Harman Becker Automotive Systems BE2728 BLUETOOTH AUTOMOTIVE INFOTAINMENT UNIT User Manual Antenna App Note

Harman Becker Automotive Systems, Inc. BLUETOOTH AUTOMOTIVE INFOTAINMENT UNIT Antenna App Note

Contents

Antenna App Note

bcant-an-001Pa © Copyright CSR 2003This material is subject to CSR’s non-disclosure agreement.CSR Unit 400 Cambridge Science ParkMilton RoadCambridgeCB4 0WHUnited Kingdom Registered in England 3665875 Tel: +44 (0)1223 692000 Fax: +44 (0)1223 692001www.csr.comBlueCore™Inverted-F and Meander Line AntennasApplication NoteJanuary 2003
Contentsbcant-an-001Pa © Copyright CSR 2003This material is subject to CSR’s non-disclosure agreement. Page 2 of 14BlueCore™ Inverted-F and Meander Line AntennasContents1Introduction.......................................................................................................................................................................32Inverted-F Antenna .........................................................................................................................................................43Meander Line Antenna ...................................................................................................................................................54Real Designs.....................................................................................................................................................................65Proximity to Metal Objects ...........................................................................................................................................76Proximity to Dielectric Materials.................................................................................................................................87Network Analyser............................................................................................................................................................98Final Tuning ................................................................................................................................................................... 109Conclusion ..................................................................................................................................................................... 12Acronyms and Definitions ................................................................................................................................................. 13Record of Changes.............................................................................................................................................................. 14List of FiguresFigure 2.1: Inverted-F Antenna...............................................................................................................................................4Figure 3.1: Meander Line Antenna.........................................................................................................................................5Figure 3.2: Input Impedance of Two Meander Line Antennas ...........................................................................................5Figure 4.1: Approximate Dimensions of Inverted-F Antenna.............................................................................................6Figure 4.2: Approximate Dimensions of Meander Line Antenna.......................................................................................6Figure 7.1: Preparation Before Measurement......................................................................................................................9Figure 7.2: Assembled System Ready to Measure.............................................................................................................9Figure 8.1: Locating Product in Far Field of Antenna....................................................................................................... 10Figure 8.2: Final Tuning Procedure .................................................................................................................................... 11
Introductionbcant-an-001Pa © Copyright CSR 2003This material is subject to CSR’s non-disclosure agreement. Page 3 of 14BlueCore™ Inverted-F and Meander Line Antennas1 IntroductionThis document outlines two types of Printed Circuit Board (PCB) antennas used by CSR.§ Inverted-F§ Meander LineAlso discussed in this document is the effect of placing metallic or dielectric materials near anantenna.
