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| [[File:FresnelSVG1.svg|thumb|500px|Fresnel zone: ''D'' is the distance between the transmitter and the receiver; ''r'' is the radius of the first Fresnel zone (n=1) at point P. P is d1 away from the transmitter, and d2 away from the receiver.]] | | [http://www.purevolume.com/clarissalashbrook/posts/6900617/Beware+The+Spilleautomater+P%C3%A5+Nettet+Scam spille automater] Hello! Allow [http://www.fizzlive.com/member/1212915/blog/view/3494922 spilleautomat] me begin by saying my title - Summer time Ard and I really [https://Www.Google.com/search?hl=en&gl=us&tbm=nws&q=feel+comfortable feel comfortable] [http://www.purevolume.com/melodymabrydn/posts/6902513/Everyone+Loves+Spilleautomaten on the web spille automater] when [http://www.fizzlive.com/member/1215044/blog/view/3504090 spilleautomat] people use the full title. To resolve [http://www.purevolume.com/marc62tyfxwi/posts/6885513/What+Ancient+Greeks+Knew+About+Les+V%C3%A5re+Artikler+Og+Bli+En+Mester+P%C3%A5+Spilleautomater+That+You. norskautomater] puzzles is one of the things she enjoys most. District of Columbia is exactly where my home is but now I'm contemplating [http://dewittdeniehy.newsvine.com/_news/2014/08/01/25111217-whispered-her-kan-du-f-vite-det-du-trenger-om-spilleautomater-p-nettet-secrets delaware beste norske spilleautomater] other choices. Hiring [http://www.purevolume.com/melodymabrydn/posts/6922473/8+Lessons+About+Casino+Spilleautomater+You+Need+To+Learn+Before+You+Hit+40 free norrman spilleautomater] has been his day job for a while but his promotion never [https://www.rebelmouse.com/shermanyfgwlxqitt/3-facts-everyone-should-know-a-666827495.html automater villig nett] arrives. I'm not great at [http://www.purevolume.com/murrayrossetti/posts/6842962/4+Surefire+Ways+Norske+Spilleautomater+Er+En+Morsom+Kilde+Til+Underholdning+Og+Her+Kan+Du+Pr%C3%B8ve. spilleautomat gällande nett] webdesign but you might want to check my web site: http://warrenmooretgqn.newsvine.com/_news/2014/08/07/25207526-seven-days-to-a-better-alle-norske-spilleautomater-som-er-p-vr-side<br><br> |
| In [[optics]] and [[radio]] [[telecommunication|communications]] (indeed, in any situation involving the radiation of waves, which includes electrodynamics, acoustics, gravitational radiation and [[Banana Doughnut theory|seismology]]), a '''Fresnel zone''' ({{IPAc-en|f|r|eɪ|ˈ|n|ɛ|l}} {{respell|fray|NEL|'}}), named for physicist [[Augustin-Jean Fresnel]], is one of a (theoretically infinite) number of concentric [[ellipsoid]]s which define volumes in the [[radiation pattern]] of a (usually) circular [[aperture (antenna)|aperture]]. Fresnel zones result from [[diffraction]] by the circular aperture.<ref>{{cite web|title= Fresnel zone|url=http://www.its.bldrdoc.gov/fs-1037/dir-016/_2398.htm|publisher=its.bldrdoc.gov|accessdate=2008-02-21}}</ref>
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| The cross section of the first (innermost) Fresnel zone is [[circle|circular]]. Subsequent Fresnel zones are [[annulus (mathematics)|annular]] (doughnut-shaped) in [[Cross section (geometry)|cross section]], and [[concentric]] with the first.
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| To maximize receiver strength, one needs to minimize the effect of obstruction loss by removing obstacles from the radio frequency [[Line-of-sight propagation|line of sight]] (RF LoS). The strongest signals are on the direct line between transmitter and receiver and always lie in the first Fresnel zone.
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| '''Fresnel zones'''
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| If unobstructed, radio waves will travel in a straight line from the transmitter to the receiver. But if there are reflective surfaces along the path, such as bodies of water or smooth terrain, the radio waves reflecting off those surfaces may arrive out of phase (by reflecting off an a surface within an even Fresnel zone) with the signals that travel directly and reduce the power of the received signal. On the other hand, the reflection (off a surface within an odd Fresnel zone) can enhance the power of the received signal if the reflection and the direct signals arrive in phase. Sometimes this results in the counter-intuitive finding that reducing the height of an antenna increases the [[signal-to-noise ratio]].
