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[[Image:Schwarzbeck BBHA 9120 D.jpg|thumb|Pyramidal microwave horn antenna, with a bandwidth of 0.8 to 18 GHz. A coaxial cable feedline attaches to the connector visible at top. This type is called a ridged horn; the curving fins visible inside the mouth of the horn increase the antenna's [[Bandwidth (signal processing)|bandwidth]]. ]]

[[Image:ATM Horn Antennas.jpg|thumb|Pyramidal horn antennas for a variety of frequencies. They have flanges at the top to attach to standard waveguides.]]

A '''horn antenna''' or '''microwave horn''' is an [[antenna (radio)|antenna]] that consists of a flaring metal [[waveguide]] shaped like a [[horn (acoustic)|horn]] to direct radio waves in a beam. Horns are widely used as antennas at [[Ultrahigh frequency|UHF]] and [[microwave]] frequencies, above 300 MHz.<ref name="Bevilaqua">{{cite web
| last = Bevilaqua
| first = Peter
| authorlink =
| coauthors =
| title = Horn antenna - Intro
| work = Antenna-theory.com website
| publisher =
| year = 2009
| url = http://www.antenna-theory.com/antennas/aperture/horn.php
| doi =
| accessdate = 2010-11-11}}</ref> They are used as [[Antenna feed|feeders]] (called [[feed horn]]s) for larger antenna structures such as [[parabolic antenna]]s, as standard calibration antennas to measure the [[Antenna gain|gain]] of other antennas, and as directive antennas for such devices as [[radar gun]]s, [[Sliding door operator|automatic door openers]], and [[microwave radiometer]]s.<ref name="Poole">{{cite web
| last = Poole
| first = Ian
| authorlink =
| coauthors =
| title = Horn antenna
| work = Radio-Electronics.com website
| publisher = Adrio Communications Ltd.
| date =
| url = http://www.radio-electronics.com/info/antennas/horn_antenna/horn_antenna.php
| doi =
| accessdate = 2010-11-11}}</ref> Their advantages are moderate directivity ([[Antenna gain|gain]]), low [[standing wave ratio]] (SWR), broad [[Bandwidth (signal processing)|bandwidth]], and simple construction and adjustment.<ref>{{cite book
| last = Narayan
| first = C.P.
| authorlink =
| coauthors =
| title = Antennas And Propagation
| publisher = Technical Publications
| year = 2007
| location =
| page = 159
| url = http://books.google.com/books?id=qlpYZVNxK1wC&pg=PA159&dq=horn+antenna#v=onepage&q=horn%20antenna&f=false
| doi =
| isbn = 81-8431-176-1}}</ref>

One of the first horn antennas was constructed in 1897 by Indian radio researcher [[Jagadish Chandra Bose]] in his pioneering experiments with microwaves.<ref>{{cite web
| last = Rodriguez
| first = Vincente
| authorlink =
| coauthors =
| title = A brief history of horns
| work = In Compliance Magazine
| publisher = Same Page Publishing
| year = 2010
| url = http://www.incompliancemag.com/index.php?option=com_content&view=article&id=487:a-brief-history-of-horns-from-early-history-to-latest-developments&catid=24:current-issue&Itemid=126
| doi =
| accessdate = 2010-11-12}}</ref><ref name="Emerson">{{cite journal
| last = Emerson
| first = D. T.
| authorlink =
| coauthors =
| title = The work of Jagadis Chandra Bose: 100 years of MM-wave research
| journal = IEEE Transactions on Microwave Theory and Research
| volume = 45
| issue = 12
| pages = 2267–2273
| publisher = IEEE
| location =
|date=December 1997
| url = http://books.google.com/books?id=09Zsv97IH1MC&pg=PA88#v=onepage&q&f=false
| issn =
| doi = 10.1109/MWSYM.1997.602853
| id =
| accessdate = March 15, 2012}} reprinted in Igor Grigorov, Ed., ''[http://books.google.com/books?id=09Zsv97IH1MC Antentop]'', Vol.2, No.3, p.87-96, Belgorod, Russia</ref> In the 1930s the first experimental research (Southworth and Barrow, 1936) and theoretical analysis (Barrow and Chu, 1939) of horns as antennas was done.<ref name="Olver">{{cite book
| last = Olver
| first = A. David
| authorlink =
| coauthors =
| title = Microwave horns and feeds
| publisher = IET
| year = 1994
| location = USA
| pages = 2–4
| url = http://books.google.com/books?id=ZZw7MHFuqF8C&pg=PA2&dq=horn+antenna+design#v=onepage&q=horn%20antenna%20design&f=false
| doi =
| isbn = 0-85296-809-4}}</ref> The development of [[radar]] in World War 2 stimulated horn research to design feed horns for radar antennas. The corrugated horn invented by Kay in 1962 has become widely used as a feed horn for microwave antennas such as [[satellite dish]]es and [[radio telescope]]s.<ref name="Olver" />

