RF Design Considerations For U-blox GNSS Receivers
GNSS antennasRF design considerations for u-blox GNSS receiversApplication noteAbstractThis document provides an overview of important antenna and interference issues to be consideredwhen integrating u-blox GNSS receivers.www.u-blox.comUBX-15030289 - R03
GNSS antennas - Application noteDocument informationTitleGNSS antennasSubtitleRF design considerations for u-blox GNSS receiversDocument typeApplication noteDocument numberUBX-15030289Revision and dateR0316-Oct-2019Disclosure restrictionThis document applies to the following products:Product nameAll u-blox GNSS productsu-blox or third parties may hold intellectual property rights in the products, names, logos and designs included in thisdocument. Copying, reproduction, modification or disclosure to third parties of this document or any part thereof is onlypermitted with the express written permission of u-blox.The information contained herein is provided “as is” and u-blox assumes no liability for its use. No warranty, either express orimplied, is given, including but not limited to, with respect to the accuracy, correctness, reliability and fitness for a particularpurpose of the information. This document may be revised by u-blox at any time without notice. For the most recentdocuments, visit www.u-blox.com.Copyright u-blox AG.UBX-15030289 - R03Page 2 of 36
GNSS antennas - Application noteContentsDocument information . 2Contents . 31Introduction . 52Antenna basics . 52.1 General considerations . 52.2 Antenna requirements . 52.3 Antenna placement . 62.4 Active and passive antennas . 63Passive antenna types . 83.1 Patch antenna . 83.2 Helix antenna . 93.3 Monopole antennas . 93.3.1Chip antenna. 93.3.2Fractal Element Antenna (FEA) .123.4 Dipole antenna .123.5 Loop antenna .133.6 Planar Inverted F Antenna (PIFA) .143.7 High-end GNSS antennas .144Which antenna is best for my application? . 154.1 Helix or patch? .154.2 Other antenna types .155Design considerations . 165.1 Patch antennas .165.1.1Ground plane .165.1.2Placement .175.1.3ESD issues .185.2 Helix antennas .185.2.1Ground plane .195.2.2Placement .195.3 Antenna matching .195.4 GSM applications .205.4.1Isolation between GNSS and GSM antenna .205.5 Dual antenna systems .206Interference issues. 236.1 Sources of noise .236.2 Eliminating digital noise sources .246.2.1Power and ground planes .246.2.2High-speed signal lines .256.2.3Decoupling capacitors .256.3 Shielding .26UBX-15030289 - R03Page 3 of 36
GNSS antennas - Application note6.3.1Feed through capacitors .276.3.2Shielding sets of sub-system assembly .286.4 Increasing jamming immunity .2976.4.1In-band jamming .296.4.2Out-band jamming .30Performance tests. 317.1 Sky View .317.2 Statistic View .317.3 Supply voltage check .327.4 Sensitivity test .327.5 Startup test.32Appendix . 33AExample of receiver signal to noise (C/No) performance calculation . 33Related documents . 35Revision history . 35Contact . 36UBX-15030289 - R03Page 4 of 36
GNSS antennas - Application note1IntroductionAntennas are a critical part of any GNSS receiver design and their importance cannot be stated highlyenough. Even the best receiver cannot bring back what has been lost due to a poor antenna, in-bandjamming, or a bad RF board design. GNSS signals are extremely weak and present unique demandson the antenna. The choice and implementation of the antenna can ultimately play a significant rolein GNSS performance.This document considers some of the RF and interference issues when implementing a GNSSantenna.2Antenna basics2.1 General considerationsA GNSS receiver needs to receive signals from as many satellites as possible. Optimal performancewill not be available in narrow streets and underground parking lots or if objects cover the antenna.Poor visibility may result in position drift or a prolonged Time-To-First-Fix (TTFF). Good sky visibilityis therefore an important advantage. A GNSS receiver will only achieve the specified performance ifthe average carrier to noise power density ratio (C/N 0) of the strongest satellites reaches at least44 dBHz. In a well-designed system, the average of the C/N0 ratio of high elevation satellites shouldbe in the range between 44 dBHz and about 50 dBHz. With a standard off-the-shelf active antenna,47 dBHz should easily be achieved.2.2 Antenna requirementsFor optimal performance, use antennas with high gain (e.g. 4 dBic) and active antennas with an LNAwith a low noise figure ( 2 dB). Ideally, the antenna has: A low level of directivity1 (see Figure 1)Good antenna visibility to the skyGood matching between antenna and cable impedanceHigh gainFilterFigure 1: Antenna radiation pattern showing low directivity (left) vs. high directivity (right) 1Appendix A provides an example of how to estimate the influence of the noise figure (NF) and gainon antenna performance.Antenna gain increases with the level of directivity.UBX-15030289 - R03ContentsPage 5 of 36
