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Thereceivercannotcompensateforthesecondeffectbecausethesignalscanceloutattheantenna,notinside

theGPSunit.However,asthereflectedsignalisusuallymuchweakerthanthedirectsignal,thetwosignalswill

notcanceloutcompletely.Thereflectedsignal willalsohavean invertedpolarity

(lefthandcircularratherthan

righthandcircular),furtherreducingthesignallevel,particularlyiftheantennahasgoodpolarizationselectivity.

Waterisaverygoodreflector;soallmarineapplicationsrequirespecialattentiontoreflectedsignalsarrivingat

theantennafromtheunderside,i.e.thewatersurface.Also,the

locationoftheantennaclosetoverticalmetal

surfacescanbeverydisruptivesincemetalisanalmostperfectreflector.Whenmountinganantennaontopofa

reflectivesurface,theantennashouldbemountedasclosetothesurfaceaspossible.Then,thereflectivesurface

willactasanextension

oftheantennasgroundplaneandnotasasourceofmulti-path.

1.5 Antennas

Even the bestreceiver cannot bring back what has been lost at the antenna. The importanceof  the attention

paidtothispartofaGPSsystemcannotbestatedhighlyenough.

1.5.1 Selecting the right Antenna

Several different antenna designs are available on theGPS applications market. The GPS signal is right-hand

circularpolarized(RHCP).Thisresultsinastyleofantennathatisdifferentfromthewell-knownwhipantennas

usedforlinearpolarizedsignals.ThemostprominentantennadesignsforGPSarethepatchantennaas

shown

inFigure 6.

 

Figure 6: Patch Antennas, EMTAC Technology Corp.

Another style is the quadrifilar helix antenna shown in Figure 7. The actual geometric size of both antenna

designsdependson thedielectricthatfillsthespacebetween theactivepartsoftheantenna.Iftheantennais

onlyloadedwithairitwillbecomparativelylarge,highdielectricconstantceramics

resultinamuchsmallerform

factor.Thesmallerthedimensionsoftheantenna,themoreperformancecriticaltightmanufacturingtolerances

become. Furthermore, a smaller antenna will present a smaller aperture to collect the signal energy from sky

resultinginaloweroverallgainoftheantenna.Thisistheresult

ofpurephysicsandthereisno“magic”toget

aroundthisproblem.Amplifyingthesignalaftertheantennawillnotimprovethesignaltonoiseratio.

GPSModules-SystemIntegrationManual(SIM)(incl.ReferenceDesign) GPSFundamentals

GPS.G4-MS4-05007-A1

  Page 16

u-blox ANTARIS 4 User Manual

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