Electromagnetic Near-field Characterization & Occupational Exposure to RF Waves in Industrial Environment Kassem Jomaa
To cite this version:
Kassem Jomaa. Electromagnetic Near-field Characterization & Occupational Exposure to RF Waves in Industrial Environment. Optics / Photonic. Université Grenoble Alpes, 2018. English. NNT : 2018GREAT111. tel-02146578
HAL Id: tel-02146578 https://tel.archives-ouvertes.fr/tel-02146578 Submitted on 4 Jun 2019
HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés.
THÈSE Pour obtenir le grade de DOCTEUR DE LA COMMUNAUTE UNIVERSITE GRENOBLE ALPES Spécialité : OPTIQUE ET RADIOFREQUENCES Arrêté ministériel : 25 mai 2016
Présentée par « Kassem JOMAA »
Thèse dirigée par « Jalal JOMAAH », et codirigée par « Fabien NDAGIJIMANA »
Préparée au sein du Laboratoire IMEP-LaHC de l’Institut National Polytechnique de Grenoble, dans l'École Doctorale Electronique, Electrotechnique, Automatique, Traitement du Signal (EEATS)
Caractérisation du Champ Proche Electromagnétique et Exposition Professionnelle aux Ondes RF en Milieu Industriel
Electromagnetic Near-field Characterization & Occupational Exposure to RF Waves in Industrial Environment
Thèse soutenue publiquement le « 14 Décembre 2018 », devant le jury composé de :
Monsieur Joe WIART Professeur à Telecom Paris Tech, Président du jury Monsieur Philippe LEVEQUE Professeur à l’Université de Limoges, XLIM, Rapporteur Monsieur Jean-Lou DUBARD Professeur à l’Université de Nice-Sophia Antipolis, LEAT, Rapporteur Monsieur Philippe DESCAMPS Professeur des Universités, Rouen, IRSEEM- ESIGELEC, Examinateur Monsieur Jalal JOMAAH Professeur à Grenoble-INP, Directeur de thèse Monsieur Fabien NDAGIJIMANA Professeur à l’Université Grenoble-Alpes, Co-directeur de thèse
Acknowledgements
First, foremost, and forever, thanks God Almighty. Thanks for blessing me much more than I deserve, thanks for everything.
The work presented in this manuscript have been carried out at IMEP-LaHC laboratory in collaboration with Pheline, a test lab that is specialized in the study of the electromagnetic fields phenomenon and its interactions with the environment, at CSTB – Grenoble. I would like to express my sincere gratitude to my supervisors, Prof. Fabien Ndagijimana and Prof. Jalal Jomaah. They have been true mentors during my doctoral study. This thesis was not to be possible without their invaluable support, guidelines and help, as well as their great advices, and unfailing commitment in my research project and beyond.
I would like to take this opportunity to thank Prof. Joe Wiart for chairing the defense jury. I would also like to sincerely thank Prof. Philippe Descamps for examining this thesis, as well as Prof. Philippe Leveque and Prof. Jean-Lou Dubard for their much appreciated involvements in my thesis review and defense.
Moreover, I would like also to express my thanks to the engineers and technicians, at IMEP-LaHC and CSTB, who offered me a strong support during this journey, as well as the administrative staff for their help in all administrative procedures.
I am also thankful for all my colleagues and friends at IMEP-LaHC as well as CSTB, for their support and their good humor. We had a lot of great discussions and we shared beautiful moments.
Finally, I would like to express a deep sense of gratitude to my parents, my sister, and my brother. You helped and supported me a lot all along my studies. I would like also to take the opportunity to express my love to my spouse, Fatima, who supported me the most, during the good and the bad days.
Grenoble, December 2018 Kassem
i
ii
Table of Contents
Acknowledgements ...... i Table of Contents ...... iii List of Figures ...... v List of Tables ...... x List of Abbreviations ...... xi General Introduction ...... 1 Chapter 1: Literature Review ...... 5 1.1. Introduction ...... 6 1.2. Electromagnetic Fields ...... 7 1.3. Electromagnetic Compatibility ...... 12 1.4. Interaction between EM fields and matter ...... 16 1.5. Radiofrequency Dosimetry ...... 18 1.6. State of the art in measurements of EM fields ...... 22 1.7. Standards and Guidelines ...... 24 1.8. Conclusion ...... 26 1.9. References ...... 27 Chapter 2: Design & Characterization of Tri-Axial Magnetic Field Probes 31 2.1. Introduction ...... 32 2.2. Electromagnetic field probes ...... 32 2.3. EM-field probes parameters ...... 35 2.4. Development of the 3D Near-field Scanning System ...... 37 2.5. Measurement Chain ...... 40 2.6. EM-field Probes Calibration techniques ...... 43 2.7. Single Magnetic field Probes ...... 49 2.8. Development of Tri-axial Magnetic field Probes ...... 62 2.9. Conclusion ...... 70 2.10. References ...... 71 Chapter 3: Radiated EMF Prediction Method Based on PWS Algorithm ... 73 3.1. Introduction ...... 74 3.2. Electromagnetic wave propagation ...... 75 3.3. Principle of the Plane Wave Spectrum Algorithm ...... 76 3.4. Algorithm Implementation ...... 80 3.5. Validation of the Algorithm ...... 83 3.6. Conclusion ...... 89
iii
3.7. References ...... 90 Chapter 4: RFID Near-field Characterization ...... 91 4.1. Introduction ...... 92 4.2. RFID Systems ...... 93 4.3. RFID Reader Antennas ...... 98 4.4. In Situ Measurements ...... 100 4.5. In-Lab Experimental Analysis ...... 106 4.6. Conclusion ...... 123 4.7. References ...... 124 Chapter 5: Industrial RF Welding Machines EMF Characterization ...... 127 5.1. Introduction ...... 128 5.2. Industrial RF Welding Machines ...... 129 5.3. Numerical Analysis ...... 133 5.4. Experimental Analysis ...... 139 5.5. Conclusion ...... 149 5.6. References ...... 150 Conclusion and Perspectives ...... 151 Appendix ...... 155 List of Publications ...... 161 Abstract ...... 163 Résume ...... 164
iv
List of Figures
Figure 1 -1: Representation of an electromagnetic wave. The electric field, in blue, and the magnetic field, in red, propagating rectilinearly and perpendicularly to each other ...... 7 Figure 1 -2: The Electromagnetic Spectrum ...... 8 Figure 1 -3: Pulsed wave signal ...... 9 Figure 1 -4: Electromagnetic field regions around a radiating source ...... 10
Figure 1 -5: Wave impedance of magnetic (ZH) and electric (ZE) dipoles as a function of k×r ...... 11 Figure 1 -6: General EMC model ...... 13 Figure 1 -7: Near-field test bench with scanning system ...... 14 Figure 1 -8: Near field system with array of probes ...... 15 Figure 1 -9: Electromagnetic incident wave decomposition ...... 16 Figure 1 -10: Examples of electric and magnetic field probes for exposure measurements ...... 22 Figure 1 -11: Narda electromagnetic field meters samples ...... 23 Figure 2 -1: Electromagnetic wave and the corresponding sensitive sensors ...... 33 Figure 2 -2: Small dipole as an example of E-field probe ...... 33 Figure 2 -3: Electric field probes ...... 34 Figure 2 -4: Magnetic field probes ...... 35 Figure 2 -5: Mechanism of the robot scanning in the plane above the DUT ...... 38 Figure 2 -6: Scanning configuration window ...... 39 Figure 2 -7: 2D Scanner for large devices ...... 40 Figure 2 -8: Measuring Near-field of passive devices via VNA ...... 41 Figure 2 -9: Measuring Near-field of active devices via SA ...... 41 Figure 2 -10: Measurement system setup ...... 42 Figure 2 -11: Typical TEM cell ...... 44 Figure 2 -12: Typical probe calibration setup using TEM cell ...... 46 Figure 2 -13: Simulation model of the used TEM cell...... 46 Figure 2 -14: (a) Transversal and (b) Lonigtuidinal cross-sections of the TEM cell ...... 47 Figure 2 -15: (a) Transversal and (b) Lonigtuidinal cross-sections of the TEM cell ...... 47 Figure 2 -16: Commercial magnetic close-field probes from Agilent ...... 48
v
Figure 2 -17: HP1141A magnetic field probe antenna factor ...... 48 Figure 2 -18: HP1140A magnetic field probe antenna factor ...... 49
Figure 2 -19: Geometrical parameters of the magnetic field probes; P1xy (left) P2xy (right) ...... 50
Figure 2 -20: magnetic field probes; P1z (left) P2z (right) ...... 50 Figure 2 -21: Simplified equivalent circuit model of a loop ...... 51 Figure 2 -22: Calibration Setup ...... 52 Figure 2 -23: Antenna factor of P1xy probe ...... 53 Figure 2 -24: Antenna factor of P2xy probe ...... 53 Figure 2 -25: Probe selectivity measurement configuration ...... 54
Figure 2 -26: P1xy Probe selectivity ...... 55
Figure 2 -27: P2xy probe selectivity ...... 56 Figure 2 -28: (a) Reference Circuit; (b) Cross section, showing the geometrical parameters ...... 57
Figure 2 -29: Validation setup using P1xy probe ...... 58
Figure 2 -30: Theoretical values compared to measurement results of Hy using P1xy at different frequencies ...... 59
Figure 2 -31: Theoretical values compared to measurement results of Hz using P1z at different frequencies ...... 59
Figure 2 -32: Theoretical values compared to measurement results of Hy using P2xy at different frequencies ...... 60
Figure 2 -33: Theoretical values compared to measurement results of Hz using P2z at different frequencies ...... 61 Figure 2 -34: Fabricated 3D H-field probe P1 ...... 62 Figure 2 -35: Fabricated 3D H-field probe P2 ...... 63 Figure 2 -36: Coupling between P1 loop elements ...... 64 Figure 2 -37: Coupling between P2 loop elements ...... 64 Figure 2 -38: Measured antenna factor versus frequency for the probe P1 ...... 65 Figure 2 -39: Measured antenna factor versus frequency for the probe P2 ...... 65 Figure 2 -40: Standard validation circuit with the tri-axial probe P1 ...... 67
