Radwin 2000 - Figure 27-1 Fresnel Zone

RADWIN2000UserManual Release2.8.30 27‐4
AbouttheFresnelZone
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Figure27‐1:Fresnelzone
Fresnellossisthepathlossoccurringfrommulti‐pathreflectionsfromreflectivesurfaces
suchaswater,andinterveningobstaclessuchasbuildingsormountainpeaks withinthe
Fresnelzone.
Radiolinksshouldbedesignedtoaccommodateobstructionsandatmosphericconditions,
weatherconditions,largebodiesofwater,andotherreflectorsandabsorbersof
electromagneticenergy.
TheFresnelzoneprovidesuswithawa ytocalculatetheamountofclearancethatawireless
waveneedsfromanobstacletoensurethattheobstacledoesnotattenuatethesignal.
ThereareinfinitelymanyFresnelzoneslocatedcoaxiallyaroundthecenterofthedirect
wave.TheouterboundaryofthefirstFresnelzoneisdefinedasthecombinedpathlengthof
allpaths,whicharehalfwavelength(1/2)ofthefrequencytransmittedlongerthanthe
directpath.Ifthetotalpathdistanceisonewavelength(1)longerthanthedirectpath,then
theouterboundaryissaidtobetwoFresnelzones.OddnumberFresnelzonesreinforcethe
directwavepathsignal;evennumberFresnelzonescancelthedirectwavepathsignal.
TheamountoftheFresnelzoneclearanceisdeterminedbythewavelengthofthesignal,the
pathlength,andthedistancetotheobstacle.Forreliability,point‐to‐pointlinksaredesigned
tohaveatleast60%ofthefirstFresnelzonecleartoavoidsignificantattenuation.
Theconceptofthe FresnelzoneisshowninFigure27‐1above.Thetopoftheobstruction
doesnotextendfarintotheFresnelzone,leaving60%oftheFresnelzoneclear;therefore,
thesignalisnotsignificantlyattenuated.
FormoreaboutFresnelzone,seehttp://en.wikipedia.org /wiki/Fresnel_zone.