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Methods for Measuring Rf Radiation Properties of Small Antennas Helsinki University of Technology Radio Laboratory Publications Teknillisen korkeakoulun Radiolaboratorion julkaisuja Espoo, October, 2001 REPORT S 250 METHODS FOR MEASURING RF RADIATION PROPERTIES OF SMALL ANTENNAS Clemens Icheln Dissertation for the degree of Doctor of Science in Technology to be presented with due permission for public examination and debate in Auditorium S4 at Helsinki University of Technology (Espoo, Finland) on the 16th of November 2001 at 12 o'clock noon. Helsinki University of Technology Department of Electrical and Communications Engineering Radio Laboratory Teknillinen korkeakoulu Sähkö- ja tietoliikennetekniikan osasto Radiolaboratorio Distribution: Helsinki University of Technology Radio Laboratory P.O. Box 3000 FIN-02015 HUT Tel. +358-9-451 2252 Fax. +358-9-451 2152 © Clemens Icheln and Helsinki University of Technology Radio Laboratory ISBN 951-22- 5666-5 ISSN 1456-3835 Otamedia Oy Espoo 2001 2 Preface The work on which this thesis is based was carried out at the Institute of Digital Communications / Radio Laboratory of Helsinki University of Technology. It has been mainly funded by TEKES and the Academy of Finland. I also received financial support from the Wihuri foundation, from HPY:n Tutkimussäätiö, from Tekniikan Edistämissäätiö, and from Nordisk Forskerutdanningsakademi. I would like to thank all these institutions, and the Radio Laboratory, for making this work possible for me. I am especially thankful for the many ideas and helpful guidance that I got from my supervisor professor Pertti Vainikainen during all my work. Of course I would also like to thank the staff of the Radio Laboratory, who provided a pleasant and inspiring working atmosphere, in which I got lots of help from my colleagues - let me mention especially Stina, Tommi, Jani, Lorenz, Eino and Lauri - without whom this work would not have been possible. And finally I want to say thanks for the encouragement I got from my parents during the last years, and for the patience my beloved Anna-Maj showed during all this time. Espoo, October 11, 2001, (Clemens Icheln) 3 Abstract In this work significant improvements for measurements of the radio-frequency (RF) radiation properties of small antennas have been proposed and investigated. The main focus is on electrically small antennas as used in mobile communications systems. The methods proposed and evaluated in this thesis allow the minimisation of the dimensions of measurement chambers, and the methods also allow pattern measurements with a minimised error from the RF feed cable and thus lower measurement uncertainty. The first two parts of the work relate to measurements performed in especially small chambers. The aim is to provide an alternative measurement environment to large, fully anechoic chambers in the special case of small antenna calibrations. The use of small chambers such as GTEM cells and small anechoic chambers is proposed. Both options have been constructed and investigated by both simulations and measurements. The results show that a GTEM cell allows the reliable measurement of the radiation pattern and 3-dB bandwidth of small antennas with a low directivity and a dynamic range of less than 20 dB. A small anechoic chamber with the largest dimension of 2.5 m was built during the work for this thesis. The results obtained when measuring the 3-D radiation pattern, efficiency and the gain of a small handset antenna in that small anechoic chamber show further that far-field measurements in such a small anechoic chamber does not result in greater measurement uncertainty than results obtained in conventional large anechoic chambers. Finally, the influence of the RF feed cables on the radiation characteristics of a small antenna under test has been reduced by a novel method. This method is based on a multi-frequency balun that efficiently suppresses the propagation of leakage and parasitic currents on the shielding of the RF feed cable. The effect of the balun has been thoroughly investigated by means of computer simulations and measurements with a prototype. Both the far field and the near field have been analysed to yield a comprehensive set of figures of merit, showing that the presented balun decreases the measurement uncertainty much better than other commonly used measures against cable effects, such as ferrite beads. The balun cannot only be used in radiation-pattern measurements in large or small anechoic chambers, but also in radio-channel measurements, in near-field scans and in Standard Absorption Rate (SAR) measurements. 4 TABLE OF CONTENTS PREFACE ...............................................................................................3 ABSTRACT .............................................................................................4 TABLE OF CONTENTS .............................................................................5 LIST OF SYMBOLS .................................................................................7 LIST OF ABBREVIATIONS .........................................................................9 1. INTRODUCTION .................................................................................... 10 2. THE SITUATION OF SMALL-ANTENNA MEASUREMENTS TODAY .......... 12 3. GTEM CELL AND SMALL ANTENNA MEASUREMENTS ........................ 17 3.1. THE GTEM-CELL.............................................................................................17 3.2. MEASUREMENTS OF FIELD PATTERNS OF MODERATELY DIRECTIVE NARROW- BAND PATCH ANTENNAS ...............................................................................................18 3.3. MEASUREMENTS OF ANTENNAS WITH SPECIAL CHARACTERISTICS IN THEIR RADIATION PATTERN.....................................................................................................21 3.4. FIELD UNIFORMITY IN THE TESTING VOLUME AT FREQUENCIES ABOVE 1 GHZ.22 3.5. ANTENNA EFFICIENCY MEASUREMENTS IN A GTEM CELL...............................25 4. SMALL ANECHOIC CHAMBERS AND THE CHARACTERISATION OF SMALL ANTENNAS AND MOBILE HANDSETS ................................................ 26 4.1. EFFECT OF THE SMALL DISTANCE BETWEEN THE ANTENNAS ............................26 4.2. EFFECT OF RF ABSORBERS IN THE VICINITY OF SMALL ANTENNAS...................32 4.2.1. Transmission coefficient measurements....................................................... 33 4.2.2. EM-field computer simulations.................................................................... 34 4.3. DESIGN, CONSTRUCTION, AND PERFORMANCE OF A SMALL SHIELDED ANECHOIC CHAMBER.....................................................................................................38 4.3.1. Field homogeneity inside the small anechoic chamber ................................ 40 4.3.2. Antenna measurements in the small anechoic chamber ............................... 44 5. QUARTER-WAVE CAP: A MEANS FOR DECREASING THE INFLUENCE OF RF CABLES............................................................................................. 50 5.1. SINGLE-BAND BALUN .......................................................................................50 5.1.1. Computer simulations................................................................................... 52 5.1.2. Optimisation ................................................................................................. 54 5.1.3. Design and application ................................................................................. 58 5.2. DUAL-BAND BALUN .........................................................................................60 5.2.1. Dual-band balun, design 1............................................................................ 60 5.2.2. Dual-band balun, design 2............................................................................ 61 5 5.3. COMPUTER SIMULATIONS.................................................................................64 5.4. MEASUREMENT SET-UPS ..................................................................................66 5.4.1. Input impedance measurements.................................................................... 67 5.4.2. 3-D pattern measurements............................................................................ 67 5.4.3. H-field scanning ........................................................................................... 70 5.4.4. Investigated configurations........................................................................... 71 5.5. MEASUREMENTS RESULTS................................................................................72 5.5.1. 3-D field pattern measurement results.......................................................... 72 5.5.2. H-field measurement results......................................................................... 80 5.5.3. Bandwidth of the balun................................................................................. 83 6. CONCLUSIONS ...................................................................................... 88 REFERENCES .........................................................................................90 6 List of Symbols A aperture C capacitance c0 speed of light in vacuum d, D distance, width dFF distance for far-field conditions E electric field strength Efar electric field strength in far field Er radial component of electric field strength Eθ theta-component of electric field strength Eφ phi-component of electric field strength erel relative error f frequency fres resonance frequency h, H height H magnetic field strength G antenna
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