Figure 2 -41: Tangential H-field component Hy for (a) Probe P1 (b) Probe P2 ...... 68
Figure 2 -42: Normal H-field component Hz at for (a) Probe P1 (b) Probe P2 ...... 69 Figure 3 -1: PWS method for magnetic field extraction ...... 79
Figure 3 -2: Spatial domain parameters representing the plane P0 at z=z0 ...... 80 Figure 3 -3: Single turn rectangular loop antenna with the corresponding radiation pattern diagram ...... 83 Figure 3 -4: RFID Loop Antenna structure ...... 84
vi
Figure 3 -5: Comparison between theoretical, simulated, and calculated values of the Hz magnetic field component at P0 (units of x & y axis is m) ...... 85 Figure 3 -6: Comparison between theoretical, simulated, and calculated magnetic field profiles at z=5cm above the DUT (units: colorbar in A/m, x & y axis in m) ...... 86 Figure 3 -7: Comparison between theoretical, simulated, and calculated magnetic field profiles at z=10cm above the DUT (units: colorbar in A/m, x & y axis in m) ...... 87 Figure 3 -8: PIFA antenna design structure ...... 87 Figure 3 -9: Simulation and calculated values of the Hz magnetic field component at P1 above the PIFA antenna (colorbar unit: A/m) ...... 88 Figure 3 -10: Simulation and calculated values of the magnetic field components at P2 above the PIFA antenna (colorbar unit: A/m) ...... 89 Figure 4 -1: RFID self-check system used in libraries ...... 92 Figure 4 -2: Main blocks of an RFID system ...... 93 Figure 4 -3: ZK-RFID406 RFID reader (a), ASR-R250G handheld RFID reader (b), ...... 95 Figure 4 -4: EAS RFID tags ...... 97 Figure 4 -5: RFID tags, for HF bands (a) and UHF bands (b) ...... 98 Figure 4 -6: 3D radiation pattern of a small loop antenna (plane of the loop is xy plane) [11] ...... 99 Figure 4 -7: EHP-200A electric and magnetic field probe ...... 101 Figure 4 -8: Measurement setup using EHP-200A probe ...... 101 Figure 4 -9: Mini RFID pad (model 210) with the presence of the EHP-200A probe ...... 102 Figure 4 -10: 3M RFID pad (model P12) with the presence of the EHP-200A probe ...... 103 Figure 4 -11: 3M self-check system (model 8420) ...... 104 Figure 4 -12: AMH Library RFID system. Area dedicated for users (left), Area dedicated for workers (right) ...... 105 Figure 4 -13: Impedance matching circuit with the loop antenna equivalent circuit ...... 107 Figure 4 -14: Simulated and measured reflection coefficient of the antenna ...... 108 Figure 4 -15: Fabricated 1-turn RFID loop antenna (top) with the HFSS model (bottom)...... 108 Figure 4 -16: Near-field mapping system with the designed RFID reader antenna ...... 109 Figure 4 -17: Magnetic field components above the RFID reader antenna measured with probe P1...... 110
vii
Figure 4 -18: Magnetic field components above the RFID reader antenna measured with probe P2 ...... 110 Figure 4 -19: Fabricated 2-turns RFID loop antenna ...... 111 Figure 4 -20: Pulsed modulated RFID reader signal ...... 112 Figure 4 -21: Magnetic field strength above the RFID reader antenna with CW signal, ...... 113 Figure 4 -22: Magnetic field strength above the RFID reader antenna with modulated signal, ...... 115 Figure 4 -23: Measurement setup for H-field characterization above 3M RFID pad P12 ...... 116 Figure 4 -24: 3M RFID reader (model 810) signals: without any tag near the antenna (blue signal) ...... 117 Figure 4 -25: Magnetic field components measured with P1 above the RFID pad P12 along y-axis ...... 118 Figure 4 -26: Magnetic field components measured with P2 above the RFID pad P12 along y-axis ...... 119 Figure 4 -27: 2D cartography of the H-field components above RFID pad P12 ...... 120
Figure 4 -28: Total H-field mapping above the RFID pad P12 at h1 & h2 ...... 120 Figure 4 -29: Measurement setup in the presence of the cylindrical phantom with TSL ...... 121 Figure 4 -30: Electrical properties of the TSL used to fill the cylindrical phantom ...... 121 Figure 4 -31: H-field components measured in a vertical plane above the RFID pad ...... 122 Figure 4 -32: Total H-field measured above the RFID pad ...... 122 Figure 5 -1: Schematic of an RF welding machine ...... 130 Figure 5 -2: RF welding machine equivalent electrical circuit ...... 130 Figure 5 -3: Small RF welding machine: with no shielding (a), with shielding (b) ...... 131 Figure 5 -4: RF welding machine with the ground plate [] ...... 132 Figure 5 -5: Duke Phantom in front of the electrode model of the welding ...... 134 Figure 5 -6: E-field distribution for different models (defined in Table 5-3) at 27.12 MHz ...... 135 Figure 5 -7: E-field vectors on the surface of the body ...... 135 Figure 5 -8: Cumulative distribution function of the electric field for the 5 different models ...... 136 Figure 5 -9: H-field distribution for different models (defined in Table 5-3) at 27.12 MH ...... 136 Figure 5 -10: Continuity of the magnetic field at 27.12 MHz ...... 137
viii
Figure 5 -11: Cumulative distribution function of the magnetic field for the 5 different models ...... 137 Figure 5 -12: RF welding machine HFSS model ...... 138 Figure 5 -13: Spectrum of the radiated signal in front of the RF welding machine, measured using EHP200A probe. The machine with the ground plate (left) and without the ground plate (right) ...... 140 Figure 5 -14: Exposure index calculated from the measurements made without ground plate (RFWM = RF welding machine position, CP = control panel position) ...... 141 Figure 5 -15: Exposure index calculated from the measurements made with ground plate (RFWM = RF welding machine position, CP = control panel position) ...... 141 Figure 5 -16: Measurement setup for H-field measurements in front of the RF welding machine using P2 probe ...... 142 Figure 5 -17: Example of the recorded waveforms for each measured point ...... 143 Figure 5 -18: Measurement planes configurations in front of the RF welding machine ...... 144 Figure 5 -19: Magnetic field plots in the planes in front of the welding machine. Measurements (top row) and simulations (bottom row) ...... 145 Figure 5 -20: Measurement planes configurations in front of the RF welding machine in the presence of the cylindrical phantom ...... 146 Figure 5 -21: Magnetic field plots in the planes in front of the welding machine in the presence of the cylindrical phantom. Measurements (top row) and simulations (bottom row) ...... 147 Figure 5 -22: Electro-optical probe used for the E-field measurements [11] ...... 147 Figure 5 -23: E-field eoProbe measurement configuration ...... 148 Figure 5 -24: Electric field plots in the planes in front of the RF welding machine ...... 149
ix
List of Tables
Table 1 -1: Occupational and general public exposure reference levels [8] ...... 24 Table 1 -2: ICNIRP Recommended ELVs [8] ...... 25 Table 2 -1: Oscilloscope technical specifications [10] ...... 42 Table 2 -2: FCC-TEM-JM1 TEM cell specifications ...... 44 Table 2 -3: Conventional magnetic probe and PCB probe main parameters ...... 50 Table 2 -4: Distance between the probes and the reference circuit ...... 58 Table 4 -1: Measured RMS field values near the mini RFID Pads, compared to Action Levels ...... 103 Table 4 -2: Measured RMS field values near the 3M RFID pad (model P12), compared to Action Levels ...... 104 Table 4 -3: Measured RMS field values near the 3M self-check system (model 8420), compared to Action Levels ...... 105 Table 4 -4: RMS measured values near the AMH RFID system, compared to Action Levels ...... 106 Table 4 -5: Recommended Exposure Limit Values at 13.56 MHz [20] ...... 113 Table 4 -6: Antenna factor of P1 and P2 at 13.56 MHz ...... 117 Table 5 -1: Gooseneck welding machine specifications ...... 132 Table 5 -2: Action levels at 27.12 MHz ...... 133 Table 5 -3: Different human models material configurations (units: length in mm, permittivity (ɛ) in F/m, and conductivity (ζ) sigma in S/m) ...... 134
x
List of Abbreviations
RF: radiofrequency EMF: electromagnetic field RFID: radiofrequency identification PCB: printed circuit board NFS: near field scan PWS: plane wave spectrum PIFA: planar inverted-F antenna PW: pulse width PRT: pulse repetition period AF: antenna factor HF: high frequency UHF: ultra-high frequency LF: low frequency EMC: electromagnetic compatibility SAR: specific absorption rate CW: continuous wave IRs: ionized radiations NRIs: non-ionized radiations AL: action levels EVL: exposure limit values TSL: tissue simulating liquid DUT: device under test AUT: antenna under test RMS: root mean square EAS: electronic article surveillance SMD: surface mounted device SA: spectrum analyzer VNA: vector network analyzer RBW: resolution bandwidth LNA: low noise amplifier USB: universal serial bus
xi
GPIB: general purpose interface bus HFSS: high frequency structure simulator FEM: finite element method MoM: method of moments FDTD: finite difference time domain TLM: transmission line matrix FIT: finite integration technique TEM: transverse electromagnetic WHO: world health organization ICNIRP: international commission on non-ionizing radiation protection CENELEC: comité Européen de normalisation électronique IEEE: institute of electrical and electronics engineers CAD: computer aided design VBS: visual basic script AMH: automated material handling IDE: integrated development environment PVC: polyvinyl chloride MRI: magnetic resonance imaging
