Design of a broadband antenna element for LTE base station antennas Master of Science Thesis MARIE STRÖM Department of Signals and Systems Division of Signal Processing and Antennas CHALMERS UNIVERSITY OF TECHNOLOGY Göteborg, Sweden, 2009 Report No. EX059/2009 Ericsson Public TECHNICAL REPORT 1 (60) Prepared (also subject responsible if other) No. EAB/FJN/AA Marie Ström EAB/FJN-09:0218 Uen Approved Checked Date Rev Reference EAB/FJN/A [Stefan Johansson] 2009-07-02 A Design of a broadband antenna element for LTE base station antennas Master of Science Thesis at the Department of Signals and Systems Chalmers University of Technology Conducted at Ericsson AB, FJN/AA, 2009 Marie Ström Ericsson Public TECHNICAL REPORT 2 (60) Prepared (also subject responsible if other) No. EAB/FJN/AA Marie Ström EAB/FJN-09:0218 Uen Approved Checked Date Rev Reference EAB/FJN/A [Stefan Johansson] 2009-07-02 A Design of a broadband antenna element for LTE base station antennas MARIE L. C. STRÖM, 2009 Department of Signals and Systems Chalmers University of Technology SE-412 96 Göteborg Sweden Telephone + 46 (0)31-772 1000 Ericsson Public TECHNICAL REPORT 3 (60) Prepared (also subject responsible if other) No. EAB/FJN/AA Marie Ström EAB/FJN-09:0218 Uen Approved Checked Date Rev Reference EAB/FJN/A [Stefan Johansson] 2009-07-02 A Abstract This report proposes a dual polarized antenna element fed by a Marchand balun, which operates in the 1710-2690 MHz frequency band. This band covers the high band frequencies for the new Long Term Evolution (LTE) system and the current mobile communication systems, 2G and 3G. The complete design of a printed dipole and a bowtie antenna is documented in a comprehensively manner. The analysis of the antennas was performed numerically using the commercial Finite Element Method (FEM) software, Ansoft High Frequency Structure Simulator (HFSS). The optimization of the Marchand balun was performed in Agilent Advance Design System (ADS). The proposed printed dipole antenna with a parasitic element, possesses an operating bandwidth of at least 47%, a +/- 45 degree linear polarization and desirable radiation pattern. A prototype of a designed antenna was built and experimentally validated, producing results that agree with the simulation result. Keywords: broadband antenna, dipole, Marchand balun, dual polarization Ericsson Public TECHNICAL REPORT 4 (60) Prepared (also subject responsible if other) No. EAB/FJN/AA Marie Ström EAB/FJN-09:0218 Uen Approved Checked Date Rev Reference EAB/FJN/A [Stefan Johansson] 2009-07-02 A Acknowledgements The thesis work has taken place at Ericsson AB, at the department FJN/AA. I would like to express my gratitude to all of the employees at FJN/AA; Anders Ek, Bo Granstam, Henrik Hallenberg, Ola Kaspersson, Håkan Karlsson, Ingolf Larsson and Lars Torstensson for their assistance in technical issues and mainly for making me feel welcome. Thanks to Anders Stjernman and Andreas Nilsson, for assistance with the simulation program HFSS. Foremost, I owe thanks to my supervisor, Henrik Jidhage, for making sure that I was on the right track and answering all of my questions. Also, to the head of department, Stefan Johansson, for giving me this opportunity and making me part of the group. Finally, thanks to my examiner, Professor Per-Simon Kildahl at the Department of Signals and Systems, Chalmers, for taking time to help me both with technical and administrative issues. Ericsson Public TECHNICAL REPORT 5 (60) Prepared (also subject responsible if other) No. EAB/FJN/AA Marie Ström EAB/FJN-09:0218 Uen Approved Checked Date Rev Reference EAB/FJN/A [Stefan Johansson] 2009-07-02 A Abbreviations ADS Agilent Advance Design System CPS CoPlanar Stripline FEM Finite Element Method HFSS Ansoft High Frequency Structure Simulator HGP Height to Ground Plane LTE Long Term Evolution QN Quasi Newton SNLP Sequential Non-Linear Programming Ericsson Public TECHNICAL REPORT 6 (60) Prepared (also subject responsible if other) No. EAB/FJN/AA Marie Ström EAB/FJN-09:0218 Uen Approved Checked Date Rev Reference EAB/FJN/A [Stefan Johansson] 2009-07-02 A Contents 1 Introduction .......................................................................................... 7 1.1 Purpose .................................................................................... 7 1.2 Limitations ............................................................................... 8 2 Technical requirements and overview ................................................ 9 2.1 Impedance ............................................................................... 9 2.2 Frequency band ....................................................................... 9 2.3 Return loss ............................................................................. 10 2.4 Horizontal 3dB beamwidth ...................................................... 10 2.5 Polarization ............................................................................ 10 2.6 Mechanical dimensions .......................................................... 10 3 Literature study .................................................................................. 11 3.1 Dipole concepts ...................................................................... 12 3.2 Microstrip antennas ................................................................ 13 3.3 Concept selection ................................................................... 13 4 Theory ................................................................................................. 15 4.1 Dipoles ................................................................................... 15 4.1.1 Broadband dipoles ................................................................. 16 4.2 Balun design .......................................................................... 16 4.2.1 Marchand balun ...................................................................... 16 4.3 Impedance matching .............................................................. 17 4.3.1 Quarter-wave transformer ...................................................... 18 4.3.2 Stub-matching ........................................................................ 18 4.4 Polarization ............................................................................ 18 5 Antenna structure and simulation result .......................................... 19 5.1 Balun design .......................................................................... 20 5.1.1 Design procedure for the Marchand balun .............................. 