An advantage of horn antennas is that since they have no [[resonant]] elements, they can operate over a wide range of [[Frequency|frequencies]], a wide [[Bandwidth (signal processing)|bandwidth]]. The usable bandwidth of horn antennas is typically of the order of 10:1, and can be up to 20:1 (for example allowing it to operate from 1 GHz to 20 GHz).<ref name="Bevilaqua" /> The input impedance is slowly varying over this wide frequency range, allowing low [[standing wave ratio|voltage standing wave ratio]] (VSWR) over the bandwidth.<ref name="Bevilaqua" /> The gain of horn antennas ranges up to 25 [[Decibel|dBi]], with 10 - 20 dBi being typical.<ref name="Bevilaqua" />

== Description ==
A horn antenna is used to transmit radio waves from a [[waveguide]] (a metal pipe used to carry radio waves) out into space, or collect radio waves into a waveguide for reception. It typically consists of a short length of rectangular or cylindrical metal tube (the waveguide), closed at one end, flaring into an open-ended conical or pyramidal shaped horn on the other end.<ref>{{cite book
| last = Graf
| first = Rudolf F.
| authorlink =
| coauthors =
| title = Modern Dictionary of Electronics
| publisher = Newnes
| year = 1999
| location = USA
| page = 352
| url = http://books.google.com/books?id=uah1PkxWeKYC&pg=PA352&dq=%22horn+antenna%22+waveguide#v=onepage&q=%22horn%20antenna%22%20waveguide&f=false
| doi =
| isbn = 0-7506-9866-7}}</ref> The radio waves are usually introduced into the waveguide by a [[coaxial cable]] attached to the side, with the central conductor projecting into the waveguide to form a [[quarter-wave antenna|quarter-wave monopole]] antenna. The waves then radiate out the horn end in a narrow beam. However in some equipment the radio waves are conducted between the [[transmitter]] or [[radio receiver|receiver]] and the antenna by a waveguide, and in this case the horn is just attached to the end of the waveguide. In horns installed outdoors, such as the [[feed horn]]s of satellite dishes, the open mouth of the horn is often covered by a plastic sheet which is transparent to the radio waves, to keep out moisture.

== How it works ==
[[Image:Hughes Direcway LNB.jpg|thumb|Corrugated conical horn antenna used as a [[feed horn]] on a Hughes Direcway home satellite dish. A transparent plastic sheet covers the horn mouth to keep out rain.]]

