u-blox ANTARIS 4 - Antennas

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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
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