Inverted-F Antennabcant-an-001Pa © Copyright CSR 2003This material is subject to CSR’s non-disclosure agreement. Page 4 of 14BlueCore™ Inverted-F and Meander Line Antennas2 Inverted-F AntennaInputOutputQuarterwaveFigure 2.1: Inverted-F AntennaThe inverted-F is a quarterwave antenna. It is bent into an L-shape. The shorter side is connected to earth. Thelonger side is left open circuit at the end. The feed point is located somewhere between the earth end and theopen end. The resulting structure resembles the letter F and possesses the properties of both a loop antenna dueto the circulating current from the feed point to ground and a whip antenna due to the open circuited straightsection.In the PCB version the antenna is printed on the top layer and a ground plane is placed near the antenna on thetop layer. There must not be a ground plane underneath the antenna.The aim is to make the quarterwave section resonate at midband frequency (which is 2441MHz for Bluetooth™).The feed point (which is the input/output connection) is connected to the L-Shape at the point corresponding to50Ω. Experiment with measurement to determine correct location for the feed point and length of this antenna.
Meander Line Antennabcant-an-001Pa © Copyright CSR 2003This material is subject to CSR’s non-disclosure agreement. Page 5 of 14BlueCore™ Inverted-F and Meander Line Antennas3 Meander Line AntennaInputOutputSGroundPlaneFigure 3.1: Meander Line AntennaThe length of the meander line antenna is difficult to predict. It is usually a bit longer than a quarterwave butdependent on its exact geometry and proximity to the ground plane.Note:In Figure 3.1 the ground plane is shown in black. S is the distance from the ground plane. See Figure 4.2 forapproximate dimensions.This type of antenna is always a PCB version. The antenna is printed on the top layer and a ground plane isplaced near the antenna on the top layer. There must be no ground plane underneath the radiating section of theantenna.ABSmith ChartFigure 3.2: Input Impedance of Two Meander Line AntennasThe real part of the impedance of this antenna is about 15-25Ω, depending on geometry and proximity to theground plane. The impedance matching is done by adjusting the length of the antenna until the input impedanceis at the unity conductance circle (when normalised to 50Ω), in the top half of the Smith chart (Point A). A shuntcapacitor is then connected between the antenna input and ground to match to 50Ω (Point B). Experimentalmeasurement is used to determine the correct design.
Real Designsbcant-an-001Pa © Copyright CSR 2003This material is subject to CSR’s non-disclosure agreement. Page 6 of 14BlueCore™ Inverted-F and Meander Line Antennas4 Real DesignsGroundPlane5..0mm6..0mmWidth=0.8mm13.5mm18.0mmNot to scaleActual SizeFigure 4.1: Approximate Dimensions of Inverted-F AntennaFeedpoint1.7mmGroundPlane2.8mmWidth=0.5mm4.4mm1.0mm1.5mm8..0mm1.5pF CapacitorPlaced immediatelyafter FeedpointNot to scaleActual SizeFigure 4.2: Approximate Dimensions of Meander Line Antenna
Proximity to Metal Objectsbcant-an-001Pa © Copyright CSR 2003This material is subject to CSR’s non-disclosure agreement. Page 7 of 14BlueCore™ Inverted-F and Meander Line Antennas5 Proximity to Metal ObjectsCSR recommends keeping metal objects as far away from the antenna as possible. Keeping metallic objects outof the near field is usually adequate.Near Field = 2D2 / λD is the largest dimension of the antenna. In the case of these antennas, this is approximately aquarterwave (λ/4).Notes:λ is the wavelength of the signal in freespace.At Bluetooth frequencies, λ=122mm in freespace.Substituting D=λ/4 into the Near Field equation gives Near Field = λ/8.Near Field = 122/8 mm = 15.25mm.
Proximity to Dielectric Materialsbcant-an-001Pa © Copyright CSR 2003This material is subject to CSR’s non-disclosure agreement. Page 8 of 14BlueCore™ Inverted-F and Meander Line Antennas6 Proximity to Dielectric MaterialsDielectric materials (like plastic or FR-4) detune an antenna by lowering its resonant frequency. The effect is notas serious as placing an antenna next to metal objects and can be corrected by reducing the length of theantenna. Therefore it is important for the antenna to be tuned when it is in the product. This is done during thedevelopment of the product.
Network Analyserbcant-an-001Pa © Copyright CSR 2003This material is subject to CSR’s non-disclosure agreement. Page 9 of 14BlueCore™ Inverted-F and Meander Line Antennas7 Network AnalyserA Vector Network Analyser (VNA) is used to perform the initial tuning of the antenna:1. The PCB track (trace), just before the antenna matching network is cut to isolate the filter and previousstages from the measurement.2. A coaxial cable is connected between the VNA and the PCB of the product. The outer conductor of thecoaxial cable is soldered to the ground plane of the PCB as close as possible to the input of the antennamatching network. The inner conductor of the coaxial cable is left floating. The coaxial cable must haveferrite beads over