A horn antenna serves the same function for [[electromagnetic wave]]s that an [[horn (acoustic)|acoustical horn]] does for [[sound wave]]s in a musical instrument such as a [[trumpet]]. It provides a gradual transition structure to match the [[Wave impedance|impedance]] of a tube to the impedance of free space, enabling the waves from the tube to radiate efficiently into space.<ref>{{cite book
| last = Stutzman
| first = Warren L.
| authorlink =
| coauthors = Gary A. Thiele
| title = Antenna theory and design
| publisher = J. Wiley
| year = 1998
| location = USA
| page = 299
| url = http://books.google.com/books?id=Av1SAAAAMAAJ
| doi =
| isbn = 0-471-02590-9}}</ref>

If a simple open-ended waveguide is used as an antenna, without the horn, the sudden end of the conductive walls causes an abrupt impedance change at the aperture, from the [[wave impedance]] in the waveguide to the [[impedance of free space]], (about 377 [[ohm]]s).<ref name="Poole" /><ref name="Bakshi">{{cite book
| last = Bakshi
| first = K.A.
| authorlink =
| coauthors = A.V. Bakshi, U.A. Bakshi
| title = Antennas And Wave Propagation
| publisher = Technical Publications
| year = 2009
| location =
| pages = 6.1–6.3
| url = http://books.google.com/books?id=XznCTvLrX2UC&pg=SA6-PA2&dq=horn+antenna#v=onepage&q=horn%20antenna&f=false
| doi =
| isbn = 81-8431-278-4}}</ref> When radio waves travelling through the waveguide hit the opening, this impedance-step reflects a significant fraction of the wave energy back down the guide toward the source, so that not all of the power is radiated. This is similar to the reflection at an open-ended [[transmission line]] or a boundary between optical mediums with a low and high [[index of refraction]], like at a glass surface. The reflected waves cause [[standing wave]]s in the waveguide, increasing the [[standing wave ratio|SWR]], wasting energy and possibly overheating the transmitter. In addition, the small aperture of the waveguide (less than one wavelength) causes significant [[diffraction]] of the waves issuing from it, resulting in a wide [[radiation pattern]] without much directivity.

To improve these poor characteristics, the ends of the waveguide are flared out to form a horn. The taper of the horn changes the impedance gradually along the horn's length.<ref name="Bakshi" /> This acts like an [[Impedance matching|impedance matching transformer]], allowing most of the wave energy to radiate out the end of the horn into space, with minimal reflection. The taper functions similarly to a tapered [[transmission line]], or an optical medium with a smoothly varying refractive index. In addition, the wide aperture of the horn projects the waves in a narrow beam

The horn shape that gives minimum reflected power is an [[Exponential function|exponential]] taper.<ref name="Bakshi" /> Exponential horns are used in special applications that require minimum signal loss, such as satellite antennas and [[radio telescope]]s. However conical and pyramidal horns are most widely used, because they have straight sides and are easier to design and fabricate.

== Radiation pattern ==
[[Image:Bocina Cónica Corrugada.JPG|thumb|Small aperture-limited horn, used as a feed horn in a radio telescope for [[millimeter wave]]s.]]
The waves travel down a horn as spherical wavefronts, with their origin at the [[apex (geometry)|apex]] of the horn, a point called the [[phase center]]. The pattern of [[Electric field|electric]] and [[magnetic field]]s at the aperture plane at the mouth of the horn, which determines the [[radiation pattern]], is a scaled-up reproduction of the fields in the waveguide. However, because the wavefronts are spherical, the [[phase (waves)|phase]] increases smoothly from the edges of the aperture plane to the center, because of the difference in length of the center point and the edge points from the apex point. The difference in phase between the center point and the edges is called the ''phase error''. This phase error, which increases with the flare angle, reduces the gain and increases the beamwidth, giving horns wider beamwidths than similar-sized plane-wave antennas such as parabolic dishes.

At the flare angle, the radiation of the beam lobe is down about -20 dB from its maximum value.<ref name="Goldsmith">{{cite book
| last = Goldsmith
| first = Paul F.
| authorlink =
| coauthors =
| title = Quasioptical Systems: Gaussian beam quasioptical propagation and applications
| publisher = IEEE Press
| year = 1998
| location = USA
| pages = 173–174
| url = http://books.google.com/books?id=qYD9n78n_vIC&pg=PA174&lpg=PA174&dq=%22aperture+limited%22+horn+antenna#v=onepage&q=%22aperture%20limited%22%20horn%20antenna&f=false
| doi =
| isbn = 0-7803-3439-6}}</ref>