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| Fresnel provided a means to calculate where the zones are, where a given obstacle will cause mostly in phase or mostly out of phase reflections between the transmitter and the receiver. Obstacles in the first Fresnel zone will create signals with a path-length phase shift of 0 to 180 degrees, in the second zone they will be 180 to 360 degrees out of phase, and so on. Even numbered zones have the maximum phase cancelling effect and odd numbered zones may actually add to the signal power.<ref>{{cite web|title=Wireless – Fresnel Zones and their Effect|url=http://www.zytrax.com/tech/wireless/fresnel.htm|publisher=zytrax.com|accessdate=2008-02-21}}</ref>
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| ==Determining Fresnel zone clearance==
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| [[Image:Fresnel zone disrupted.png|thumb|300px|Several examples of how the Fresnel zone can be disrupted.]]
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| The concept of Fresnel zone clearance may be used to analyze [[Interference (communication)|interference]] by obstacles near the path of a radio beam. The first zone must be kept largely free from obstructions to avoid interfering with the radio reception. However, some obstruction of the Fresnel zones can often be tolerated, as a [[rule of thumb]] the maximum obstruction allowable is 40%, but the recommended obstruction is 20% or less.
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| For establishing Fresnel zones, first determine the RF Line of Sight (RF LoS), which in simple terms is a straight line between the transmitting and receiving antennas. Now the zone surrounding the RF LoS is said to be the Fresnel zone.<ref>{{cite web|title=Fresnel Zone Clearance|url=http://www.softwright.com/faq/engineering/Fresnel%20Zone%20Clearance.html|publisher=softwright.com|accessdate=2008-02-21}}</ref>
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| The general equation for calculating the Fresnel zone radius at any point P in between the endpoints of the link is the following:
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| :<math>F_n = \sqrt{\frac{n \lambda d_1 d_2}{d_1 + d_2}} </math>
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| where,
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| F<sub>n</sub> = The nth Fresnel Zone radius in metres
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| d<sub>1</sub> = The distance of P from one end in metres
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| d<sub>2</sub> = The distance of P from the other end in metres
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| <math>\lambda</math> = The wavelength of the transmitted signal in metres
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| The cross sectional radius of each Fresnel zone is the longest in the center of the RF LoS, shrinking to a point at the antenna on each end. For practical applications, it is often useful to know the maximum radius of the first Fresnel zone. From the above formula, the following formulas can be derived, using <math>d_1 = d_2</math>, <math>D = d_1 + d_2</math>, and <math>\lambda = \frac{c}{f}</math>. Now we have an easy way to calculate the radius of the first Fresnel zone (F<sub>1</sub> in the above equation), knowing the distance between the two antennas and the frequency of the transmitted signal.
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| In [[International System of Units|SI]]:
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| :<math>r = 8.657 \sqrt{{D} \over f}</math>
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| :where
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| :* ''r'' = radius in metres
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| :* ''D'' = total distance in kilometres
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| :* ''f'' = frequency transmitted in gigahertz.
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| Or in [[imperial units]]:
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| :<math>r = 36.03 \sqrt{{D} \over f}</math>
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| :where
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| :* ''r'' = [[radius]] in feet
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| :* ''D'' = total distance in miles
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| :* ''f'' = frequency transmitted in [[Gigahertz]].
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| ==See also==
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| {{MultiCol}}
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| * [[Diversity scheme]]
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| * [[Fresnel integral]]
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| {{ColBreak}}
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| * [[Fresnel number]]
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| * [[Fresnel diffraction]]
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| * [[Microwave]]
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| * [[Near and far field|Near field]]
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| {{ColBreak}}
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| * [[Path loss]]
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| * [[Rain fade]]
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| * [[Weissberger's Model]]
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| * [[Zone plate]]
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| {{EndMultiCol}}
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| ==References==
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| {{reflist}}
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| *{{FS1037C MS188}}
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| ==External links==
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| *[http://www.geocontext.org/publ/2010/04/profiler/en/?a=51.493505021366715,-0.010830760002136&b=51.476855897332214,-0.000549852848053&ab=1&maptype=hybrid&f=2400-5-0-m Fresnel Zone Calculator and Elevation Chart]
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| *[http://www.afar.net/fresnel-zone-calculator/ Fresnel Zone Calculator]
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| *[http://wisp-router.com/wirelesscalculators.php#fresnel FEN Fresnel Zone Calculator]
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| *[http://www.maxstream.net/support/knowledgebase/article.php?kb=90 More Fresnel Zone Details]
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| *[http://www.searchanddiscovery.net/documents/geophysical/sheriff/index.htm R.E. Sherriff, Understanding the Fresnel zone]
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| *[http://www.tapr.org/ve3jf.dcc97.html VHF/UHF/Microwave Radio Propagation: A Primer for Digital Experimenters]
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| {{DEFAULTSORT:Fresnel Zone}}
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| [[Category:Diffraction]]
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| [[Category:Radio frequency propagation]]
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