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| {{Refimprove|date=December 2009}}
| | Luke is usually a superstar within the producing and the profession advancement first 2nd to his 3rd studio album, & , is definitely the evidence. He burst to the picture in 2007 with his amusing mixture of straight down-house availability, movie celebrity great looks and words, is defined t in the significant way. The new recor about the region graph or chart and #2 in the take charts, producing it the next maximum debut at that time of 2008 for the nation artist. <br><br> |
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| In [[electromagnetics]] and [[antenna (radio)|antenna]] theory, '''antenna aperture''' or '''effective area''' is a measure of how effective an antenna is at receiving the power of [[radio wave]]s. The aperture is defined as the area, oriented perpendicular to the direction of an incoming radio wave, which would intercept the same amount of power from that wave as is produced by the antenna receiving it. At any point, a beam of radio waves has an ''[[irradiance]]'' or ''power flux density'' ''(PFD)'' which is the amount of radio power passing through a unit area. If an antenna delivers an output power of ''P<sub>o</sub>'' [[watt]]s to the load connected to its output terminals when irradiated by a uniform field of power density ''PFD'' watts per square metre, the antenna's aperture ''A<sub>eff</sub>'' in square metres is given by:<ref name="Bakshi">{{cite book
| | The kid of a , knows perseverance and determination are important elements when it comes to an effective profession- . His to start with record, Stay Me, produced the Top reaches “All My Buddies Say” and “Country Gentleman,” while his effort, Doin’ Point, discovered the performer-a few right No. 7 single men and women: More Contacting Is really a Great Point.”<br><br>Inside the [http://okkyunglee.com luke bryan concert tours] tumble of 2009, Tour: Luke Bryan & which in fact had an impressive set of , such as City. “It’s much like you are getting a approval to go to the next level, states individuals performers that were an element of the [http://www.netpaw.org luke bryan concert tickets cheap] Tourmore than into a bigger degree of luke bryan concert [http://www.ladyhawkshockey.org meet and greet tickets luke bryan] tour schedule - [http://lukebryantickets.asiapak.net lukebryantickets.asiapak.net] - musicians.” It packaged among the most successful tours in its 15-year historical past.<br><br>my blog post; luke bryan tickets ([http://www.ffpjp24.org relevant internet page]) |
| | last = Bakshi
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| | first = K.A.
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| | authorlink =
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| | coauthors = A.V.Bakshi, U.A.Bakshi
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| | title = Antennas And Wave Propagation
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| | publisher = Technical Publications
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| | year = 2009
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| | location =
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| | pages = 1.17
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| | url = http://books.google.com/books?id=XznCTvLrX2UC&pg=SA6-PA2&dq=antenna+aperture&hl=en&ei=0cfcTLbQMs-PnweHhOAW&sa=X&oi=book_result&ct=result&resnum=3&ved=0CDwQ6AEwAg#v=onepage&q=horn%20antenna&f=false
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| | doi =
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| | id =
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| | isbn = 81-8431-278-4}}</ref>
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| :<math>A_{eff} = \frac {P_o}{PFD} \,</math> .
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| So the power output of an antenna in watts is equal to the power density of the radio waves in watts per square metre, multiplied by its aperture in square metres. The larger an antenna's aperture is, the more power it can collect from a given field of radio waves. To actually obtain the predicted power available ''P<sub>o</sub>'', the [[polarization (waves)|polarization]] of the incoming waves must match the polarization of the antenna, and the load (receiver) must be [[impedance match]]ed to the antenna's feedpoint impedance.
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| Although this concept is based on an antenna receiving a radio frequency wave, knowing ''A<sub>eff</sub>'' directly supplies the [[antenna gain|(power) gain]] of that antenna. Due to [[reciprocity (electromagnetism)|reciprocity]], an antenna's gain in receiving and transmitting are identical. Therefore ''A<sub>eff</sub>'' can just as well be used to compute the performance of a transmitting antenna. Note that ''A<sub>eff</sub>'' is a function of the direction of the radio wave relative to the orientation of the antenna, since the gain of an antenna varies according to its [[radiation pattern]]. When no direction is specified, ''A<sub>eff</sub>'' is understood to refer to its maximum value, with the antenna oriented so its [[main lobe]], the axis of maximum sensitivity, is directed toward the source.
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| ==Aperture efficiency==
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| In general, the aperture of an antenna is not directly related to its physical size.<ref name="Narayan">{{cite book
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| | last = Narayan
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| | first = C.P.