As the size of a horn in wavelengths is increased, the phase error increases, giving the horn a wider radiation pattern. Keeping the beamwidth narrow requires a longer horn (smaller flare angle) to keep the phase error constant. The increasing phase error limits the aperture size of practical horns to about 15 wavelengths; larger apertures would require impractically long horns.<ref name="Meeks">{{cite book
| last = Meeks
| first = Marion Littleton
| authorlink =
| coauthors =
| title = Astrophysics, Volume 12 of Methods of experimental physics, Part 2
| publisher = Academic Press
| year = 1976
| location = USA
| page = 11
| url = http://books.google.com/books?id=Acyhg3xq73AC&pg=PA11&dq=%22horn+antenna%22#v=onepage&q=%22horn%20antenna%22&f=false
| doi =
| isbn = 0-12-475952-1}}</ref> This limits the gain of practical horns to about 1000 (30 dBi) and the corresponding minimum [[beamwidth]] to about 5 - 10°.<ref name="Meeks" />

==Types==
[[File:Horn antenna types.svg|upright=0.7|thumb|Horn antenna types]]

These are the common types of horn antenna. Horns can have different flare angles as well as different expansion curves (elliptic, hyperbolic, etc.) in the E-field and H-field directions, making possible a wide variety of different beam profiles.
:'''Pyramidal horn''' (a, right) – a horn antenna with the horn in the shape of a four-sided pyramid, with a rectangular cross section. They are a common type, used with rectangular waveguides, and radiate linearly polarized radio waves.<ref name="Bakshi" />
:'''Sectoral horn''' – A pyramidal horn with only one pair of sides flared and the other pair parallel. It produces a fan-shaped beam, which is narrow in the plane of the flared sides, but wide in the plane of the narrow sides. These types are often used as feed horns for wide search radar antennas.
::'''E-plane horn''' (b) – A sectoral horn flared in the direction of the electric or [[Electric field|E-field]] in the waveguide.
::'''H-plane horn''' (c) – A sectoral horn flared in the direction of the magnetic or [[Magnetic field|H-field]] in the waveguide.
:'''Conical horn''' (d) – A horn in the shape of a [[Cone (geometry)|cone]], with a circular cross section. They are used with cylindrical waveguides.
:'''Exponential horn''' (e) – A horn with curved sides, in which the separation of the sides increases as an exponential function of length. Also called a ''scalar horn'', they can have pyramidal or conical cross sections. Exponential horns have minimum internal reflections, and almost constant impedance and other characteristics over a wide frequency range. They are used in applications requiring high performance, such as feed horns for communication satellite antennas and radio telescopes.
:'''Corrugated horn''' – A horn with parallel slots or grooves, small compared with a wavelength, covering the inside surface of the horn, transverse to the axis. Corrugated horns have wider bandwidth and smaller sidelobes and cross-polarization, and are widely used as feed horns for [[Satellite dish antenna|satellite dishes]] and [[radio telescope]]s.
:'''Ridged horn''' – A pyramidal horn with ridges or fins attached to the inside of the horn, extending down the center of the sides. The fins lower the cutoff frequency, increasing the antenna's bandwidth.
:'''Septum horn''' – A horn which is divided into several subhorns by metal partitions (septums) inside, attached to opposite walls.
:'''Aperture-limited horn''' – a long narrow horn, long enough so the phase error is a negligible fraction of a wavelength,<ref name="Goldsmith">{{cite book
| last = Goldsmith
| first = Paul F.
| authorlink =
| coauthors =
| title = Quasioptical Systems: Gaussian beam quasioptical propagation and applications
| publisher = IEEE Press
| year = 1998
| location = USA
| page = 174
| url = http://books.google.com/books?id=qYD9n78n_vIC&pg=PA183&lpg=PA183&dq=%22aperture+limited%22+horn+antenna#v=onepage&q=%22aperture%20limited%22%20horn%20antenna&f=false
| doi =
| isbn = 0-7803-3439-6}}</ref> so it essentially radiates a plane wave. It has an aperture efficiency of 1.0 so it gives the maximum gain and minimum [[beamwidth]] for a given aperture size. The gain is not affected by the length but only limited by diffraction at the aperture.<ref name="Goldsmith" /> Used as feed horns in [[radio telescope]]s and other high-resolution antennas.

