Radwin 2000 D+ - Figure 19-1 Fresnel Zone

RADWIN2000D+UserManual Release3.6.00 19‐4
AbouttheFresnelZone Chapter19
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Figure19‐1: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ar e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 19‐1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Fre snelzone,seehttp://en.wikipedia.org/wiki/Fresnel_zone.