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[[File:T antenna.svg|thumb|upright=1.8|Types of T antennas: ''(a)'' simple ''(b)'' multiwire.  <font color="red">Red</font> parts are [[Electrical insulator|insulators]], <font color="grey">grey</font> are supporting towers.]] 
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A '''T-aerial''' or '''flat-top aerial''' is a simple wire radio aerial ([[antenna (radio)|antenna]])<ref name="Graf">{{cite book 
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  | last = Graf
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  | first = Rudolf F.
* Note: you need not enter a email address (nor any other private information). Please do not use a password that you use elsewhere.
  | authorlink =
  | coauthors =
  | title = Modern dictionary of electronics, 7th Ed.
  | publisher = Newnes
  | year = 1999
  | location = USA
  | pages = 761
  | url = http://books.google.com/books?id=uah1PkxWeKYC&pg=PA761&dq=%22T+antenna%22+capacitance&hl=en&sa=X&ei=LUNGT-O7DePniALn8JjaDQ&ved=0CE4Q6AEwBDgK#v=onepage&q=%22T%20antenna%22%20capacitance&f=false
  | doi =
  | id =
  | isbn = 0-7506-9866-7}}</ref> used in the [[very low frequency|VLF]], [[low frequency|LF]], [[medium frequency|MF]] and [[shortwave]] bands.<ref name="Chatterjee" >{{cite book 
  | last = Chatterjee
  | first = Rajeswari
  | authorlink =
  | coauthors =
  | title = Antenna theory and practice, 2nd Ed.
  | publisher = New Age International
  | year = 2006
  | location = New Delhi
  | pages = 243–244
  | url = http://books.google.com/books?id=J4YcUA-rxJoC&pg=PA244&dq=%22low+frequency+antennas%22&hl=en&sa=X&ei=B61DT4P4Jc3WiALH_ayVAg&ved=0CHAQ6AEwCQ#v=onepage&q=%22low%20frequency%20antennas%22&f=false
  | doi =
  | id =
  | isbn = 81-224-0881-8}}</ref><ref name="Rudge">{{cite book 
  | last = Rudge
  | first = Alan W.
  | authorlink =
  | coauthors =
  | title = The Handbook of Antenna Design, Vol. 2
  | publisher = IET
  | year = 1983
  | location =
  | pages = 578–579
  | url = http://books.google.com/books?id=QjYtNJZmWLEC&pg=PA546&dq=%22low+frequency%22+antenna&hl=en&sa=X&ei=qUpGT-mmM-PWiAK_7snbDQ&ved=0CFAQ6AEwAjge#v=onepage&q=%22low%20frequency%22%20antenna&f=false
  | doi =
  | id =
  | isbn = 0-906048-87-7}}</ref><ref name="Edwards">{{cite web
  | last = Edwards
  | first = R.J.Edwards G4FGQ
  | authorlink =
  | coauthors =
  | title = The Simple Tee Antenna
  | work = Antenna design library
  | publisher = [http://www.smeter.net/ S meter website]
  | date = August 1, 2005
  | url = http://www.smeter.net/antennas/simple-t-antenna.php
  | format =
  | doi =
  | accessdate = 2012-02-23}}</ref>  T-aerials are widely used as receiving aerials for [[shortwave listening]], and transmitting aerials for [[amateur radio]] stations<ref name="ARRL">{{cite book 
  | last = Straw
  | first = R. Dean, Ed.
  | authorlink =
  | coauthors =
  | title = The ARRL Antenna Book, 19th Ed.
  | publisher = American Radio Relay League
  | year = 2000
  | location = USA
  | pages = 6.36
  | url =
  | doi =
  | id =
  | isbn = 0-87259-817-9}}</ref> and [[long wave]] and [[medium wave]] broadcasting stations.


It consists of a horizontal wire suspended between two [[radio masts and towers|radio masts]] or buildings and insulated from them at the ends.<ref name="Graf" /><ref name="Edwards" />    A vertical wire is connected to the center of the horizontal wire and hangs down close to the ground, where it is connected to the [[transmitter]] or [[radio receiver|receiver]]. The two wires form a 'T' shape, hence the name.  The transmitter power is applied, or the receiver is connected, between the bottom of the vertical wire and a [[ground (electricity)|ground]] connection.  Sometimes multiple parallel horizontal wires are used, connected together at the center wire.
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The T-aerial functions as a [[monopole antenna]] with capacitive top-loading; other antennas in this category include the inverted-L, [[umbrella antenna|umbrella]], delta, and triatic antennas.  It was invented during the first decades of radio, the [[wireless telegraphy]] era before 1920.
'''MathML'''
:<math forcemathmode="mathml">E=mc^2</math>


