Radwin 2000 C PLUS

RADWIN2000CPlusUserManual Release3.5.70 18‐4
AbouttheFresnelZone Chapter18
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Figure18‐1: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way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 18‐1above.Thetopoftheobstruction
doesnotextendfarintotheFresnelzone,leav ing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.