== Optimum horn ==
[[Image:Rillenhorn.jpg|thumb|Corrugated horn antenna with a bandwidth of 3.7 to 6 GHz designed to attach to SMA waveguide feedline. This was used as a feedhorn for a parabolic antenna on a British military base.]]

For a given frequency and horn length, there is some flare angle that gives minimum reflection and maximum gain. The internal reflections in straight-sided horns come from the two locations along the wave path where the impedance changes abruptly; the mouth or aperture of the horn, and the throat where the sides begin to flare out. The amount of reflection at these two sites varies with the ''flare angle'' of the horn (the angle the sides make with the axis). In narrow horns with small flare angles most of the reflection occurs at the mouth of the horn. The [[Antenna gain|gain]] of the antenna is low because the small mouth approximates an open-ended waveguide. As the angle is increased, the reflection at the mouth decreases rapidly and the antenna's gain increases. In contrast, in wide horns with flare angles approaching 90° most of the reflection is at the throat. The horn's gain is again low because the throat approximates an open-ended waveguide. As the angle is decreased, the amount of reflection at this site drops, and the horn's gain again increases.

This discussion shows that there is some flare angle between 0° and 90° which gives maximum gain and minimum reflection.<ref name="Tasuku" >{{cite book
| last = Tasuku
| first = Teshirogi
| authorlink =
| coauthors = Tsukasa Yoneyama
| title = Modern millimeter-wave technologies
| publisher = IOS Press
| year = 2001
| location = USA
| pages = 87–89
| url = http://books.google.com/books?id=ijP52rsxTN8C&pg=PA86&dq=horn+antenna#v=onepage&q=horn%20antenna&f=false
| doi =
| isbn = 1-58603-098-1}}</ref> This is called the ''optimum horn''. Most practical horn antennas are designed as optimum horns. In a pyramidal horn, the dimensions that give an optimum horn are:<ref name="Tasuku" /><ref name="Narayan 2007, p. 168">[http://books.google.com/books?id=qlpYZVNxK1wC&pg=PA168&dq=horn+antenna&hl=en&ei=0cfcTLbQMs-PnweHhOAW&sa=X&oi=book_result&ct=result&resnum=5&ved=0CEcQ6AEwBA#v=onepage&q=horn%20antenna&f=false Narayan 2007, p. 168]</ref>

:<math>a_E = \sqrt{2 \lambda L_E} \qquad a_H = \sqrt{3 \lambda L_H}</math>

For a conical horn, the dimensions that give an optimum horn are:<ref name="Tasuku" />

:<math>d = \sqrt{3 \lambda L}</math>

where
:''aE'' is the width of the aperture in the E-field direction
:''aH'' is the width of the aperture in the H-field direction
:''LE'' is the slant length of the side in the E-field direction
:''LH'' is the slant length of the side in the H-field direction.
:''d'' is the diameter of the cylindrical horn aperture
:''L'' is the slant length of the cone from the apex.
:''λ'' is the wavelength

An optimum horn does not give maximum gain for a given ''aperture size''; this is achieved by a very long horn (an ''aperture limited'' horn). It gives the maximum gain for a given horn ''length''. Tables showing dimensions for optimum horns for various frequencies are given in microwave handbooks.

[[Image:Green Banks - Ewen-Purcell Horn Antenna.jpg|thumb|upright=0.50|Large pyramidal horn used in 1951 to detect the 21 cm (1.43 GHz) radiation from hydrogen gas in the [[Milky Way]] galaxy.]]

