u-blox ANTARIS 4 User Manual

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Figure 7: Quadrifilar Helix Antenna, Sarantel, Ltd.
In contrast to helix antennas, patch antennas require a ground plane for operation. Helix antennas can be
designedforusewithorwithoutagroundplane.
For precisionapplicationssuch assurveyingortiming, somevery high-endsystemsdoexist.Commonto these
designsare large size,high power consumptionand
highcost. These designsarehighlyoptimizedtosuppress
multi-pathsignalsreflectedfromtheground(chokeringantennas,multi-pathlimitingantennas,MLA).Another
areaofoptimizationisaccuratedeterminationofthephasecenteroftheantenna.ForprecisionGPSapplications
withpositionresolutioninthemillimeterrangeitisimportant
thatsignalsfromsatellitesatallelevationsvirtually
ltonoiseratiowillbelost.
Activeantennashaveanintegratedlow-noiseamplifier.Thisisbeneficialintworespects.Firstly,thelossesofthe
noisefigureoftheGPSreceiversystem.Secondly,thereceivernoisefigurecan
ingperformance.Therefore,somereceiverswillonlyworkwithactiveantennas.
sufficient,evenforcablelengthsupto5m.There’snoneedfortheantennaLNAgaintoexceed
26dB for usewith u-blox receivers. Withshorter cables anda gain above 25dB, an overload condition might
occuronsomereceivers.
When comparing
 gain measures of active and passive antennas one has to keep in mind that the gain of an
activeantennaiscomposedoftwocomponents,theantennagainofthepassiveradiator,givenindBic,andthe
LNApowergaingivenindB.A lowantennagaincannotbe compensated
byhighLNAgain.Ifamanufacturer
provides one total gain  figure, this is not sufficient to judge the quality of the antenna. One would need
informationonantennagain(indBic),amplifiergain,andamplifiernoisefigure.
1.5.3 Patch Antennas
Patchantennasareidealforanapplicationwheretheantennasitsonaflatsurface,e.g.theroofofacar.Patch
antennas can demonstrate a very high gain, especially if they are mounted on top of a  large ground plane.
Ceramicpatchantennasareverypopularbecauseoftheir small
size,typicallymeasuring25x25mm
2
downto
meet at exactly the same point inside the antenna. For these types of applications receivers with multiple
antennainputsareoftenrequired.
Atthe lowend ofthe spectrumofpossibleantennasolutions -if theuseris willingtoaccept significantsignal
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losses -a simple linear polarizedwhip or stripantenna willwork. Compared to acircular polarizedantenna,a
minimumof3dBofsigna
1.5.2 Active and Passive Antennas
Passive antennas contain only the radiating element, e.g. the ceramic patch or the helix structure. Sometimes
theyalsocontainapassivematchingnetworktomatchtheelectricalconnectionto50Ohmsimpedance.
cable
nolongeraffecttheoverall
bemuchhigherwithoutsacrific
ActiveantennasrequireapowersupplythatcontributestototalGPSsystempowerconsumption,typicallyinthe
regionof5to20mA.Usually,thesupplyvoltageisfedtotheantennathroughthecoaxialRFcable.Insidethe
antenna, the DC component on the inner conductor will be separated from the RF signal and routed to the
supplypinoftheLNA.
The useof an active
 antenna isalways advisable if the RF-cable length between receiver and antennaexceeds
approximately 10cm. Care should be taken that the gain of the LNA inside the antenna does not lead to an
overload conditionat thereceiver.Forreceiversthat also workwithpassive antennas,an antenna LNA
gain of
15dBisusually
GPSModules-SystemIntegrationManual(SIM)(incl.ReferenceDesign) GPSFundamentals
GPS.G4-MS4-05007-A1
  Page 17

u-blox ANTARIS 4 Specifications

General

Brand	u-blox
Model	ANTARIS 4
Category	GPS
Language	English