==How it works==
<!--'''PNG''' (currently default in production)
[[File:T antenna vs vertical antenna.svg|thumb|upright=1.5|RF current distributions <font color="red">''(red)''</font> in a vertical antenna ''(a)'' and the T antenna ''(b)'', showing how the horizontal wire serves to improve the efficiency of the vertical radiating wire.<ref name="Huang" /> The width of the red area perpendicular to the wire at any point is proportional to the current.  At resonance the current is the tail part of a sinusoidal [[standing wave]].  In the vertical antenna, the current must go to zero at the top.  In the T, the current flows into the horizontal wire, increasing the current in the top part of the vertical wire.  The [[radiation resistance]] and thus the radiated power in each, is proportional to the square of the area of the vertical part of the current distribution.]]
:<math forcemathmode="png">E=mc^2</math>


When the length of the wire segments are shorter than a quarter [[wavelength]] (λ/4) of the radio waves, as is typical for use below 1&nbsp;MHz, the antenna functions as a vertical [[electrical length|electrically short]] [[monopole antenna]] with capacitive top-loading.<ref name="Rudge" />    Because the two arms of the "T" have equal but oppositely-directed currents in them, which causes the radio waves from them to cancel far from the antenna,  and because of similar cancelling ground currents, the horizontal wire radiates little radio power.<ref name="Rudge, 1983, p.554">[http://books.google.com/books?id=QjYtNJZmWLEC&pg=PA546  Rudge, 1983, p.554]</ref>  Instead it serves to add [[capacitance]] to the top of the antenna.<ref name="Huang">{{cite book 
'''source'''
  | last = Huang
:<math forcemathmode="source">E=mc^2</math> -->
  | first = Yi
  | authorlink =
  | coauthors = Kevin Boyle
  | title = Antennas: from theory to practice
  | publisher = John Wiley & Sons
  | year = 2008
  | location =
  | pages = 299–301
  | url = http://books.google.com/books?id=W0i43vVWVaUC&pg=PA299&dq=%22t+antenna%22&hl=en&sa=X&ei=_vlBT7KgAszYiAKlk5S0AQ&ved=0CE8Q6AEwAg#v=onepage&q=%22t%20antenna%22&f=false
  | doi =
  | id =
  | isbn = 0-470-51028-5}}</ref><ref name="Rudge, 1983, p.554"/>          This increases the currents in the upper portion of the vertical wire ''(see drawing at right)'', increasing the [[radiation resistance]] and thus its efficiency,<ref name="Huang" /> allowing it to radiate more power, or in a receiving antenna be more sensitive to incoming radio signals. The top load wire can increase radiated power by 2 to 4 times (3 to 6 dB).
 
However, the antenna is still typically not as efficient as a full-height λ/4 vertical [[Monopole antenna|monopole]],<ref name="ARRL" />  and has a higher [[Q factor|Q]] and thus a narrower [[bandwidth (signal processing)|bandwidth]].  T antennas are typically used at low frequencies where it is not practical to build a quarter-wave vertical antenna because of its height,<ref name="Edwards" /><ref name="Griffith">{{cite book 
  | last = Griffith
  | first =  B. Whitfield
  | authorlink =
  | coauthors =
  | title = Radio-Electronic Transmission Fundamentals, 2nd Ed.
  | publisher = SciTech Publishing
  | year = 2000
  | location = USA
  | pages = 389–391
  | url = http://books.google.com/books?id=m5DIroWLw2EC&pg=PA391&dq=%22t+antenna%22&hl=en&sa=X&ei=X2ZDT7_wFKLTiAK3k8WhAQ&ved=0CHYQ6AEwCTgU#v=onepage&q=%22t%20antenna%22&f=false
  | doi =
  | id =
  | isbn = 1-884932-13-4}}</ref> and the vertical radiating wire is often very [[Electrical length|electrically short]], only a small fraction of a wavelength long, 0.1λ or less.  Since the radiation resistance and efficiency increases with height, the antenna should be suspended as high as possible.


