequencies ε < 0 and µ < 0 and n < 0. Like the DPS, the NIM has intrinsic impedance that is equal to the outside, and, therefore, is also lossless. The direction of power flow (i.e., the Poynting vector) in the first slab should be the same as that in the second one, because the power of the incident wave enters the first slab (without any reflection at the first interface), traverses the first slab, exits the second interface, enters the second slab and traverses it, and finally leaves the second slab. However, as stated earlier, the direction of power is anti-parallel to the direction of phase velocity. Therefore, the wave vector k<sub>2</sub> is in the opposite direction of k<sub>1</sub>. Furthermore, whatever phase difference is developed by traversing the first slab can be decreased and even cancelled by traversing the second slab. If the ratio of the two thicknesses is '''''d<sub>1</sub> / d<sub> 2</sub> = n<sub>2</sub> / n<sub>1</sub>''''', then the total phase difference between the front and back faces is zero.<ref name=cavity-resonator/> This demonstrates how the NIM slab at chosen frequencies acts as a phase compensator. It is important to note that this phase compensation process is only on the ratio of '''''d<sub>1</sub> / d<sub> 2</sub>''''' rather than the thickness of '''''d<sub>1</sub> + d<sub>1</sub>'''''. Therefore, '''''d<sub>1</sub> + d<sub>1</sub>''''' can be any value, as long as this ratio satisfies the above condition. Finally, even though this two-layer structure is present, the wave traversing this structure would not experience the phase difference.

Following this, the next step is the subwavelength cavity resonator.<ref name=cavity-resonator>
{{cite journal| last = Engheta| first =Nader
| title =An Idea for Thin Subwavelength Cavity Resonators Using Metamaterials With Negative Permittivity and Permeability| journal =IEEE Antennas and Wireless Propagation Letters| volume =1
| issue =1|pages =10–13|year =2002
| url =http://repository.upenn.edu/cgi/viewcontent.cgi?article=1011&context=ese_papers
| doi =10.1109/LAWP.2002.802576
| access-date =2009-10-08|bibcode = 2002IAWPL...1...10E| s2cid =12554352
}}</ref>

====Compact subwavelength 1-D cavity resonators using metamaterials====

The phase compensator described above can be used to conceptualize the possibility of designing a compact 1-D cavity resonator. The above two-layer structure is applied as two perfect
reflectors, or in other words, two perfect conducting plates. Conceptually, what is constrained in the resonator is '''''d<sub>1</sub> / d<sub>2</sub>''''', not '''''d<sub>1</sub> + d<sub>2</sub>'''''. Therefore, in principle, one can have a thin subwavelength cavity resonator for a given frequency, if at this frequency the second layer acts a metamaterial with negative permittivity and permeability and the ratio correlates to the correct values.<ref name=cavity-resonator/>

The cavity can conceptually be thin while still resonant, as long as the ratio of thicknesses is satisfied. This can, in principle, provide possibility for subwavelength, thin, compact cavity resonators.<ref name=cavity-resonator/>

=== Miniature cavity resonator utilizing FSS ===

Frequency selective surface (FSS) based metamaterials utilize ''equivalent'' LC circuitry configurations. Using FSS in a cavity allows for miniaturization, decrease of the resonant frequency, lowers the cut-off frequency and smooth transition from a fast-wave to a slow-wave in a waveguide configuration.<ref name=FSS-cavity-2004>
{{Cite journal|last1 = Caiazzo|first1 =Marco |title =A Metamaterial Surface for Compact Cavity Resonators|journal =IEEE Antennas and Wireless Propagation Letters|volume =3|issue =14 |page =261
| year =2004|url =http://repository.upenn.edu/cgi/viewcontent.cgi?article=1191&context=ese_papers
| doi =10.1109/LAWP.2004.836576|last2 = Maci|first2 = S.|last3 = Engheta|first3 = N.|bibcode = 2004IAWPL...3..261C|s2cid =25842956 }}</ref>