xii
General Introduction
Over the past few decades, the increasing usage of electromagnetic waves has spawned a proliferation of new applications creating a real revolution, particularly in areas related to wireless communications. Some of these applications have spread rapidly and massively, resulting in increasing exposure of people to electromagnetic waves. Among this rapid evolution several questions raises about their possible health effects, especially in the industrial environment where the workers are exposed daily and for a long period of time to electromagnetic waves. The proximity of the human body to the radiating source makes the tissues of the body a potential target for the electromagnetic energy emitted.
Different studies are done on the effects of radio frequencies on the brain or the head focusing on cell phones and mobile applications. Epidemiological studies have mainly focused on the incidence of brain tumors since the increase in the use of mobile telephony. On the other hand, few studies have examined the exposure to industrial systems that contribute to the electromagnetic phenomenon. In view of the general anxiety generated by the industrial sector, it is necessary to verify their safety in real conditions of use. At each site, safety zones should be defined in relation to the level of exposure and the access perimeters should be determined. Staffs working in the most exposed areas are thus strictly regulated. At the same time, the European directive 2013/35/EU, released in June 2013, defines the reference and action levels in terms of electric and magnetic fields applicable in the industrial sector, making the employers responsible of their applications.
It is in this perspective that this thesis work was done, as it focuses on the exposure assessments and characterization of the emitted electromagnetic fields, in particular in the industrial environment and provides the employers the necessary toolstounderstandworker‘sexposure. The analysis was performed mainly on two applications; the first was the characterization of electromagnetic fields near RFID systems especially those used in libraries, while the other application was the industrial RF welding machines. This analysis made it possible to evaluate the levels
General Introduction 1
of the electromagnetic field in the near-field region and to check if it meets the reference levels given in the international guidelines and directives. A special effort has been made to establish experimental conditions that are consistent with reality. This work was under two projects, expo@Lyon and EXTI, supported and funded by ANSES (French Agency for Food, Environmental and Occupational Health & Safety).
This manuscript is composed of five chapters. The first one is a reminder of electromagnetic fields in general where their main characteristics are recalled. It also addresses the possible interactions between electromagnetic fields and the matter, where the general parameters that make it possible to quantify this energy are presented. In addition of introducing the main international guidelines and European directives that specifies the exposure limit values and action levels. This chapter was followed by the description of several techniques for measuring electromagnetic fields in the near field region, as well as the electromagnetic field probes in general. Then the development of a new 3D scanning system for near-field measurements is presented. After that, and for high precision measurement purposes, a new tri-axial magnetic field probes has been designed. These probes measure the three components of the magnetic field simultaneously, that reduces the time of scanning by a factor of 3. Two different probes was designed, the first consists of 3 conventional loops that are combined in a plastic cube with a total dimension of 10×12×13 mm3 , whereas the second probe is a PCB probe printed on an FR4 substrate with a reduced dimension of 9×9×3.2 mm3. These probes were calibrated and the whole measurement system was validated.
The third chapter presents a reconstruction algorithm that requires just the tangential components of the magnetic field in an initial plane to reconstruct the 3D magnetic field distribution above the radiating device. In addition of the tri-axial probe presented in chapter 2 that reduce the scanning time, sometimes multiple scans is needed to build the 3D map of the radiated electromagnetic fields and this is time consuming especially if high resolution is needed. Hence the proposed algorithm solves this problem and reduces the scanning time to the minimum.
2 General Introduction
The fourth chapter is devoted for the near field characterization of RFID systems used in Libraries. In site measurements was carried out at different libraries in Lyon, using a commercial electromagnetic field probe. And for more detailed study, the analysis was done in the Lab using the fabricated probes near different antenna types including a commercial RFID loop antenna.
Finally, the exposure analysis near RF welding machines is presented in chapter five.ThiskindofmachinesgeneratesastrongRFenergywhileit‘sinoperation,as consequence workers are exposed to this energy. This work aims to characterize the field levels in the vicinity of these machines. The effect of a human body immersed in an RF electromagnetic field was studied by simulating a virtual phantom having different material layers. Moreover a numerical model of the machine was developed to compare the experimental data with simulations. Three measurement campaigns were done near a real machine that exists at the site of INRS, where the magnetic and electric fields were characterized.
At the end, the conclusion of the work done within this thesis is presented as well as some perspectives.
General Introduction 3
Chapter 1: Literature Review
Contents
Chapter 1: Literature Review ...... 5 1.1. Introduction ...... 6 1.2. Electromagnetic Fields ...... 7 1.2.1. Electromagnetic Waves ...... 7 1.2.2. Continuous and Pulsed Waves ...... 8 1.2.3. EM Wave Propagation Zones ...... 9 1.3. Electromagnetic Compatibility ...... 12 1.3.1. Near-field characterization ...... 14 1.4. Interaction between EM fields and matter ...... 16 1.5. Radiofrequency Dosimetry ...... 18 1.5.1. Quantification Parameters ...... 18 1.5.2. Numerical Dosimetry ...... 20 1.5.3. Experimental Dosimetry ...... 21 1.6. State of the art in measurements of EM fields ...... 22 1.7. Standards and Guidelines ...... 24 1.8. Conclusion ...... 26 1.9. References ...... 27
Literature Review 5
1.1. INTRODUCTION
Electromagnetic fields nowadays are found everywhere in our environment. They are used in different types of applications, starting from the electrical power lines and not ending with telecommunications and medical applications. The extremely rapid evolution of different technologies that radiate electromagnetic fields has led to an increase in the number of users exposed to these waves, and raises several questions about the possible effects from this exposure. Public concerns about the effects of electromagnetic waves emitted by the cell phone on human health are growing especially their long-term effects. In this context, numerous studies have already been carried out and others are in progress [1]–[3]. Various studies have been done on different biological tissues that are affected by the radiation, focusing more specifically on the human head tissues. The thermal effect depends on the amount of radiation absorbed by the human body. The respect of safety standards makes it possible to protect oneself from it. On the other hand, the non-thermal effects are not clearly established. In industrial environment, workers are exposed to electromagnetic fields generated from different kinds of applications such as: induction heating, welding, glue drying, sealing, electrolysis, etc. Workers are usually more close to the sources; hence they may be exposed to higher levels than the general public. Moreover, each equipment has its own characteristics, such as the type of the signal generated (continuous, modulated, pulsed…), that can affect the exposure analysis. Imperceptible in most work situations, electromagnetic fields can, beyond certain thresholds, have effects on the health. Overexposure effects varies from one source to another, for example radiations from high frequency applications may leads to severe burns while the induced currents from low frequency applications may affects the nervous system and the exposed person may experience vertigo and nausea. It is therefore important to recall a few notions in order to assess the risks associated with exposure to electromagnetic fields at the workplace and in the work environment. This assessment serves as a basis for the implementation of preventive measures to reduce occupational exposures. In the first part of this chapter, the main characteristics of the electromagnetic fields are recalled, with the possible interaction of EMFs with matter. In order to highlight the different elements of this interaction, the second part of this chapter will
6 Literature Review
be devoted to RF dosimetry reminders and to the general parameters that make it possible to quantify the power absorbed and the rise of temperature in the corresponding biological objects.