21 5.2 Dipole concept investigation ................................................... 22 5.2.1 Printed dipole ......................................................................... 23 5.2.2 Bowtie antenna....................................................................... 25 5.2.3 Printed dipole with a parasitic element ................................... 28 5.2.4 Bowtie with a parasitic element .............................................. 30 5.2.5 Bowtie with a parasitic element and a tuning slot .................... 32 5.2.6 Summary ................................................................................ 34 5.3 Detailed analysis for the bowtie antenna ................................ 35 5.4 Detailed analysis for the printed dipole ................................... 39 5.4.1 Removing the parasitic element ............................................. 44 6 Prototype manufacturing and measurements ................................. 45 6.1 Comparison between measurements and simulations ............ 47 7 Discussion .......................................................................................... 49 8 Conclusions ....................................................................................... 50 9 Proposed future work ........................................................................ 51 10 References ......................................................................................... 52 Ericsson Public TECHNICAL REPORT 7 (60) Prepared (also subject responsible if other) No. EAB/FJN/AA Marie Ström EAB/FJN-09:0218 Uen Approved Checked Date Rev Reference EAB/FJN/A [Stefan Johansson] 2009-07-02 A 1 Introduction This report examines the possibility of developing a broadband antenna element for base station antenna applications, which operates in the 1710- 2690 MHz frequency band. This band covers the high band frequencies for the current 2G and 3G system, as well as the requirements for the coming LTE system. An important factor considered is the mechanical dimensions; it is important that the antenna can fit in a base station antenna array. To reduce cost and space, the antenna should be dual polarized to support polarization diversity instead of space diversity. Other important characteristics for the antenna are low return loss and proper radiation pattern. The investigation aims to evaluate solutions for designing a broadband antenna, with an emphasis on accomplishing dual polarization in combination with a desirable return loss. Different antenna solutions are investigated, to establish which accomplishes the broadband characteristics and the other specific requirements (described in section 2). In section 3, a short introduction to different antenna concepts, including the findings from the literature study on investigated antennas, is presented. A brief theory behind dipoles, baluns and matching techniques relevant for this project can be found in section 4. Section 5 shows the complete design for a printed dipole and a bowtie antenna in the simulation program HFSS. The design process is described in a comprehensive manner from the basic structure to the final version of the antenna. The results, including impedance matching, radiation properties, beamwidth and polarization are included in the design process. Section 6 discusses the manufacturing of a prototype for a designed antenna. Simulations carried out are presented and compared with the results of physical testing of the antenna. This is followed by a discussion, conclusions and suggestions for future improvements. 1.1 Purpose Nowadays, the market demand for wireless communication systems is growing rapidly. The development of mobile broadband systems requires higher data rate to satisfy the customer demand. In order to meet these demands, the new LTE system has started to be introduced to the market. The LTE system will provide a higher data rate, lower delay, improved coverage and spectral efficiency compared to the 3G system. The current mobile communication systems, 2G and 3G, are already fully established on the market. When implementing the new LTE system, it is necessary to ensure compatibility between it and the existing systems, as the LTE technique is introduced successively. One way for the systems to co-exist could be to mount an extra LTE antenna on the site. However, this can be difficult due to the limited site space and increased site rent. Another solution is to develop a broadband antenna that makes it possible for the systems to share the same antenna. The possibility for such a solution is examined in this report. Another positive factor is a reduction in the number of possible product variants. Ericsson Public TECHNICAL REPORT 8 (60) Prepared (also subject responsible if other) No. EAB/FJN/AA Marie Ström EAB/FJN-09:0218 Uen Approved Checked Date Rev Reference EAB/FJN/A [Stefan Johansson] 2009-07-02 A 1.2 Limitations For an antenna to function properly, the radiation properties for both the co- polarization and cross-polarization need to be evaluated. The cross- polarization is always undesirable and should be much lower than the maximum gain of the co-polarization. Back radiation is also undesirable and should be as low as possible. In this report, only a small amount of attention is given to analyze the cross-polarization and back radiation. Ericsson Public TECHNICAL REPORT 9 (60) Prepared (also subject responsible if other) No. EAB/FJN/AA Marie Ström EAB/FJN-09:0218 Uen Approved Checked Date Rev Reference EAB/FJN/A [Stefan Johansson] 2009-07-02 A 2 Technical requirements and overview The requirements for a base station antenna element operating in the LTE and 3G systems can be seen in Table 1. The limitations for the parameters are set so that the antenna element can operate in an antenna array. Parameter Requirement Impedance 50 Ω Frequency band 1710 - 2690 MHz Return loss >15 dB Horizontal 3dB beamwidth 60 – 70° Polarization +/- 45° Mechanical dimensions − Element separation <105 mm − Element height <50 mm Table 1: Project requirements. 2.1 Impedance The antenna will be connected to a 50Ω coaxial cable. It is therefore important for the antenna element to be matched to an input impedance of 50Ω. 2.2 Frequency band The frequency band 1710 to 2690 MHz includes the current 2G, 3G and the standard for the next generation of wireless mobile system, LTE. In this frequency range, the return loss criterion is 15dB. The broad frequency spectrum requires a high fractional bandwidth, calculated to 45 % with Eq. 1 ( f =2690, f =1710 and f = 2200 MHz). H L 0 f − f fb= H L Eq. 1 f 0