=== Composite metamaterial based cavities ===

As an LHM application four different cavities operating in the microwave regime were fabricated and experimentally observed and described.<ref name=composite-cavity-11-2009>
{{Cite journal| last1 = Caglayan| first1 = Humeyra| title = Experimental observation of cavity formation in composite metamaterials| journal = Optics Express| pages = 11132–40| volume = 16| issue = 15| date = 2008-07-21| doi = 10.1364/OE.16.011132| pmid = 18648427| last2 = Bulu| first2 = I| last3 = Loncar| first3 = M| last4 = Ozbay| first4 = E| bibcode = 2008OExpr..1611132C| hdl = 11693/13532| doi-access = free}}</ref>

== Metamaterial ground plane ==

=== Leaky mode propagation with metamaterial ground plane ===

A magnetic dipole was placed on metamaterial (slab) ground plane. The metamaterials have either constituent parameters that are both negative, or negative permittivity or negative permeability. The dispersion and radiation properties of leaky waves supported by these metamaterial slabs, respectively, were investigated.<ref name=leaky-waves-MM-gp>
{{Cite journal| last1 = Baccarelli| first1 =Paolo| title =Effects of Leaky-Wave Propagation in Metamaterial Grounded Slabs Excited by a Dipole Source| journal =IEEE Transactions on Microwave Theory and Techniques| volume =53| issue =1| page =32| date =2005-01-17| doi =10.1109/TMTT.2004.839346| last2 = Burghignoli| first2 = P.| last3 = Frezza| first3 = F.| last4 = Galli| first4 = A.| last5 = Lampariello| first5 = P.| last6 = Lovat| first6 = G.| last7 = Paulotto| first7 = S.|bibcode = 2005ITMTT..53...32B| s2cid =14055916}}</ref>

== Patented systems ==

[[File:Phased array metamaterial system.jpg|thumb|300px|[[Microstrip]] line ('''400''') for a phased array metamaterial antenna system. '''401''' represents unit-cell circuits composed periodically along the microstrip.''' 402''' series capacitors. '''403''' are T-junctions between capacitors, which connect ('''404''') spiral inductor delay lines to 401. 404 are also connected to ground vias '''405'''.]]
Multiple systems have [[patent]]s.

Phased array systems and antennas for use in such systems are well known in areas such as telecommunications and [[radar]] applications. In general phased array systems work by coherently reassembling signals over the entire array by using circuit elements to compensate for relative phase differences and time delays.<ref name=metamaterial-phased-arrayed-antenna/>

=== Phased array antenna ===

Patented in 2004, one phased array antenna system is useful in automotive radar applications. By using NIMs as a [[biconcave lens]] to focus microwaves, the antenna's [[sidelobe]]s are reduced in size. This equates to a reduction in radiated energy loss, and a relatively wider useful bandwidth. The system is an efficient, dynamically ranged [[phased array radar]] system.<ref name=metamaterial-phased-arrayed-antenna/>

In addition, signal amplitude is increased across the [[microstrip]] transmission lines by suspending them above the ground plane at a predetermined distance. In other words, they are not in contact with a solid substrate. Dielectric signal loss is reduced significantly, reducing signal attenuation.<ref name=metamaterial-phased-arrayed-antenna/>

This system was designed to boost the performance of the [[Monolithic microwave integrated circuit]] (MMIC), among other benefits. A transmission line is created with photolithography. A metamaterial lens, consisting of a thin wire array focuses the transmitted or received signals between the line and the emitter / receiver elements.<ref name=metamaterial-phased-arrayed-antenna>{{Cite web|url=https://patents.google.com/patent/US6958729B1/en|title=Phased array metamaterial antenna system}}</ref>

The lens also functions as an input device and consists of a number of periodic unit-cells disposed along the line. The lens consists of multiple lines of the same make up; a plurality of periodic unit-cells. The periodic unit-cells are constructed of a plurality of electrical components; capacitors and inductors as components of multiple [[distributed-element circuit]]s.<ref name=metamaterial-phased-arrayed-antenna/>

The metamaterial incorporates a conducting transmission element, a substrate comprising at least a first ground plane for grounding the transmission element, a plurality of unit-cell circuits composed periodically along the transmission element and at least one [[via (electronics)|via]] for electrically connecting the [[transmission medium|transmission element]] to at least the first ground plane. It also includes a means for suspending this transmission element a predetermined distance from the substrate in a way such that the transmission element is located at a second predetermined distance from the ground plane.<ref name=metamaterial-phased-arrayed-antenna/>