To increase the top-load capacitance, several parallel horizontal wires are often used, connected together at the center where the vertical wire attaches.<ref name="ARRL" /> This increases the radiation resistance, and thus efficiency and bandwidth. The capacitance does not increase proportionally with the number of wires, however, because each wire's electric field is partially shielded from the ground by proximity to the adjacent wires.<ref name="ARRL" />
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==Radiation pattern==
==Demos==
Since the vertical wire is the actual radiating element, the antenna radiates [[vertical polarization|vertically polarized]] radio waves in an [[omnidirectional antenna|omnidirectional]] [[radiation pattern]], with equal power in all azimuthal directions.<ref name="Barclay">{{cite book 
  | last = Barclay
  | first = Leslie W.
  | authorlink =
  | coauthors =
  | title = Propagation of radiowaves
  | publisher = Institution of Electrical Engineers
  | year = 2000
  | location =
  | pages = 379–380
  | url = http://books.google.com/books?id=fBoTO48FBD8C&pg=PA380&dq=ELF+propagation+transmitter&hl=en&sa=X&ei=Zgg4T8nxPKWeiQKBpdyiCg&ved=0CFIQ6AEwAzgK#v=onepage&q=ELF%20propagation%20transmitter&f=false
  | doi =
  | id =
  | isbn = 0-85296-102-2}}</ref>  The axis of the horizontal wire makes little difference.  The power is maximum in a horizontal direction or at a shallow elevation angle, decreasing to zero at the zenith.  This makes it a good antenna at LF or MF frequencies, which propagate as [[ground wave]]s with vertical polarization, but it also radiates enough power at higher elevation angles to be useful for [[sky wave]] ("skip") communication.  The effect of poor ground conductivity is generally to tilt the pattern up, with the maximum signal strength at a higher elevation angle.


==Transmitting antennas==
Here are some [https://commons.wikimedia.org/w/index.php?title=Special:ListFiles/Frederic.wang demos]:
[[Image:Titanic antenne T 01.JPG|thumb|upright=2.0|One of the first uses of T-aerials in the early 20th century was on ships, since they could be strung between masts. This is the antenna of the RMS Titanic, which broadcast the rescue call during her sinking in 1912.  It was a multiwire T with a 50 m vertical wire and four 120 m horizontal wires. ]]


Since it is shorter than λ/4 the T antenna has a high [[capacitive reactance]].  In transmitting antennas, to make the antenna [[resonant]] so it can be driven efficiently, this capacitance must be canceled out by adding an [[inductor]], a [[loading coil]], in series with the bottom of the antenna.  Particularly at lower frequencies, the high inductance and capacitance compared to its low radiation resistance makes the loaded antenna behave like a high [[Q factor|Q]] [[tuned circuit]], with a narrow bandwidth over which it will remain [[impedance matching|impedance matched]] to the transmission line, compared to a λ/4  monopole.  To operate over a large frequency range the loading coil often must be adjustable, and adjusted when the frequency is changed to keep the [[standing wave ratio|SWR]] low.  The high Q also causes high voltages and currents on the antenna, roughly Q times the driving-point voltage and current.  The insulators at the ends must be designed to withstand these voltages.  In high power transmitters the output power is often limited by the onset of [[corona discharge]] on the wires.<ref name="AntennaReactance" >{{cite web
  | last = LaPorte
  | first = Edmund A.
  | authorlink =
  | coauthors =
  | title = Antenna Reactance
  | work = Radio Antenna Engineering
  | publisher = [http://vias.org  Virtual Institute of Applied Science]
  | year = 2010
  | url = http://www.vias.org/radioanteng/radio_antenna_engineering_01_08_01.html
  | doi =
  | accessdate = 2012-02-24}}</ref> 