the outer sleeve of the coaxial cable. The ferrite beads help to prevent RF currentsfrom flowing on the outside of the coaxial cable (which would disturb the measurement).3. A One-Port calibration is performed on the VNA with Open, Short, Loads connected at the end of thecoaxial cable inside the product.4. The inner conductor of the coaxial cable is soldered to the input of the antenna matching network.5. The antenna is tuned by adjusting the values of any “matching network” components, the feed point ofthe antenna or the length of the antenna until the S11 trace (displayed on the VNA) is at the centre ofthe Smith chart at the midband frequency 2441MHz.6. The antenna is now roughly tuned and the cut track can be repaired by putting a small amount of solderover the cut.Figure 7.1: Preparation Before MeasurementFigure 7.2: Assembled System Ready to Measure
Final Tuningbcant-an-001Pa © Copyright CSR 2003This material is subject to CSR’s non-disclosure agreement. Page 10 of 14BlueCore™ Inverted-F and Meander Line Antennas8 Final TuningAfter tuning the antenna using the VNA procedure, it is necessary to perform fine tuning. This will yield a smallimprovement and will be the final optimisation of the antenna. It is best to perform this procedure in an anechoicchamber, but when this is not possible an indoor or outdoor test range can be used. It is important to minimiseradio signal reflections. Avoid metallic objects such as lab-benches, filing cabinets, lampposts and cars.Figure 8.1: Locating Product in Far Field of Antenna
Final Tuningbcant-an-001Pa © Copyright CSR 2003This material is subject to CSR’s non-disclosure agreement. Page 11 of 14BlueCore™ Inverted-F and Meander Line AntennasFigure 8.2: Final Tuning ProcedureConnect omni directional receiveantenna to a spectrum analyserPlace fully assembled Bluetooth productapproximately 2m away from receive antennaPut Bluetooth product into continuous transmitWatch power level of received signal on the specturm analyser whilemoving receive antenna ±10cm in each of x,y,z planes to ensure it is notlocated in a null point. A sudden dip in received power indicates a null pointIs antenna located ina null point?Rotate Bluetooth product around in the x,y,z planes until the maximumpower level is observed on the spectrum analyser. This ensures thatthe dominant polarisation mode of the antenna is measured.Has the maximum possiblepower level been observed onthe spectrum analyser?Record power level observedon the spectrum analyser.Turn off Bluetooth product, disassemble itand make adjustments to length of antenna,feed point or matching componentsReassemble Bluetooth product and place it inthe same location and orientation as beforeEnd of procedureNoYesYesRepeat process on multiple devicesto ensure results are repeatableYesNoHas the maximum receivepower been obtained?No
Conclusionbcant-an-001Pa © Copyright CSR 2003This material is subject to CSR’s non-disclosure agreement. Page 12 of 14BlueCore™ Inverted-F and Meander Line Antennas9 ConclusionMetal objects should be kept at least 15.25mm away from the Inverted-F and Meander Line types of antennas inthe Bluetooth frequency band in order for the antenna to work efficiently. If that is not possible, then extraexperimentation is required to determine an acceptable trade-off between antenna performance and product size.Even if these rules are followed, antenna detuning can occur. Usually the resonant frequency of the antenna willbe lowered. This can be corrected by reducing the length of the antenna.
Acronyms and Definitionsbcant-an-001Pa © Copyright CSR 2003This material is subject to CSR’s non-disclosure agreement. Page 13 of 14BlueCore™ Inverted-F and Meander Line AntennasAcronyms and DefinitionsBlueCoreGroup term for CSR’s range of Bluetooth chipsBluetoothSet of technologies providing audio and data transfer over short-range radio connectionsCSR Cambridge Silicon RadioPCB Printed Circuit BoardRF Radio FrequencyVNA Vector Network Analyser
Record of Changesbcant-an-001Pa © Copyright CSR 2003This material is subject to CSR’s non-disclosure agreement. Page 14 of 14BlueCore™ Inverted-F and Meander Line AntennasRecord of ChangesDate: Revision Reason for Change:24 JAN 03 aOriginal publication of this document. (CSR reference bcant-an-001Pa).BlueCore™Inverted-F and Meander Line AntennasApplication Notebcant-an-001PaJanuary 2003Bluetooth™ and the Bluetooth logos are trademarks owned by Bluetooth SIG Inc, USA and licensed to CSR.BlueCore is a trademark of CSR.All other product, service and company names are trademarks, registered trademarks or service marks of theirrespective owners.CSR’s products are not authorised for use in life-support or safety-critical applications.

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