=== ENG and MNG waveguides and scattering devices ===

This structure was designed for use in waveguiding or scattering of waves. It employs two adjacent layers. The first layer is an epsilon-negative (ENG) material or a mu-negative (MNG) material. The second layer is either a double-positive (DPS) material or a double-negative (DNG) material. Alternatively, the second layer can be an ENG material when the first layer is an MNG material or the reverse.<ref name=US-Patent-7218190>[[Nader Engheta|Engheta, Nader]]; Alù, Andrea "Waveguides and scattering devices incorporating epsilon-negative and/or mu-negative slabs" {{US patent|7218190}} publication date May 15, 2007</ref>

== Reducing interference ==
[[File:SundryKeyFobs2154.jpg|thumb|A [[keyless entry system]] [[key fob]]]]
Metamaterials can reduce interference across multiple devices with smaller and simpler shielding. While conventional absorbers can be three inches thick, metamaterials can be in the millimeter range&mdash;2&nbsp;mm (0.078&nbsp;in) thick.<ref name=poliferate>{{Cite news
| last =Matthew
| first =Finnegan
| title =Metamaterials to revolutionize wireless infrastructure
| newspaper =TechEye
| quote =With the increasing proliferation of wireless devices inside and out of the home and workplace there are concerns over how interference from the external electromagnetic environment can cause problems for the connectivity of devices in the future.
| publisher =JAM IT Media Ltd
| date =December 10, 2010
| url =http://www.techeye.net/internet/metamaterials-to-revolutionise-wireless-infrastructure
| access-date =2010-12-30}}</ref>

== See also ==
{{div col|colwidth=30em}}

* [[Acoustic metamaterials]]
* [[Chirality (electromagnetism)]]
* [[Kymeta]]
* [[Metamaterial]]
* [[Metamaterial cloaking]]
* [[Metamaterials surface antenna technology]]
* [[Negative index metamaterials]]
* [[Nonlinear metamaterials]]
* [[Photonic metamaterials]]
* [[Photonic crystal]]
* [[Quantum metamaterials]]
* [[Seismic metamaterials]]
* [[Split-ring resonator]]
* [[Superlens]]
* [[Tunable metamaterials]]
* [[Transformation optics]]
* [[Acoustic dispersion]]
* [[Coplanar waveguide]]
:::: '''Books'''
* [[Metamaterials Handbook]]
* [[Metamaterials: Physics and Engineering Explorations]]

{{div col end}}

== References ==
{{Reflist|35em}}

=== General references ===
*{{Cite journal
 |last1       = Ziolkowski
 |first1      = R. W.
 |last2      = Lin
 |first2     = Chia-Ching
 |last3      = Nielsen
 |first3     = Jean A.
 |last4      = Tanielian
 |first4     = Minas H.
 |last5      = Holloway
 |first5     = Christopher L.
 |title      = Design and Experimental Verification of
 |journal    = [[IEEE Antennas and Wireless Propagation Letters]]
 |volume     = 8
 |pages      = 989–993
 |date       = August–September 2009
 |doi        = 10.1109/LAWP.2009.2029708
 |bibcode    = 2009IAWPL...8..989Z
|citeseerx      = 10.1.1.205.4814
 |s2cid = 7804333
 }}

== External links ==
* [https://web.archive.org/web/20110605114601/http://www.wpafb.af.mil/news/story_print.asp?id=123120782 U.S. Air Force Research Lab] Demonstrated metamaterials technology transforms antenna radiation pattern
* [http://www.turpion.org/php/paper.phtml?journal_id=pu&paper_id=3699 The electrodynamics of substances with simultaneously negative values of ε and μ] Victor G. Veselago.
* [http://lib.tkk.fi/Diss/2009/isbn9789512299874/isbn9789512299874.pdf Microwave transmission-line networks for backward-wave media and reduction of scattering]
* [https://spectrum.ieee.org/computing/hardware/air-power Radiating power through air]
* [http://www.ntia.doc.gov/osmhome/redbook/redbook.html NTIA] Manual of Regulations and Procedures for Federal Radio Frequency Management. 2011.

{{DEFAULTSORT:Metamaterial Antennas}}
[[Category:Radio frequency antenna types]]
[[Category:Metamaterials]]