1.2. ELECTROMAGNETIC FIELDS
1.2.1. Electromagnetic Waves An electromagnetic wave is the combination of an electric field and a magnetic field propagating in the free space at the speed of light. These waves are characterized mainly by their: Frequency: which is the number of oscillations at a given point during one second; it is denoted by f and expressed in hertz [Hz]. Period: denoted by T and expressed in seconds [s], and it is defined as the inverse of the frequency with the relation: Tf1/ . Wavelength: that is the distance that separates two corresponding points of oscillation. It characterizes in particular the distance between two nodes or twovalleysofawave.Itisdenotedbyλandexpressedinmeters[m].This quantity is inversely proportional to the frequency in the vacuum. It is related to the frequency and the speed of propagation of the light c by the relation: c f (1-1)
Electric Field (E )
Magnetic Direction of Field (H ) Propagation (k)
Figure 1-1: Representation of an electromagnetic wave. The electric field, in blue, and the magnetic field, in red, propagating rectilinearly and perpendicularly to each other
Electromagnetic waves extend over a continuous set of frequencies called the electromagnetic spectrum. It is mainly splitted into two categories, ionizing and non- ionizing radiations, depending on their energy and frequency. Ionizing radiations
Literature Review 7
(IRs) are the waves having their wavelength below 10 nm. Examples of these radiations are X-rays and γ-rays which can break the molecular bonds, normally these waves are used for medical imaging applications [1][2]. On the other hand, the non-ionized radiations (NIRs) include static electric and magnetic fields, low frequency electromagnetic fields, radio frequencies, infrared radiation, visible light and ultraviolet light. These electromagnetic fields can be from a natural or a manmade origin and they are used for many applications (Figure 1-2). Electromagnetic waves in this range can cause biological effects due to the heating produced when interacting with biological objects. The domain of interest concerns the range of radio frequencies (RF) which extends from 30 kHz to 300 GHz.
Figure 1-2: The Electromagnetic Spectrum [6]
1.2.2. Continuous and Pulsed Waves The emission of the electromagnetic signals can be continuous or discontinuous. The continuous wave (CW) signals are those emitted continuously without any interruption. The discontinuous signals, on the other hand, are those emitted in a sequence of finite duration pulses, separated by time in which the transmitter is off, they are also called pulsed signals. The number of pulses emitted per second
8 Literature Review
corresponds to the pulse repetition frequency (PRF) and it is expressed in hertz. The ratio of the pulse width (PW) η over the pulse repetition interval (PRI) or the period is called the duty cycle D: PW (1-2) D PRI
The PRF corresponds to the inverse of the PRI: 1 (1-3) PRF PRI
In a pulsed electromagnetic wave, the peak or the maximum power Pmax of a pulse is large compared to the average power Pavg. The latter corresponds to the integration of the instantaneous power over the signal period, and we can say that it is the peak power multiplied by the duty cycle; the lower the duty cycle is the lower the average power. 1 T P P() t dt avg Pavg Pmax D P max PW PRF (1-4)
T 0
PRI PW
Pmax Power
Pavg ……
Time Figure 1-3: Pulsed wave signal
1.2.3. EM Wave Propagation Zones
The electromagnetic wave does not have the same propagation properties in the entire space around a radiating source. Starting from the different components of the electromagnetic field of an elementary dipole or loop, we find that these zones are characterized by the wave impedance. The behavior of transmitted fields is dependent on the nature and the frequency of sources that generates these waves. Moving away from the radiating source, we distinguish mainly two propagation zones, the near-field and the far-field zones, Figure 1-4.
Literature Review 9
Far-Field (Fraunhoffer zone)
Radiative Near-Field (Fresnel zone)
Reactive Near-Field
Antenna
Figure 1-4: Electromagnetic field regions around a radiating source
1.2.3.1. Near field zone
The near field region is mainly defined when the distance from the radiating source r is: r ( 1-5 ) 2
In the near field zone, the radiated electric and magnetic fields are independent of each other. The origin of the radiated field depends on the nature of the source. Indeed, the magnetic loop (magnetic dipole) essentially provides a magnetic field and in the same way an electric dipole mainly provides an electric field. The wave impedance is obtained with equations ( 1-6 ) and ( 1-7 ) for the electric and magnetic dipoles respectively, with approximation according to ( 1-5 ), that is kr 1, where k 2/ is the wave number [7].
11 1 2 jkr kr Z ( 1-6 ) E 1 kr 1 jkr
10 Literature Review
1 1 jkr ZH kr 11 ( 1-7 ) 1 2 jkr kr
In this zone, the characteristic impedance is proportional to 1/kr for the electric dipole and to kr for the elementary magnetic loop. For the value of r /2 , the impedances tend towards the intrinsic impedance . This distance is commonly used in electromagnetic compatibility to define the boundary between the near field area and the far field regions.
The characteristic impedance modules of the elementary electric dipole ZE, the elementary magnetic loop ZH and the vacuum are presented in Figure 1-5.
Near field Far field
Transition zone
η
r Reactive zone 2
Figure 1-5: Wave impedance of magnetic (ZH) and electric (ZE) dipoles as a function of k×r
The near field zone is divided into two regions, reactive near field and radiating near field. The reactive near field region is very thin and is at a distance R1 from the antenna:
d 3 R 0.62 1 ( 1-8 )
Literature Review 11
where d is the largest dimension of the antenna. In this zone, the waves are evanescent and the phenomenon of propagation is negligible. The radiative near field region, also called Fresnell zone, is an intermediate zone located between R1 and R2. d 2 R 2 2 ( 1-9 )
1.2.3.2. Far field zone
The far-field region, also known by Fraunhofer zone, lies beyond the distance R2, or when the distance r is: r 2 ( 1-10 )
The far-field zone is introduced to simplify the formulation of wave equations. The radiated energy is confined in a conical beam and the waves are locally almost planar. In this zone, the notion of separation between purely electrical behavior and purely magnetic behavior is no longer valid. Indeed, the wave impedance is equal to the characteristic impedance of the vacuum , that is equal to 120π or 377 Ω,and the wave formed by the electric and the magnetic fields is a transverse plane wave. In addition, the E and H fields are orthogonal and their amplitudes decrease in a ratio of 1/r . In this work, we are interested in the near field region where the transverse wave hypothesis is no longer valid. The calculation of the electromagnetic fields then requires the implementation of direct numerical calculation of the Maxwell‘s differential equations.
1.3. ELECTROMAGNETIC COMPATIBILITY
Electromagnetic Compatibility (EMC) is the ability of a system to function satisfactorily and without producing intolerable electromagnetic disturbances for any other system in its environment. Human beings are considered as systems as well, so human safety is also a subject of EMC analysis.
12 Literature Review
A general model of all EMC configurations mainly consists of: Source: the transmitter of the disturbing or parasitic signal Coupling: the interaction between the source and the victim Victim: the receiver of the transmitted signal
Coupling Path Source -Conductive Victim -Radiative
Figure 1-6: General EMC model
Electromagnetic waves are capable of influencing the behavior of an electronic system or a modification in the medium of propagation. These effects are called electromagnetic perturbations. Sources of these perturbations can be either from a natural origin, such as: Lightning (electrostatic discharge between clouds or between cloud and ground) Cosmic (in particular solar radiation) or from a man-made sources. In this case, we can differentiate two categories: Unintentional man-made sources, such as perturbations resulting from the use ofelectricity,powerlines,powerswitches,andhighspeedlogiccircuits… Intentional man-made sources, that consists of all the devices that are made for generating RF signals including: telecommunication systems, microwave links,radarapplications,RFIDsystems…
Perturbations are then classified according to their mode of transmission: Radiated emissions: when the source produces an electromagnetic field that propagates in the space. The effect on the victim depends on the nature, frequency and distance from the radiating source. Conducted emissions: here the electromagnetic disturbances are transmitted via the electrical connections between different devices.