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| | authorlink =
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| | coauthors =
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| | title = Antennas And Propagation
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| | publisher = Technical Publications
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| | year = 2007
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| | location =
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| | pages = 51
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| | url = http://books.google.com/books?id=qlpYZVNxK1wC&pg=PA51&dq=FRIIS&hl=en&ei=0cfcTLbQMs-PnweHhOAW&sa=X&oi=book_result&ct=result&resnum=5&ved=0CEcQ6AEwBA#v=onepage&q=horn%20antenna&f=false
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| | doi =
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| | id =
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| | isbn = 81-8431-176-1}}</ref> However some types of antennas, for example [[Parabolic antenna|parabolic dishes]] and [[Horn antenna|horns]], have a physical aperture (opening) which collects the radio waves. In these ''aperture antennas'', the effective aperture ''A<sub>eff</sub>'' must always be less than the area of the antenna's physical aperture ''A<sub>phys</sub>'', as can be seen from the definition above. An antenna's ''aperture efficiency'', e<sub>a</sub> is defined as the ratio of these two areas:
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| :<math>e_a = \frac {A_{eff}} {A_{phys}} \,</math>
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| The aperture efficiency is a dimensionless parameter between 0 and 1.0 that measures how far the antenna falls short of using all the radio power entering its physical aperture. If the antenna were perfectly efficient, all the radio power falling within its physical aperture would be converted to electrical power delivered to the load attached to its output terminals, so these two areas would be equal ''A<sub>eff</sub>'' = ''A<sub>phys</sub>'' and the aperture efficiency would be 1.0. But all antennas have losses, such as power dissipated as heat in the resistance of its elements, nonuniform illumination by its [[Antenna feed|feed]], and radio waves scattered by structural supports and diffraction at the aperture edge, which reduce the power output. Aperture efficiencies of typical antennas vary from 0.35 to 0.70 but can range up to 0.90.
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| ==Aperture and gain==
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| The [[directivity]] of an antenna, its ability to direct radio waves in one direction or receive from a single direction, is measured by a parameter called its [[antenna gain|gain]], which is the ratio of the power received by the antenna to the power that would be received by a hypothetical [[isotropic radiator|isotropic antenna]], which receives power equally well from all directions.
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| It can be shown that the aperture of a lossless isotropic antenna, which by definition has unity gain, is:
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| :<math>A_{eff} = \frac {\lambda^2}{4 \pi} \,</math> | |
| where ''λ'' is the [[wavelength]] of the radio waves. So the gain of any antenna is proportional to its aperture:
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| :<math>G = \frac {4 \pi A_{eff} } { \lambda^2 } = \frac {4 \pi A_{phys} e_a } { \lambda^2 } \,</math>
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| So antennas with large effective apertures are [[high gain antenna]]s, which have small angular [[beam width]]s. Most of their power is radiated in a narrow beam in one direction, and little in other directions. As receiving antennas, they are most sensitive to radio waves coming from one direction, and are much less sensitive to waves coming from other directions. Although these terms can be used as a function of direction, when no direction is specified, the gain and aperture are understood to refer to the antenna's axis of maximum gain, or [[antenna boresight|boresight]].