At low frequencies the radiation resistance is very low; often less than an ohm,<ref name="ARRL" /><ref name="Balanis">{{cite book 
* accessibility:
  | last =  Balanis
** Safari + VoiceOver: [https://commons.wikimedia.org/wiki/File:VoiceOver-Mac-Safari.ogv video only], [[File:Voiceover-mathml-example-1.wav|thumb|Voiceover-mathml-example-1]], [[File:Voiceover-mathml-example-2.wav|thumb|Voiceover-mathml-example-2]], [[File:Voiceover-mathml-example-3.wav|thumb|Voiceover-mathml-example-3]], [[File:Voiceover-mathml-example-4.wav|thumb|Voiceover-mathml-example-4]], [[File:Voiceover-mathml-example-5.wav|thumb|Voiceover-mathml-example-5]], [[File:Voiceover-mathml-example-6.wav|thumb|Voiceover-mathml-example-6]], [[File:Voiceover-mathml-example-7.wav|thumb|Voiceover-mathml-example-7]]
  | first = Constantine A.
** [https://commons.wikimedia.org/wiki/File:MathPlayer-Audio-Windows7-InternetExplorer.ogg Internet Explorer + MathPlayer (audio)]
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** [https://commons.wikimedia.org/wiki/File:MathPlayer-SynchronizedHighlighting-WIndows7-InternetExplorer.png Internet Explorer + MathPlayer (synchronized highlighting)]
  | coauthors =
** [https://commons.wikimedia.org/wiki/File:MathPlayer-Braille-Windows7-InternetExplorer.png Internet Explorer + MathPlayer (braille)]
  | title = Modern Antenna Handbook
** NVDA+MathPlayer: [[File:Nvda-mathml-example-1.wav|thumb|Nvda-mathml-example-1]], [[File:Nvda-mathml-example-2.wav|thumb|Nvda-mathml-example-2]], [[File:Nvda-mathml-example-3.wav|thumb|Nvda-mathml-example-3]], [[File:Nvda-mathml-example-4.wav|thumb|Nvda-mathml-example-4]], [[File:Nvda-mathml-example-5.wav|thumb|Nvda-mathml-example-5]], [[File:Nvda-mathml-example-6.wav|thumb|Nvda-mathml-example-6]], [[File:Nvda-mathml-example-7.wav|thumb|Nvda-mathml-example-7]].
  | publisher = John Wiley & Sons
** Orca: There is ongoing work, but no support at all at the moment [[File:Orca-mathml-example-1.wav|thumb|Orca-mathml-example-1]], [[File:Orca-mathml-example-2.wav|thumb|Orca-mathml-example-2]], [[File:Orca-mathml-example-3.wav|thumb|Orca-mathml-example-3]], [[File:Orca-mathml-example-4.wav|thumb|Orca-mathml-example-4]], [[File:Orca-mathml-example-5.wav|thumb|Orca-mathml-example-5]], [[File:Orca-mathml-example-6.wav|thumb|Orca-mathml-example-6]], [[File:Orca-mathml-example-7.wav|thumb|Orca-mathml-example-7]].
  | year = 2011
** From our testing, ChromeVox and JAWS are not able to read the formulas generated by the MathML mode.
  | location =
  | pages = 2.8–2.9 (Sec. 2.2.2)
  | url = http://books.google.com/books?id=UYpV8L8GNCwC&pg=SA9-PA9&dq=%22Top+loading%22+monopole&hl=en&sa=X&ei=zUhGT_DAKKKPigK7kvnaDQ&ved=0CHAQ6AEwCDgU#v=onepage&q=%22low%20frequency%22%20antenna&f=false
  | doi =
  | id =
  | isbn = 1-118-20975-3}}</ref> so the efficiency is limited by other resistances in the antenna. The input power is divided between the radiation resistance and the ohmic resistances of the antenna-ground circuit, chiefly the coil and the ground. The resistance in the coil and particularly the ground system must be kept very low to minimize the power dissipated in them.


It can be seen that at low frequencies the design of the loading coil can be challenging:<ref name="ARRL" /> it must have high inductance but very low losses at the transmitting frequency (high [[Q]]), must carry high currents, withstand high voltages at its ungrounded end, and be adjustable.<ref name="Griffith" />  It is often made of [[litz wire]].<ref name="Griffith" />
==Test pages ==


At low frequencies the antenna requires a good low resistance [[ground (electricity)|ground]] to be efficient.  The RF ground is typically constructed of a "star" of many radial copper cables buried about 1&nbsp;ft. in the earth, extending out from the base of the vertical wire, and connected together at the center.  The radials should ideally be long enough to extend beyond the [[displacement current]] region near the antenna.    At VLF frequencies the resistance of the soil becomes a problem, and the radial ground system is sometimes raised and mounted a few feet above ground, insulated from it, to form a [[counterpoise]].
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==Equivalent circuit==
*[[Inputtypes|Inputtypes (private Wikis only)]]
The power radiated (or received) by an electrically short vertical antenna like the T is proportional to the square of the "effective height"  of the antenna,<ref name="ARRL" />  so the antenna should be made as high as possible.  Without the horizontal wire, the RF current distribution in the vertical wire would decrease linearly to zero at the top (see drawing ''a'' above), giving an effective height of half the physical height of the antenna.  With an ideal "infinite capacitance" top load wire, the current in the vertical would be constant along its length, giving an effective height equal to the physical height, therefore increasing the power radiated fourfold.  So the power radiated (or received) by a T antenna is up to four times that of a vertical monopole of the same height.
*[[Url2Image|Url2Image (private Wikis only)]]
 