== Gain ==
Horns have very little loss, so the [[directivity]] of a horn is roughly equal to its [[Antenna gain|gain]].<ref name="Bevilaqua" /> The [[Antenna gain|gain]] ''G'' of a pyramidal horn antenna (the ratio of the radiated power intensity along its beam axis to the intensity of an [[isotropic radiator|isotropic antenna]] with the same input power) is:<ref name="Narayan 2007, p. 168"/>

:<math>G = \frac{4 \pi A}{\lambda^2} e_A </math>

For conical horns, the gain is:<ref name="Tasuku" />

:<math>G = \left ( \frac{\pi d}{\lambda} \right )^2 e_A </math>

where
:''A'' is the area of the aperture,
:''d'' is the aperture diameter of a conical horn
:''λ'' is the [[wavelength]],
:''eA'' is a dimensionless parameter between 0 and 1 called the ''[[aperture efficiency]]'',

The aperture efficiency ranges from 0.4 to 0.8 in practical horn antennas. For optimum pyramidal horns, ''eA'' = 0.511.,<ref name="Tasuku" /> while for optimum conical horns ''eA'' = 0.522.<ref name="Tasuku" /> So an approximate figure of 0.5 is often used. The aperture efficiency increases with the length of the horn, and for aperture-limited horns is approximately unity.

==Horn-reflector antenna==

A type of antenna that combines a horn with a [[parabolic reflector]] is the Hogg or horn-reflector antenna, invented by Alfred C. Beck and [[Harald T. Friis]] in 1941<ref name="Beck">[http://www.google.com/patents?id=7mluAAAAEBAJ&printsec=frontcover&dq=2416675&hl=en&sa=X&ei=7hPwTs2FHIStiQLmypyaDg&ved=0CDMQ6AEwAA U. S. patent no. 2416675 ''Horn antenna system'', filed November 26, 1941, Alfred C. Beck, Harold T. Friis] on Google Patents</ref> and further developed by David C. Hogg at [[Bell labs]] in 1961,<ref name="Crawford">{{cite journal
| last = Crawford
| first = A.B.
| authorlink =
| coauthors = D. C. Hogg, and L. E. Hunt
| title = Project Echo: A Horn-Reflector Antenna for Space Communication
| journal = Bell System Technical Journal
| volume = 40
| issue =
| pages = 1095–1099
| publisher = AT&T
| location = USA
| date = July 1961
| url = http://www3.alcatel-lucent.com/bstj/vol40-1961/articles/bstj40-4-1095.pdf
| issn =
| doi =}} on [http://www.alcatel-lucent.com/bstj/ Alcatel-Lucent website]</ref> and is therefore typically known as the Hogg horn, or colloquially the "sugar scoop" due to its characteristic shape. It consists of a horn antenna with a reflector mounted in the mouth of the horn at a 45 degree angle so the radiated beam is at right angles to the horn axis. The reflector is a segment of a parabolic reflector, and the focus of the reflector is at the apex of the horn, so the device is equivalent to a [[parabolic antenna]] fed off-axis.<ref name="Meeks, 1976, p.13">[http://books.google.com/books?id=Acyhg3xq73AC&pg=PA13&dq=horn+antenna+hogg&hl=en&ei=hdgmTJzAE83nnQeZ0qniBQ&sa=X&oi=book_result&ct=result&resnum=1&ved=0CCYQ6AEwADgK#v=onepage&q=horn%20antenna%20hogg&f=true Meeks, 1976, p.13]</ref> The advantage of this design over a standard parabolic antenna is that the horn shields the antenna from radiation coming from angles outside the main beam axis, so its radiation pattern has very small [[Side lobe|sidelobes]].<ref name="Pattan">{{cite book
| last = Pattan
| first = Bruno
| authorlink =
| coauthors =
| title = Satellite systems: principles and technologies
| publisher = Springer
| year = 1993
| location = USA
| page = 275
| url = http://books.google.com/books?id=0GJWEro9ea4C&pg=PA275&dq=horn+antenna#v=onepage&q=horn%20antenna&f=false
| doi =
| isbn = 0-442-01357-4}}</ref> Also, the aperture isn't partially obstructed by the feed and its supports, as with ordinary front-fed parabolic dishes, allowing it to achieve aperture efficiencies of 70% as opposed to 55-60% for front-fed dishes.<ref name="Meeks, 1976, p.13"/> The disadvantage is that it is far larger and heavier for a given aperture area than a parabolic dish, and must be mounted on a cumbersome turntable to be fully steerable. This design was used for a few [[radio telescope]]s and [[communication satellite]] ground antennas during the 1960s. Its largest use, however, was as fixed antennas for microwave relay links in the [[AT&T Long Lines]] microwave network.<ref name="Crawford" /><ref name="Pattan" /><ref name="KS15676">{{cite web
| title = KS-15676 Horn-Reflector Antenna Description
| work = Bell System Practices, Issue 3, Section 402-421-100
| publisher = AT&T Co.
| date = September 1975
| url = http://long-lines.net/tech-equip/radio/BSP402421100/p01.html
| format = PDF
| doi =
| accessdate = 2011-12-20}} on Albert LaFrance [long-lines.net] website</ref> Since the 1970s this design has been superseded by shrouded [[Parabolic antenna|parabolic dish antennas]], which can achieve equally good sidelobe performance with a lighter more compact construction. Probably the most photographed and well-known example is the 15 meter (50 foot) long [[Holmdel Horn Antenna]]<ref name="Crawford" /> at Bell Labs in Holmdel, New Jersey, with which [[Arno Penzias]] and [[Robert Woodrow Wilson|Robert Wilson]] discovered cosmic [[microwave background radiation]] in 1965, for which they won the 1978 [[Nobel Prize in Physics]].