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| ==Friis transmission equation==
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| {{main|Friis transmission equation}}
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| The fraction of the power delivered to a transmitting antenna that is received by a receiving antenna is proportional to the product of the apertures of both the antennas. This is given by a form of the [[Friis transmission equation]]:.<ref name="Narayan" />
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| :<math>P_r = \frac { A_t A_r }{ r^2 \lambda^2 } P_t \,</math>
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| where
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| :''P<sub>r</sub>'' is the power delivered by the receiving antenna in watts
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| :''P<sub>t</sub>'' is the power applied to the transmitting antenna in watts
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| :''A<sub>r</sub>'' is the aperture of the receiving antenna in m<sup>2</sup>
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| :''A<sub>t</sub>'' is the aperture of the transmitting antenna in m<sup>2</sup>
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| :''r'' is the distance between the antennas in m
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| :''λ'' is the wavelength of the radio waves in m
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| ==Thin element antennas==
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| In the case of thin element antennas such as monopoles and dipoles, there is no simple relationship between physical area and effective area. However, the effective areas can be calculated from their power gain figures:<ref>Orfanidis, Sophocles J. (2010) ''Electromagnetic Waves and Antennas'' chapter 15 page 609, retrieved 2011-04-05 from http://www.ece.rutgers.edu/~orfanidi/ewa/</ref>
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| {| border="1" class="wikitable" align="center"
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| !Wire antenna
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| !Power gain
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| !Effective area
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| |-
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| |Short dipole ([[Hertzian dipole]])
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| |1.5
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| |0.1194 <math>\lambda</math><sup>2</sup>
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| |-
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| |[[Dipole antenna|Half-wave dipole]]
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| |1.64
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| |0.1305 <math>\lambda</math><sup>2</sup>
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| |-
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| |[[Monopole antenna|Quarter-wave monopole]]
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| |1.28
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| |0.1025 <math>\lambda</math><sup>2</sup>
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| |}
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| This assumes that the monopole antenna is mounted above an infinite [[ground plane]] and that the antennas are lossless. When resistive losses are taken into account, particularly with small antennas, the [[antenna gain]] might be substantially less than the [[directivity]], and the effective area is less by the same factor.<ref>Weeks, W.L. (1968) ''Antenna Engineering'', McGraw Hill Book Company, chapters 8, pp. 297-299 and 9, pp. 343-346.</ref>
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| ==Effective length==
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| {{main|Antenna factor}}
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| {{move section portions|Antenna factor|date=July 2013}}
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| For antennas which are not defined by a physical area, such as [[Monopole antenna|monopoles]] and [[Dipole antenna|dipoles]] consisting of thin rod [[electrical conductor|conductors]], the aperture bears no obvious relation to the size or area of the antenna. An alternate measure of antenna gain that has a greater relationship to the physical structure of such antennas is ''effective length'' ''l<sub>eff</sub>'' measured in metres, which is defined for a receiving antenna as:<ref name="Rudge">{{cite book
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| | last = Rudge
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| | first = Alan W.
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| | authorlink =
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| | coauthors =
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| | title = The Handbook of Antenna Design, Vol. 1
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| | publisher = IET
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| | year = 1982
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| | location = USA
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| | pages = 24
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| | url = http://books.google.com/books?id=njTri1942n4C&pg=PA562&dq=horn+antenna&hl=en&ei=W7QmTP3hMoSBnQe3uMC8Bg&sa=X&oi=book_result&ct=result&resnum=8&ved=0CFkQ6AEwBw#v=onepage&q=antenna%20aperture&f=false
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| | doi =
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| | id =
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| | isbn = 0-906048-82-6}}</ref>
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| :<math>l_{eff} = V_0 / E_s \, </math>
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| where
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| :''V<sub>0</sub>'' is the open circuit voltage appearing across the antenna's terminals
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| :''E<sub>s</sub>'' is the electric [[field strength]] of the radio signal, in [[volt]]s per metre, at the antenna.
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| The longer the effective length the more voltage and therefore the more power the antenna will receive. Note, however, that an antenna's gain or ''A<sub>eff</sub>'' increases according to the ''square'' of ''l<sub>eff</sub>'', and that this proportionality also involves the antenna's [[radiation resistance]]. Therefore this measure is of more theoretical than practical value and is not, by itself, a useful figure of merit relating to an antenna's directivity.
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| ==References==
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| {{reflist|2}}
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| ==See also==
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| *[[Antenna (radio)]]
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| {{DEFAULTSORT:Antenna Aperture}}
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| [[Category:Antennas|Aperture]]
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| [[de:Apertur]]
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Luke is usually a superstar within the producing and the profession advancement first 2nd to his 3rd studio album, & , is definitely the evidence. He burst to the picture in 2007 with his amusing mixture of straight down-house availability, movie celebrity great looks and words, is defined t in the significant way. The new recor about the region graph or chart and #2 in the take charts, producing it the next maximum debut at that time of 2008 for the nation artist.
The kid of a , knows perseverance and determination are important elements when it comes to an effective profession- . His to start with record, Stay Me, produced the Top reaches “All My Buddies Say” and “Country Gentleman,” while his effort, Doin’ Point, discovered the performer-a few right No. 7 single men and women: More Contacting Is really a Great Point.”
Inside the luke bryan concert tours tumble of 2009, Tour: Luke Bryan & which in fact had an impressive set of , such as City. “It’s much like you are getting a approval to go to the next level, states individuals performers that were an element of the luke bryan concert tickets cheap Tourmore than into a bigger degree of luke bryan concert meet and greet tickets luke bryan tour schedule - lukebryantickets.asiapak.net - musicians.” It packaged among the most successful tours in its 15-year historical past.
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