Literature Review 13
Obviously currents and parasitic voltages circulating in the cables interconnecting equipments will radiate themselves. Similarly, the radiated disturbances will be able to induce stray currents and voltages in the various interconnections. The conducted and radiated perturbations are therefore intimately coupled. EMC refers, in general, to the effects on equipments, devices, electronic systems. However, several electromagnetic phenomena are to be considered: thermal and non-thermal effects on biological tissues. In this case another definition is used, bio-electromagnetic compatibility.
1.3.1. Near-field characterization Near-field characterization is a necessary step in EMC measurements as well as exposure analysis of electromagnetic radiation in the space surrounding a device under test (DUT). Near-field measurements are carried out using a bench that consists of probes, sensitive to different components of the radiated electric and/or magnetic fields, and a scanning system that maps the amplitude and/or phase of the radiated field. These measurements make it possible to locate the sources of radiation in electronic cards and to extract the electromagnetic behavior near different devices. A near field test bench is adapted to perform automatic measurements in the space surrounding the DUT.
PC
Analyzer Positioning System
Probe
DUT Figure 1-7: Near-field test bench with scanning system
There are several techniques for measuring electromagnetic fields in the near field region. We are interested in those using electromagnetic field probes because it
14 Literature Review
is the easiest to implement. Here we can differentiate between two configurations of the used probe; the first one is the direct probe method that uses a small antenna or probe. This probe is placed in the environment close to the device to be characterized and the mapping of the electromagnetic field is obtained by scanning the probe above the surface of the device (Figure 1 -7) or by moving the device under the probe. The second one is to use an array of probes (Figure 1-8) to capture the radiated electromagnetic field. In the latter, the field is captured by a one-shot or multi-shots depending on the dimensions of the probe array as well as those for the DUT.
Control System
Analyzer Array of Probes
PC
DUT Display Figure 1-8: Near field system with array of probes
These measurements allow the characterization of the near-field electromagnetic radiation emitted by different devices. The mapping of different components of the radiated field can then be done at different heights above the DUT. It is then possible to locate the sources of the radiation, to know the distribution and the value of the electromagnetic field as well as the paths of currents. The near-field scan (NFS) measurement technique relies on the use of measurement probes based on miniature magnetic loops, for magnetic field measurements, or electric dipoles, for electric field measurements. The detection of the electromagnetic field is carried out using, for example, a diode detector, spectrum analyzer or network analyzer. The probes are connected using transmission lines such as coaxial cables and an amplifier can be used to improve the sensitivity of the test bench.
Literature Review 15
1.4. INTERACTION BETWEEN EM FIELDS AND MATTER
The propagating electromagnetic wave can meet different obstacles. Depending on its wavelength and the electromagnetic characteristics of the material encountered, such as permittivity, different types of interactions can occurs as reflection, refraction, diffraction, diffusion, and absorption. This section aims to provide a better understanding of the specific properties of NIRs in the radiofrequency range.
Figure 1-9: Electromagnetic incident wave decomposition
In contact with matter, the electromagnetic wave is splitted into a reflected wave and a transmitted wave (Figure 1-9). The transmitted wave is the most significant parameter for quantifying the dissipation of energy within the material. Depending on the frequency range, wave penetration and interactions with biological tissues are not identical and the wave-matter interactions of NIRs do not rely on the same physical properties of the wave. As a result, the physical quantities are not the same and the biological effects vary according to the type of radiation. Sensitive tissues and organs vary depending on the type of interaction. For the low frequencies, the main effects concern the stimulation of excitable tissues such as the muscles, the cardiovascular system, and the central nervous system. The higher frequencies cause a warming (temperature increase) of the tissues with a penetration that is lower as the frequency is high.
16 Literature Review
To know precisely the effects produced by an electromagnetic field in a biological medium, it is important to know the electrical characteristics (relative permittivity εr and the conductivity ζ) of this medium. Biological media have both free charges and bound charges which, under the effect of an applied electric field, are responsible for conduction and polarization currents. The human body is globally considered non-magnetic with regard to the study of induced electromagnetic fields; the magnetic permeability of biological media is thus assimilated to that of vacuum
μ0. In biological tissues, the free charges that can create conduction current are ionic in nature. These ions are more or less free to move under the effect of an applied electric field and are subject to friction forces and stresses due to the structure of the medium. Their mobility is a function of the frequency of the applied electric field. The presence of polar molecules of different size, also subjected to friction forces, gives biological media lossy dielectric behavior. A thermal effect is caused by a heating of biological tissues. Indeed, because of their polarized nature, water molecules subjected to an oscillating electric field tend to follow the orientation of this field. Due to the intermolecular friction generated by sufficient power, a rise in temperature occurs. This heating varies according to the power of the electromagnetic field. The threshold of appearance of a thermal effect corresponds to a rise of 1°C in body temperature in humans [8]. Below this value, the body's thermoregulatory system is able to maintain body temperature. A non-thermal or athermic effect is not induced by a significant rise in temperature. It appears at exposure levels for which the thermoregulatory system of the exposed organism is able to regulate the temperature and to dissipate a possible heating. In recent years, many results on wave-living interactions have been published. The study of physical and biological phenomena is very complex and therefore imposes extreme rigor in the development of experimental, measurement and observation procedures. In this work we are not going to check the effect of the electromagnetic fields on the tissues and biological organisms, rather the aim is to quantify the exposure parameters refereeing to the international guidelines and standards that defines the corresponding limits that may leads to thermal effects.
Literature Review 17
1.5. RADIOFREQUENCY DOSIMETRY
RF dosimetry is the quantification of the electromagnetic fields being absorbed by biological objects that are exposed to electromagnetic waves. RF dosimetry is the study that addresses the effects of electromagnetic radiations on the living organisms. Different dosimetric quantities are used to characterize the radiation levels. Depending on the considered frequency band, the interactions of an EM wave of NIRs with matter are not based on the same physical quantity. The relevant measurement units to characterize the interactions are thus not the same and biological effects vary depending on the type of radiation. The purpose of the dosimetric study is to evaluate the impact of electromagnetic radiation on living beings by quantifying the levels of EM fields as well as the absorbed powers. It can be conducted both experimentally, requiring the implementation of specific exposure systems and the use of dedicated sensors, and by numerical simulation. Dosimetric studies are also essential for defining and updating national and international standards and recommendations related to the use of radiofrequency technologies.
1.5.1. Quantification Parameters
1.5.1.1. Electric field (E) The electric field is defined by the force F that appears due to an electric charge q. An electric field appears as soon as electric charges are in motion. This field propagates at the speed of light in the free space. The magnitude of the electric field at a distance r from a charge q canbederivedfromcoulomb‘slawasfollows: q E 2 ( 1-11 ) 40 r r
It is expressed in volt per meter [V/m]
18 Literature Review
1.5.1.2. Magnetic field (H) Magnetic fields appear when an electric current passes through a conductor; its intensity is expressed in ampere per meter [A/m]. Certain materials such as those intended to manufacture magnets emit in their environment a magnetic induction without the presence of a current. As an example, suppose that we have a wire with a radius r, the passing electrical current I through this wire generate a magnetic field in the region surrounding the wire with an intensity H given by: I H 2 r ( 1-12 )
1.5.1.3. Specific Absorption Rate (SAR) At the interface between the biological medium and the external environment, part of the radiated electromagnetic wave is transmitted within the tissues. Faced with the difficulty of formally correlating the external electric and magnetic fields with the biological effects generated, the quantification of the energy absorbed by the tissues proves to be the most coherent approach for the study of the interactions between waves and the biological medium. SAR is the appropriate parameter for quantifying the tissue absorption of electromagnetic fields. It is rigorously defined as the time derivative of the quantity of energy absorbed dW by a tissue of mass dm contained in an volume dV of a known density density ρ: d dW d dW SAR dt dm dt dV ( 1-13 )
To evaluate SAR expression, it is necessary to know the physical quantities involved in the interaction phenomena between the electromagnetic wave and the biological medium. The electric field within the tissues has the effect of causing a heating of the medium. Thus the power absorbed by the tissues is evaluated in two ways: either by the determination of the electric field in situ or by the characterization of the temperature rise induced by the RF exposure: by the electric field in situ: E 2 SAR ( 1-14 ) where ρ and ζ are respectively the density and the electrical conductivity of the medium and E is the effective value of the electric field
Literature Review 19
by the temperature rise in the tissue: T SAR C t (1-15) where ∂T is the temperature rise (in degrees Kelvin, K) during the exposure time ∂t (in second, s) and C the heat capacity mass of the tissue (in J/(kg.K)). SAR has two forms: average SAR: it is averaged over the whole body and corresponds to exposures whole body (case of the base stations for example) local SAR: it is averaged over 1 g or 10 g and corresponds to localized exposures (case of mobile phones for example).