GNSS antennas - Application note2.3 Antenna placementThe position of the antenna mounting is crucial for an optimal performance of the GNSS receiver.When using patch antennas, the antenna plane should be parallel to the geographic horizon. Theantenna must have full view of the sky ensuring a direct line-of-sight with as many visible satellitesas possible.1st choice placement2nd choice placementRecommended antenna positionsPerformance may be degraded!If recommended placements are not available, these may also beviable.Note:Window and roof reduce GNSS signal and obstruct sky view2Note: There may be multipath signals and an obstructed sky viewNote:Fiberglass airfoil attenuates the GNSS signalFigure 2: Recommended antenna position Place the antenna as far away as possible from radiating or jamming signals.2.4 Active and passive antennasPassive 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 50Ohms impedance. Active antennas have an integrated Low-Noise Amplifier (LNA). This is beneficial2Some cars have a metallic coating on the windscreens. GPS/GALILEO reception may not be possible in such a car without theuse of SuperSense Technology. There is usually a small section, typically behind the rear view mirror, reserved for mobile phoneand GPS/GALILEO antennas.UBX-15030289 - R03ContentsPage 6 of 36
GNSS antennas - Application notein two respects. First, the losses of the cable after the LNA no longer affect the overall noise figure ofthe GNSS receiver system. Secondly, the LNA in the antenna helps to reduce the overall noise figureof the system resulting in a better sensitivity. Some receivers are designed so that they will only workwith active antennas.Active antennas need a power supply that will contribute to GNSS system power consumption,typically in the order of 3 to 20 mA. Usually, the supply voltage is fed to the antenna through thecoaxial RF cable. Inside the antenna, the DC component on the inner conductor will be separated fromthe 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 the receiver andantenna exceeds about 10 cm. Care should be taken that the gain of the LNA inside the antenna doesnot lead to an overload condition at the receiver. For receivers that also work with passive antennasan antenna LNA gain of 15 dB is usually sufficient, even for cable lengths up to 5 m. There is no needfor the antenna LNA gain to exceed 26 dB for use with u-blox receivers (at the RF input). With shortercables and a gain above 35 dB, an overload condition might occur on some receivers.When comparing the gain figures of active and passive antennas, keep in mind that the gain of anactive antenna is composed of two components, the antenna gain of the passive radiator, given indBic, and the LNA power gain, given in dB. A low antenna gain cannot be compensated by high LNAgain. It is not possible to judge the quality of the antenna if a manufacturer provides one total gainfigure. Information is needed on the antenna gain (in dBic), the amplifier gain, and the amplifier noisefigure.Active antennaPassive antennaNeeds more power (10 – 60 mW) than a passive antenna.Is more tolerant to minor impedance miss-match or cablelength than a passive antenna (see section 5.3).Helps to keep the receiver noise figure low.Is less affected by jamming into the antenna cable than apassive antenna (if equipped with filter).Does not add anything to the power budget.Antenna must be connected with a carefully designed microstrip or strip line of maximum 10 cm to the GNSS receiver toensure good GNSS performance.Jamming signals coupled into the micro-strip or strip linenegatively affect the performance.RF design experience is required to properly design a passiveantenna.Table 1: Active vs. passive antennaUBX-15030289 - R03ContentsPage 7 of 36
