u-blox ANTARIS 4 User Manual

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antennaswitha “reasonable”sizewillthereforetypicallyshow alowersensitivity comparedto a“reasonably”
sizedpatchantenna.
Ahelixantennamightresultina“moresatellitesonthescreen”situationindifficultsignalenvironmentswhen
directlycomparedwithapatchantenna.Thisisduetothefactthatthehelix willmoreeasily pickup
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signalsthroughitsomnidirectionalradiationpattern.However,thepracticaluseofthesesignalsis verylimited
because of the uncertain path of the reflected signals. Therefore, the receivers can see more satellites but the
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tsa50Ohmssourceimpedance. Typicallywhathappens isthatthecenterfrequency
of the antenna is shifted away from GPS frequency - usually towards lower frequencies – by some external
i asonsforthiseffectareprimarilydisturbancesin thenear fieldoftheantenna.Thiscaneither
bea groundplane,thatdoesnothavethesizeforwhichtheantennawasdesigned,oritcanbeanenclosure
rease with decreasingantennasize, making it harder to
achieveoptimumtuning.
navigationsolutionwillbedegradedbecauseofdistortedrangemeasurementsinamulti-pathenvironment.
If possibletestthe actualperformanceof differentantennatypes inareallifeenvironmentbefore starting the
mechanicaldesignoftheGPSenabledproduct.
1.5.8 Antenna Matching
AllcommonGPSantennasaredesignedfora50Ohmselectricalload.Therefore,oneshouldselecta50Ohms
cable to connect the antenna to the receiver. However, there are several circumstances under which the
matching impedance of the antenna might shift considerably. Expressed in other words, this means that the
antennanolonger presen
nfluence.There
withadifferentdielectricconstantthanair.
In order to analyze effects like this one would normally employ electrical field simulations, which will result in
exactrepresentationoftheelectricfieldsinthenearfieldoftheantenna.Furthermore,thesedistortionsofthe
near field will also show their effect in the far field, changing the radiation pattern of the antenna.
Unfortunately, there is no simple formula to calculate the frequency shift of a given antenna in any specified
environment.Soonemustdoeitherextensivesimulationorexperimentalwork.Usually,antennamanufacturers
offer a selection of pre-tuned antennas, so the user can test and select the version that best fits the given
environment.However,testingequipmentsuchasascalarnetworkanalyzerisneededtoverifythematching.
Again,itmustbepointedoutthatthesmallerthesizeoftheantenna,themoresensitiveitwillbetodistortions
in thenearfield. Also theantenna bandwidth will dec
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Figure 11: Dependency of center frequency on ground plane dimension for a 25 x 25 mm
2
patch, EMTAC
ALNAplacedveryclosetotheantennacanhelptorelievethematchingrequirements.Iftheinterconnectlength
betweenantennaandLNAismuchshorterthanthewavelength(9.5cmonFR-4),thematchinglossesbecome
lessimportant.UndertheseconditionsthematchingoftheinputtotheLNAbecomesmoreimportant.Withina
reasonable mismatch range, integrated LNAs can show a gain decrease in the order of a few dBs versus an
increaseofnoisefigureintheorderofseveraltenthsofadB.Ifyourapplicationrequiresa verysmallantenna,a
GPSModules-SystemIntegrationManual(SIM)(incl.ReferenceDesign) GPSFundamentals
GPS.G4-MS4-05007-A1
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u-blox ANTARIS 4 Specifications

General

Brand	u-blox
Model	ANTARIS 4
Category	GPS
Language	English