1.5.2. Numerical Dosimetry Numerical dosimetry is the numerical modeling of the exposure system with the biological medium exposed to an electromagnetic field. Numerical methods have been developed to solve Maxwell's equations in the biological medium taking into account the electrical properties (electrical conductivity and dielectric permittivity) of this medium. The interest is then on the evaluation of the intensity of the electric and magnetic fields within the structure. In numerical calculations a segmentation process is applied on the studied medium, so it is decomposed into elementary volumes called voxels. Since the late 1970s, numerical computational methods have developed rapidly, with major interest in modeling human and animal anatomy. One of them is the method of moments (MoM) in which the human body is divided into blocks each containing its own electrical properties [9][10]. This method was widely used in the 1980s for the numerical evaluation of the average SAR. Since the 1990s, it has been superseded by the finite difference time-domain method (FDTD), first introduced by Kane Yee in 1966 [11]. It allows, at any point of the discretized space, to calculate the components of the electromagnetic field at a time t from those calculated at the previous step at the neighboring points. The advantages of this method are the relatively simple formulation, the robustness of the algorithm, and the resolution capability over wide frequency bands. This method allows modeling both the sources of exposure and the propagation of electromagnetic waves in vacuum or in complex materials (such as the biological human tissues) in the temporal and spatial domain. The spatial approach therefore implies the possibility of determining the SAR at any
20 Literature Review
point in the structure from the components of the electric field calculated in each cell and the dielectric parameters of the studied tissues or numerical models. There are also other SAR evaluation methods based on the hybridization of the FDTD method as we can found in [12][13]. Numerous software programs are available for electromagnetic computation where the interaction between electromagnetic fields and different objects can be analyzed, based on different methods, including the numerical mapping of the, E- field, H-field, and SAR within a biological structure. For example, High Frequency Structure Simulator (HFSS) is a software from ANSYS that is based on the finite element method (FEM), also CST Microwave Studio is based on different methods mainly finite integration technique (FIT) for transient solver, FEM for frequency domain solver, and MoM for integral equation solver as well as the transmission line matrix method (TLM).
1.5.3. Experimental Dosimetry Experimental dosimetry is complementary to the numerical one. The experimental approach encounters difficulties related to in vivo measurements, since there are some problems related to the implantation of the measuring instrument in a living organism and also due to the strong heterogeneity of the tissues. On the other hand, in vitro experiments, does not encounter these difficulties. The instrumentation required for the measurement of the electromagnetic field will be presented in the next chapter. In order to ensure a good repeatability of the measurements and duo to the difficulty of in vivo measurements, phantoms have been developed. These phantoms are filled with liquids or gel substances simulating the electrical properties of the human body tissues. Numerous experimental evaluations have been conducted on phantoms [14]–[16]. The made phantoms are mostly homogeneous because it is difficult to manufacture heterogeneous structures based on liquid or gelled substances. However, some heterogeneous achievements have emerged, notably using solid materials for bones and liquids for tissues containing a lot of water [10],[11]. The tissue simulated liquid (TSL) depends on the type of each tissue and on the frequency of study. Numerous studies have established suitable compositional formulas for the fabrication of the materials used in these phantoms. These materials are referenced in international standards for RF dosimetry [19], [20]. The solutions
Literature Review 21
contained in these phantoms are generally composed of high permittivity liquids, such as water, in which liquids having lower permittivity are added. The value of the conductivity ζ is then adjusted by adding salt.
1.6. STATE OF THE ART IN MEASUREMENTS OF EM FIELDS
The evaluation of exposure to electromagnetic fields consists of measuring the intensity of electric and magnetic fields at a given point away from the radiating source. In the far-field region it is enough to measure only one of them, either the electric field or the magnetic field, and then the other can be calculated based on the relation between them in the fee space that is E =120π×H.
HMDA 1545
AP2000 ETSI 3125 series
Figure 1-10: Examples of electric and magnetic field probes for exposure measurements
However, in the near-field region it is not the case, where both the electric and the magnetic fields should be measured separately. Different equipments have been used to measure electric or/and magnetic fields levels near radiating devices. For electric field measurements in the frequency range of 440 MHz to 3 GHz, ETS- Lindgren's 3125 balanced dipole series [21] can be used, these antennas are designed to match the operating frequencies of mobile phones and wireless applications, also bi-conical antennas like AP2000 can be used for environment exposure measurement in the frequency range from 60 MHz to 2.5 GHz [22]. On the other hand, for magnetic field measurements different loop antennas are used, for example the active
22 Literature Review
loop antenna HMDA 1545 is a magnetic field strength meter that measures the field in the frequency range of 9 kHz to 80 MHz [23].
NIM-511 NBM 520 SRM 3006
Figure 1-11: Narda electromagnetic field meters samples
This kind of probes needs to be connected to a measuring instrument, like spectrum analyser, to record the field levels. However, there exists different kind of meters that integrates their own analyser like NIM-511, NBM 520, SRM3006, and others from Narda test solutions (Figure 1 -11), where both electric and magnetic field probes can be found for different frequency ranges for these meters [24].
These measuring instruments are used in wide variety of applications, from EMC testing to exposure measurements. Nevertheless, their spatial resolution is a limitation in some applications as their sizes is not so compact, in addition to their high price. This leads us for fabricating other compact probes for EMC and exposure measurement applications that will be presented in chapter 2.
Literature Review 23
1.7. STANDARDS AND GUIDELINES Different guidelines and standards have been established to manage people's exposure to electromagnetic radiations. In 1998, ICNIRP (International Commission on Non-Ionizing Radiation Protection) established in its guidelines the exposure limit values for a spectrum extending up to 300 GHz [8]. ICNIRP defines the reference levels for the general public and workers (Table 1 -1). Exposure limit values (ELVs) are defined and reassessed regularly in articles published in peer-reviewed scientific journals on the thermal and non-thermal effects of radiation. The standards are based on the assessment of the biological effects whose health consequences have been established. Table 1-1: Occupational and general public exposure reference levels [8] Electric field intensity Magnetic field intensity Power density [W/m2] Frequency [V/m] [A/m] Band General General General Occupational Occupational Occupational Public Public Public 10 MHz - 61 28 0.16 0.073 10 2 400 MHz 400 MHz - 3f 1.375f 0.008f 0.0037f f/40 f/200 2 GHz 2 GHz - 137 61 0.36 0.16 50 10 300 GHz
Reference levels were used as the basis for the definition of the European Recommendation 1999/519/CE [25], setting the limit of public exposure to electromagnetic fields, and the Directive 2004/40/CE concerning the minimum safety and health requirements relating to worker exposure [26]. CENELEC (Comité Européen de Normalisation ELECtronique) has also taken these values into standardization [27]. In the United States, IEEE (the Institute of Electrical and Electronics Engineers) itself has published several standards [19], [28], [29]. The reference levels or the electric and magnetic fields limit values for the protection of users against the harmful effects of non-ionizing radiation, ensure compliance with the basic restriction. These limit values are established on the basis of the proven thermal effect of microwaves and are accompanied by a margin of safety between workers and the general public. For example, for whole body SAR, the critical biological effects are notable, in animals, from 4 W/kg, ICNIRP considers that the
24 Literature Review
first effects appear from this threshold of SAR which corresponds to a rise of 1° C in temperature. In humans, and based on a safety factor of 10, the exposure limit was set at 0.4 W/kg (whole body SAR). For general public, an additional safety factor of 5 in relation to the occupational exposure limit has been introduced. The safety factor with respect to the occurrence of proven effects is therefore 50. The SAR limit of whole body exposure for the public was therefore set at 0.08 W/kg. These reduction factors are introduced to account for scientific uncertainties related to experimental errors as well as taking into account people who are not informed about their environment and the varying sensitivities of people according to their physiological or pathological state (sick, elderly, children, etc.). ICNIRP also recommends limits on local SAR in the case of inhomogeneous exposure (Table 1 -2).