GNSS antennas - Application note3Passive antenna typesThe GNSS signal is right-hand circular polarized (RHCP). This results in a style of antenna that isdifferent from the well-known whip antennas used for linear polarized signals.3.1 Patch antennaThe most common antenna type for GNSS applications is the patch antenna. Patch antennas are flat,generally have a ceramic and metal body and are mounted on a metal base plate. They are often castin a housing.Patch antennas are ideal for situations where the antenna is mounted on a flat surface, e.g. the roofor the dashboard of a car. Patch antennas can show a very high gain, especially if they are mountedon top of a large ground plane (70 x 70 mm). Ceramic patch antennas are very popular because of thelow costs and the huge variation of available sizes (40 x 40 mm down to 10 x 10 mm; typical 25 x 25mm).Antenna type25 x 25 mm patchApplication exampleu-blox reference design with 25 x 25 mm patch (C04-5H)Figure 3: Examples of patch antennasA smaller antenna will present a smaller aperture to collect the signal energy from the sky, resultingin a lower overall gain of the antenna. This is the result of pure physics and there is no “magic” to getaround this problem. Amplifying the signal after the antenna will not improve the signal to noise ratio. Patch antennas of 25 mm by 25 mm show optimal performance and are cost-efficient. Patchessmaller than 17 mm by 17 mm tend to demonstrate moderate navigation performance3.Performance is dependent on the ground plane size. For more information about u-blox reference designs see our website at www.u-blox.com.3Unless enhanced by u-blox SuperSense technology.UBX-15030289 - R03ContentsPage 8 of 36
GNSS antennas - Application note3.2 Helix antennaAnother style is the quadrifilar helix antenna. The actual geometric sizedepends on the dielectric that fills the space between the active parts ofthe antenna. If the antenna is only loaded with air it will be comparativelylarge (60 mm length and 45 mm diameter), high dielectric constantceramics result in a much smaller form factor. The smaller thedimensions of the antenna, the more performance-critical tightmanufacturing tolerances become.Figure 4: Helix antennaAs with patch antennas, filling the antenna with a high dielectricconstant material can reduce the size of helix antennas. Sizes in the orderof 18 mm length and 10 mm diameter are being offered to the market.Again, antenna gain will decrease with decreasing size of the antenna. Helical antennas are typically used in applications where multiple antenna orientations arepossible. They are robust and demonstrate good navigation performance.Antenna typeHelix: GeoHelix-P2 (Sarantel)Application exampleu-blox reference design with helix antenna (C05-5H)Figure 5: Examples of helix antenna For more information about u-blox reference designs see our website at www.u-blox.com.3.3 Monopole antennas3.3.1 Chip antennaChip antennas are becoming increasingly important for GNSS designs. Their low cost and extremelysmall size (down to 3.2 x 1.6 x 1.1 mm), as well as high gain and omni-directional radiation patternsmake them particularly attractive in consumer electronic applications such as mobile telephones andPNDs. Due to their miniature size, a variety of factors influence the performance of chip antennas.These factors include the footprint, ground plane size, isolation distance (typical 5 mm) and mountingof the chip antenna and GNSS device. The isolation distance or “keep-out area” can have an importantimpact on antenna efficiency, and thus on GNSS performance, and needs to be carefully consideredUBX-15030289 - R03ContentsPage 9 of 36
GNSS antennas - Application notein designs. Isolation distances must be included to avoid deviations in antenna performance. Evenwith these measures acceptable performance cannot always be guaranteed due to potential detuningeffects created by nearby objects.Even if some antenna manufacturers claim that a ground plane is not required, the available groundplane has a sig
All u-blox GNSS products u-blox or third parties may hold intellectual property rights in the products, names, logos and designs included in this document. Copying, reproduction, modification or disclosure to third parties of this document or any part thereof is only permitted with the express writte
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