BlueCore™ Inverted-F and Meander Line Antennas Application Note January 2003 CSR Unit 400 Cambridge Science Park Milton Road Cambridge CB4 0WH United Kingdom Registered in England 3665875 Tel: +44 (0)1223 692000 Fax: +44 (0)1223 692001 www.csr.com bcant-an-001Pa © Copyright CSR 2003 This material is subject to CSR’s non-disclosure agreement.
Contents Contents Introduction.......................................................................................................................................................................3 2 Inverted-F Antenna .........................................................................................................................................................4 3 Meander Line Antenna .............................................................................................................
Introduction 1 Introduction This document outlines two types of Printed Circuit Board (PCB) antennas used by CSR. § Inverted-F § Meander Line bcant-an-001Pa © Copyright CSR 2003 This material is subject to CSR’s non-disclosure agreement. Page 3 of 14 BlueCore™ Inverted-F and Meander Line Antennas Also discussed in this document is the effect of placing metallic or dielectric materials near an antenna.
Inverted-F Antenna 2 Inverted-F Antenna Quarterwave Figure 2.1: Inverted-F Antenna The inverted-F is a quarterwave antenna. It is bent into an L-shape. The shorter side is connected to earth. The longer side is left open circuit at the end. The feed point is located somewhere between the earth end and the open end.
Meander Line Antenna 3 Meander Line Antenna S Input Output Figure 3.1: Meander Line Antenna The length of the meander line antenna is difficult to predict. It is usually a bit longer than a quarterwave but dependent 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 for approximate dimensions. This type of antenna is always a PCB version.
Real Designs 4 Real Designs 18.0mm 13.5mm . 5 .0mm . Not to scale Ground Plane Actual Size Figure 4.1: Approximate Dimensions of Inverted-F Antenna 1.0mm 8 .0 mm 1.5mm 4.4mm Width=0.5mm 2.8mm 1.7mm . Not to scale Ground Plane 1.5pF Capacitor Placed immediately after Feedpoint Feedpoint Actual Size Figure 4.2: Approximate Dimensions of Meander Line Antenna bcant-an-001Pa © Copyright CSR 2003 This material is subject to CSR’s non-disclosure agreement.
Proximity to Metal Objects 5 Proximity to Metal Objects CSR recommends keeping metal objects as far away from the antenna as possible. Keeping metallic objects out of the near field is usually adequate. Near Field = 2D2 / λ 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 Materials 6 Proximity to Dielectric Materials Dielectric materials (like plastic or FR-4) detune an antenna by lowering its resonant frequency. The effect is not as serious as placing an antenna next to metal objects and can be corrected by reducing the length of the antenna. Therefore it is important for the antenna to be tuned when it is in the product. This is done during the development of the product.
Network Analyser 7 Network Analyser A Vector Network Analyser (VNA) is used to perform the initial tuning of the antenna: The PCB track (trace), just before the antenna matching network is cut to isolate the filter and previous stages from the measurement. 2. A coaxial cable is connected between the VNA and the PCB of the product. The outer conductor of the coaxial cable is soldered to the ground plane of the PCB as close as possible to the input of the antenna matching network.
Final Tuning 8 Final Tuning After tuning the antenna using the VNA procedure, it is necessary to perform fine tuning. This will yield a small improvement and will be the final optimisation of the antenna. It is best to perform this procedure in an anechoic chamber, but when this is not possible an indoor or outdoor test range can be used. It is important to minimise radio signal reflections. Avoid metallic objects such as lab-benches, filing cabinets, lampposts and cars.
Final Tuning Connect omni directional receive antenna to a spectrum analyser Place fully assembled Bluetooth product approximately 2m away from receive antenna Put Bluetooth product into continuous transmit BlueCore™ Inverted-F and Meander Line Antennas Watch power level of received signal on the specturm analyser while moving receive antenna ±10cm in each of x,y,z planes to ensure it is not located in a null point.
Conclusion 9 Conclusion Metal objects should be kept at least 15.25mm away from the Inverted-F and Meander Line types of antennas in the Bluetooth frequency band in order for the antenna to work efficiently. If that is not possible, then extra experimentation is required to determine an acceptable trade-off between antenna performance and product size. bcant-an-001Pa © Copyright CSR 2003 This material is subject to CSR’s non-disclosure agreement.
Acronyms and Definitions Acronyms and Definitions Group term for CSR’s range of Bluetooth chips Bluetooth Set of technologies providing audio and data transfer over short-range radio connections CSR Cambridge Silicon Radio PCB Printed Circuit Board RF Radio Frequency VNA Vector Network Analyser bcant-an-001Pa © Copyright CSR 2003 This material is subject to CSR’s non-disclosure agreement.
Record of Changes Record of Changes Date: Revision 24 JAN 03 a Reason for Change: Original publication of this document. (CSR reference bcant-an-001Pa). Inverted-F and Meander Line Antennas Application Note bcant-an-001Pa January 2003 Bluetooth™ 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 their respective owners.