Table 1-2: ICNIRP Recommended ELVs [8] Whole body SAR Local SAR (10g) Occupational 0.1 W/kg 4 W/kg General Public 0.08 W/kg 2 W/kg
However, we must be careful if we want to compare the limit values of different countries because of the large number of different parameters (calculation methods, sources taken into account, and places of application of more restrictive values...). It is therefore difficult to finally determine which countries apply more drastic exposure rules than others. Disparities in electromagnetic field standards between some countries contribute to public fears. Since 2008, the World Health Organization (WHO) has been asking the different countries for a global harmonization of regulatory exposures that would establish a framework of reference.
The professional protection regulations are mainly based on the European Directive 2013/35/EU of the European Parliament and of the Council of 26 June 2013 [30]. This Directive is part of the package of four directives on the exposure of workers to the risks arising from physical agents: noise, vibration, EMFs and optical radiation. It is an individual directive under Framework Directive 89/391/EEC on the introduction of measures to encourage improvements in the safety and health of workers at work. It is based on ICNIRP 2009 and 2010 guidelines. It introduces a
Literature Review 25
system of ELVs on the basis of biophysical and biological considerations, in particular on the basis of scientifically well-established short-term and acute direct effects, i.e., thermal effects and electrical stimulation of tissues. However, it does not address long-term effects since there is currently not enough scientific evidence for this. The Directive also allows for exceptions in medical applications using magnetic resonance imaging (MRI), for some special sectors subject to the Member State‘s decision, but only if the circumstances duly justify exceeding the ELVs, and this on a temporary basis [31]. In order to facilitate assessment, the Directive gives ELVs for health effects as well as sensory effects and action levels (ALs) for the EMFs. Since the exposure limits values are given in quantities that are not possible to measure in a workplace, the action levels will become in most cases the de facto limits. The ALs are given for electric and magnetic fields, induced and contact currents which are measurable quantities [31].
1.8. CONCLUSION
In this chapter we have presented a general review about the electromagnetic field phenomenon including the possible interaction between these waves and the biological mediums. Then we introduced the RF dosimetry in its two branches the numerical and experimental. As well as the used probes and field measurements devices that exists nowadays for exposure analysis. The end of the chapter was about the existing standards and international guidelines that give the exposure limit values and the reference levels to be respected from the general public as well as the professionals. The next chapters will be mainly divided into 2 parts. The first is about the development of a near-field measurement system with a new tri-axial magnetic field probe, and also an algorithm for predicting the 3D magnetic field distribution above a radiating device starting from the tangential components at an initial plane. The second part is dedicated for the exposure assessment of two applications, RFID systems used in libraries, and RF welding machines used in the industrial sector.
26 Literature Review
1.9. REFERENCES
[1] B.Szentpali,―Humanexposuretoelectromagnetic fields from mobile phones,‖1999,vol.1,pp.222–231.
[2] M.M.R.Moussa,―Reviewonhealtheffectsrelatedtomobilephones.Part II:resultsandconclusions,‖J. Egypt. Public Health Assoc., vol. 86, no. 5 and 6, pp. 79–89, Dec. 2011.
[3] P. Frei, A. H. Poulsen, C. Johansen, J. H. Olsen, M. Steding-Jessen, and J. Schuz,―Useofmobilephonesandriskofbraintumours:updateofDanishcohort study,‖BMJ, vol. 343, no. oct19 4, pp. d6387–d6387, Oct. 2011.
[4] J. Sabol, R. Ralbovska, and J. Hudzietzova,―Importantroleofradiation protection in specific applications of X-raysandradionuclidesinbioengineering,‖ 2013, pp. 1–4.
[5] X.Huang,C.Kim,andP.Jiang,―Effectsofhigh-dose gamma ray irradiation on the physicochemical properties and water-treeing deterioration of cross-linked polyethylenecableinsulation,‖IEEE Electr. Insul. Mag., vol. 27, no. 4, pp. 17–25, 2011.
[6] ―SafeLevelsofElectromagneticRadiation.‖DefenderShield website, 07- Apr-2014.
[7] V.Benjamin,―Identificationdesources électromagnétiques multipolaires équivalentes par filtrage spatial: Application à la CEM rayonnée pour les convertisseursd‘électroniquedepuissance,‖InstitutNationalPolytechniquede Grenoble - INPG, 2009.
[8] ICNIRP,―Guidelinesforlimitingexposure to time-varying electric, magnetic,andelectromagneticfields(upto300GHz).‖HealthPhysics74(4):494- 522, 1998.
[9] D.E.LivesayandK.Chen,―ElectromagneticFieldsInducedInside ArbitrarilyShapedBiologicalBodies,‖IEEE Trans. Microw. Theory Tech., vol. 22, no. 12, pp. 1273–1280, Dec. 1974.
[10] M.J.Hagmann,O.P.Gandhi,andC.H.Durney,―NumericalCalculationof ElectromagneticEnergyDepositionforaRealisticModelofMan,‖IEEE Trans. Microw. Theory Tech., vol. 27, no. 9, pp. 804–809, Sep. 1979.
[11] KaneYee,―Numericalsolutionofinitialboundaryvalueproblemsinvolving maxwell‘sequationsinisotropicmedia,‖IEEE Trans. Antennas Propag., vol. 14, no. 3, pp. 302–307, May 1966.
Literature Review 27
[12] E. Piuzzi, S. Pisa, M. Cavagnaro, and P. Bernardi,―Humanexposuretoradio base-stationantennasinurbanenvironment,‖IEEE Trans. Microw. Theory Tech., vol. 48, no. 11, pp. 1996–2002, Nov. 2000.
[13] S.Mochizuki,S.Watanabe,M.Taki,Y.Yamanaka,andH.Shirai,―ANew Iterative MoM/FDTD Formulation for Simulating Human Exposure to ElectromagneticWaves,‖IEICE Trans. Electron., vol. E87–C, no. No. 9, pp. 1540– 1547, Sep. 2004.
[14] C.Gabriel,―Tissueequivalentmaterialforhandphantoms,‖Phys. Med. Biol., vol. 52, no. 14, pp. 4205–4210, Jul. 2007.
[15] G.Fixter,A.S.Treen,I.J.Youngs,andS.Holden,―Designofsolid broadbandhumantissuesimulantmaterials,‖IEE Proc. - Sci. Meas. Technol., vol. 149, no. 6, pp. 323–328, Nov. 2002.
[16] M. Lazebnik, E. L. Madsen, G. R. Frank, and S. C. Hagness,―Tissue- mimicking phantom materials for narrowband and ultrawideband microwave applications,‖Phys. Med. Biol., vol. 50, no. 18, pp. 4245–4258, Sep. 2005.
[17] M.Takahashi,I.Ida,K.Ito,andY.Okano,―TheSARevaluationmethodby a combination of thermographic experiments and biological tissue-equivalent phantoms,‖IEEE Trans. Microw. Theory Tech., vol. 48, no. 11, pp. 2094–2103, Nov. 2000.
[18] M. A. Stuchly, A. Kraszewski, S. S. Stuchly, G. W. Hartsgrove, and R. J. Spigel,―RFEnergyDepositionina Heterogeneous Model of Man: Near-Field Exposures,‖IEEE Trans. Biomed. Eng., vol. BME-34, no. 12, pp. 944–950, Dec. 1987.
[19] ―IEEEC95.1-2005 Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3 kHz to 300GHz.‖IEEE Standards Association, 2005.
[20] IEC 62209-1:2016,―Measurementprocedurefortheassessmentofspecific absorption rate of human exposure to radio frequency fields from hand-held and body-mounted wireless communication devices - Part 1: Devices used next to the ear (Frequencyrangeof300MHzto6GHz).‖IEC Standards, 06-Jul-2016.
[21] "DIPOLES 3125 Precision Dipole Antenna" ETS-Lindgren [Online]. Available: www.ets-lindgren.com/datasheet/antennas/dipoles/4014/401402
[22] "AP2000 - Biconical Antenna" Clampco Sistemi [Online] Available: www.clampco.it/measurement-antennas/biconical-antenna-AP2000.html
[23] "Active Loop Antennas / Magnetic Field Probes" Schwarzbeck [Online] Available: www.schwarzbeck.de/en/antennas/loop-antennas/active-loop-antennas- magnetic-field-probes.html
28 Literature Review
[24] Narda Safety Test Solutions [Online] Available: www.narda-sts.com
[25] Recommandation1999/519/CE,―RecommandationduConseiln° 1999/519/CEdu12/07/99relativeàlalimitationdel‘expositiondupublicaux champsélectromagnétiques(de0Hzà300GHz).‖OfficialJournaloftheEuropean Communities, 30-Jul-1999.