==Bug reporting==
The [[radiation resistance]] of an ideal T antenna with very large top load capacitance is<ref name="Huang" />
If you find any bugs, please report them at [https://bugzilla.wikimedia.org/enter_bug.cgi?product=MediaWiki%20extensions&component=Math&version=master&short_desc=Math-preview%20rendering%20problem Bugzilla], or write an email to math_bugs (at) ckurs (dot) de .
:<math>R_R = 80\pi^2 \left ( \frac {h}{\lambda} \right )^2  \,</math>
so the radiated power is
:<math>P = 80\pi^2  \left ( \frac {hI_0}{\lambda} \right )^2  \,</math>
where '''''h''''' is the height of the antenna, '''''λ''''' is the wavelength, and '''''I<sub>0</sub>''''' is the RMS input current in amperes.
 
The equivalent circuit of the antenna (including loading coil) is the series combination of the capacitive reactance of the antenna, the inductive reactance of the loading coil, and the radiation resistance and the other resistances of the antenna-ground circuit.  So the input impedance is
 
:<math>z = R_C + R_D + R_L + R_G + R_R + j \omega L - \frac {1}{j \omega C}  \, </math>
 
At resonance the capacitive reactance of the antenna is cancelled by the loading coil so the input impedance at resonance '''''z<sub>0</sub>'''''  is just the sum of the resistances in the antenna circuit<ref name="RadiationEfficiency" >{{cite web
  | last = LaPorte
  | first = Edmund A.
  | authorlink =
  | coauthors =
  | title = Radiation Efficiency
  | work = Radio Antenna Engineering
  | publisher = [http://vias.org  Virtual Institute of Applied Science]
  | year = 2010
  | url = http://www.vias.org/radioanteng/radio_antenna_engineering_01_05.html
  | doi =
  | accessdate = 2012-02-24}}</ref>
 
:<math>z_0 = R_C + R_D + R_L + R_G + R_R \, </math>
 
So the efficiency ''η'' of the antenna, the ratio of radiated power to input power from the feedline, is
 
:<math>\eta = \frac {R_R}{R_C + R_D + R_L + R_G + R_R} \, </math>
 
where
:'''''R<sub>C</sub>''''' is the ohmic resistance of the antenna conductors (copper losses)
:'''''R<sub>D</sub>''''' is the equivalent series dielectric losses
:'''''R<sub>L</sub>''''' is the equivalent series resistance of the loading coil
:'''''R<sub>G</sub>''''' is the resistance of the ground system
:'''''R<sub>R</sub>''''' is the radiation resistance
:'''''C''''' is the capacitance of the antenna at the input terminals
:'''''L''''' is the inductance of the loading coil
 
It can be seen that, since the radiation resistance is usually very low, the major design problem is to keep the other resistances in the antenna-ground system low to obtain the highest efficiency.<ref name="RadiationEfficiency" />
 
==Multiple-tuned antenna==
The "multiple-tuned antenna" is a variant of the T antenna used in high power low frequency transmitters to reduce ground power losses.<ref name="Griffith" />  It consists of a long capacitive top-load consisting of multiple parallel wires supported by a line of transmission towers, sometimes several miles long.  Several vertical radiator wires hang down from the top-load, each attached to its own ground through a loading coil.  The antenna is driven either at one of the radiator wires, or more often at one end of the top-load, by bringing the wires of the top-load diagonally down to the transmitter.  Although the vertical wires are separated, the distance between them is small compared to the length of the LF waves, so the currents in them are in phase and they can be considered as one radiator.    Since the antenna current flows into the ground through N parallel grounds rather than one, the equivalent ground resistance, and therefore the power dissipated in the ground, is reduced to 1/N that of a simple T antenna.<ref name="Griffith" />
 
==See also==
*[[Dipole antenna]]
*[[Longwave]]
*[[Mast radiator]]
 
==References==
{{reflist}}
 
{{Antenna Types}}
 
{{DEFAULTSORT:T-Aerial}}
[[Category:Power cables]]
[[Category:Radio frequency antenna types]]
 
[[de:T-Antenne]]
[[fr:Antenne en T]]

Latest revision as of 23:52, 15 September 2019

This is a preview for the new MathML rendering mode (with SVG fallback), which is availble in production for registered users.

If you would like use the MathML rendering mode, you need a wikipedia user account that can be registered here [[1]]

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