{{multiple image
|align = center
| footer = Horn-reflector antennas
| image1 =Horn_Antenna-in_Holmdel,_New_Jersey.jpeg
| caption1 = 50 ft. [[Holmdel Horn Antenna|Holmdel horn antenna]] at [[Bell labs]] in Holmdel, New Jersey, USA, with which [[Arno Penzias]] and [[Robert Woodrow Wilson|Robert Wilson]] discovered cosmic [[microwave background radiation]] in 1964.
| width1 = 200
| image2 = Relay 1 antenna USA.jpg
| caption2 = Large 177 ft. horn reflector antenna at [[Andover Earth Station|AT&T satellite communications facility]] in Andover, Maine, USA, used in 1960s to communicate with the first direct relay [[communications satellite]], [[Telstar]].
| width2 = 270
| image3 = Hogg horn antennas.jpg
| caption3 = AT&T Long-Lines KS-15656 C-band (4-6 GHz) microwave relay horns<ref name="KS15676" /> on roof of AT&T telephone switching center, Seattle, Washington, USA
| width3 = 120
}}

==See also==
*[[Holmdel Horn Antenna]]

==External links==
*[http://www.antenna-theory.com/antennas/aperture/horn.php Horn Antennas] Antenna-Theory.com
*{{cite web
| title = KS-15676 Horn-Reflector Antenna Description
| work = Bell System Practices, Issue 3, Section 402-421-100
| publisher = AT&T Co.
| date = September 1975
| url = http://long-lines.net/tech-equip/radio/BSP402421100/p01.html
| format = PDF
| doi =
| accessdate= }} on Albert LaFrance [long-lines.net] website
*[http://www.google.com/patents?id=7mluAAAAEBAJ&printsec=frontcover&dq=2416675&hl=en&sa=X&ei=7hPwTs2FHIStiQLmypyaDg&ved=0CDMQ6AEwAA U. S. patent no. 2416675 ''Horn antenna system'', filed November 26, 1941, Alfred C. Beck, Harold T. Friis] on Google Patents

==References==
{{reflist|2}}

{{Antenna Types}}

{{DEFAULTSORT:Horn Antenna}}
[[Category:Telecommunications equipment]]
[[Category:Radio frequency antenna types]]

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