[26] DIRECTIVE2004/40/CE,―Rectificatifàladirective2004/40/CEdu Parlement européen et du Conseil du 29 avril 2004 concernant les prescriptions minimalesdesécuritéetdesantérelativesàl‘expositiondestravailleursauxrisques dusauxagentsphysiques(champsélectromagnétiques).‖Journalofficieldel‘Union européenne, 24-May-2004.
[27] CENELEC,―Humanexposuretoelectromagneticfields,ENV50166–1(low frequencies) and ENV 50166–2(highfrequencies).‖CENELEC,Bruxelles,1995.
[28] ―IEEEC95.1- 1999 Standard for Safety Levels with Respect to Human ExposuretoElectric,MagneticandElectromagneticFields,0Hzto300GHz.‖IEEE Standards Association, 1999.
[29] ―IEEEC95.3-2002 Recommended practice for measurements and computations of radiofrequency electromagnetic fields with respect to human exposure to such fields, 100 kHz-300GHz.‖IEEEStandardsAssociation,2002.
[30] ―Directive2013/35/EU of the European Parliament and of the Council of 26 June 2013 on the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (electromagnetic fields) (20th individual Directive within the meaning of Article 16(1) of Directive 89/391/EEC) andrepealingDirective2004/40/EC.‖,26-Jun-2013.
[31] "Electromagnetic fields in working life A guide to risk assessment", Kjell Hansson Mild, and Monica Sandström, European Trade Union Institute, 2015
Literature Review 29
Chapter 2: Design & Characterization of Tri- Axial Magnetic Field Probes
Contents
Chapter 2: Design & Characterization of Tri-Axial Magnetic Field Probes 31 2.1. Introduction ...... 32 2.2. Electromagnetic field probes ...... 32 2.2.1. Electric-field Probes ...... 33 2.2.2. Magnetic-field Probes ...... 34 2.3. EM-field probes parameters ...... 35 2.3.1. Antenna Factor ...... 35 2.3.2. Selectivity ...... 36 2.3.3. Sensitivity ...... 36 2.3.4. Spatial resolution ...... 36 2.3.5. Frequency band ...... 37 2.4. Development of the 3D Near-field Scanning System ...... 37 2.5. Measurement Chain ...... 40 2.6. EM-field Probes Calibration techniques ...... 43 2.7. Single Magnetic field Probes ...... 49 2.7.1. Probes Description ...... 49 2.7.2. Probes Calibration ...... 52 2.7.3. Selectivity ...... 54 2.7.4. Validation ...... 56 2.8. Development of Tri-axial Magnetic field Probes ...... 62 2.8.1. Design ...... 62 2.8.2. Coupling Analysis ...... 64 2.8.3. Calibration ...... 65 2.8.4. Validation ...... 66 2.9. Conclusion ...... 70 2.10. References ...... 71
Design & Characterization of Tri-Axial Magnetic Field Probes 31
2.1. INTRODUCTION
Different types of probes are used to measure the components of the electric and magnetic fields (coaxial probes, electro-optical probes, PCB probes…). These sensors convert the local electromagnetic energy radiated by the device into a voltage to be measured by the receiver. These probes have the main characteristic of being of small dimensions with respect to the wavelength in order to limit the induced disturbances and to allow a local detection of the electromagnetic field with a good resolution. NFS method has the advantage of being transposable in an industrial environment. However, the many metal parts present at the probe and the connection cables disturb the local electromagnetic field causing a bias in the measurement. In addition, the presence of flexible transmission lines can lead to precision problems with amplitudes and phases measured at high frequencies.
This chapter has been devoted to the description of the developed near-field measurement test bench. To do this, we will begin with an introduction on the electromagnetic field probes in general. Next, we describe the developed near field test bench that includes the measuring instrument and two scanning systems. After that we present the designed tri-axial magnetic field probe as a new three axis probe that measures the three components of the magnetic field simultaneously, as well as the calibration method of the measurement chain. In the last part, the validation of the whole system is shown.
2.2. ELECTROMAGNETIC FIELD PROBES
An electromagnetic field probe is a sensor that captures the field radiated by an electrical device or system in operation. The electric or magnetic character of this field depends respectively on the variation of voltage and the fluctuations of current inside this device. Once produced, the radiation of the field is propagated in a given direction in the space. It is therefore essential to choose a specific probe for the detection of this field. There are two types of electromagnetic field probes: electric field probes and magnetic field probes.
32 Design & Characterization of Tri-Axial Magnetic Field Probes
Electric Field (E ) Axis sensitive to Electric fields
Axis sensitive to Magnetic fields
Magnetic Direction of Field (H ) Dipole Propagation antenna
Loop antenna
Figure 2-1: Electromagnetic wave and the corresponding sensitive sensors
2.2.1. Electric-field Probes Generally the electric field probes are in the form of a dipole, or monopole, and they are sensitive to voltage variations. When subjected to an electric field, a potential difference appears at its terminals. V
x h Figure 2-2: Small dipole as an example of E-field probe
To capture the x component of the electric field (Ex) for example, the probe is constructed in the form of an electric dipole (Figure 2-2). Each strand of the dipole has a length of h/2. The main parameter for this sensor is the length h of the dipole. Indeed, the voltage across the probe is related to the electric field by the following relation [1],[2]:
h/2 h Vsin k x E x dx x ( 2-1 ) h/2 2
This relationship shows that the sensitivity of this sensor depends on the length of the dipole. The longer the strand, the more sensitive the probe is. On the other hand, the shorter the strand is the better the spatial resolution. The orientation of the probe specifies the corresponding E-field component measured. The orientation of the
Design & Characterization of Tri-Axial Magnetic Field Probes 33
probe with respect to x and y axes makes it possible to detect the tangential component of the desired electric field Ex or Ey respectively. To measure the normal component of the electric field Ez itisbettertouseamonopolethatcanbea50Ω characteristic impedance open coaxial cable.
PCB probe Coaxial probe Figure 2-3: Electric field probes
Electric field probes can be made with a coaxial cable or printed circuit board (Figure 2-3).
2.2.2. Magnetic-field Probes
In general, magnetic field probes are in the form of a loop, they are sensitive to current variations. When a magnetic field crosses the loop, an induced electromotive force (emf) appears at its terminals. This emf can be measured using an oscilloscope or spectrum analyzer in the form of an electrical voltage. The amplitude of this voltage depends on the surface of the loop and the amplitude of the magnetic field crossing the surface of the loop. Magnetic field passing through a loop generates a voltage across its terminals accordingtoFaraday‘slaw,whichstatesthattheinducedvoltageisproportionalto the rate of change of magnetic flux through a circuit loop [2]:
dd V B. dS emf dt dt S ( 2-2 )
34 Design & Characterization of Tri-Axial Magnetic Field Probes
PCB probes Coaxial probe Figure 2-4: Magnetic field probes
For a given probe, there is a relation between the voltage obtained at its terminals and the field value which passes through it. These two quantities are linked by the antenna factor which is presented in the next section. Several architectures can be used to design near-field probes (Figure 2-4). Coaxial probes can be made with or without shielding. Without shielding, the loop may be small, but it has a higher capacitive coupling coefficient than shielded probes. However, shielded probes do not provide good spatial resolution due to the large size of their loop. A printed circuit can also be used to make a magnetic field probe. The orientation of the probe specifies the corresponding H-field component measured, the axis that it is orthogonal to the loop surface is the measured component.
2.3. EM-FIELD PROBES PARAMETERS
Different parameters define the performance of the electric and magnetic field probes. We recall these parameters below:
2.3.1. Antenna Factor The antenna factor (AF) represents the ratio between the amplitude of the magnetic or electric field and the amplitude of the voltage induced across the probe terminals. E V/ m AF1/ m ( 2-3 ) VVind.
Design & Characterization of Tri-Axial Magnetic Field Probes 35
H A/ m AF S/ m ( 2-4 ) VVind.
The probe should be placed in an area that we can know the H or E fields inside it either theoretically or by calculation. Then the AF can be calculated by evaluating the ratio between the theoretical or simulated value of the fields of the radiating circuit and the voltage measured by the corresponding probe (Vind.) in its frequency band. The analytical expressions of the various EM field components are a function of the geometrical parameters and the power injected to the used circuit. The process of extracting the AF of probe also called calibration; we will talk about different techniques in section 2.7.2 .
2.3.2. Selectivity The selectivity corresponds to the separation of the probe between the components of the electric and / or magnetic field. In other words, it represents the ability of a probe to measure predominantly a component of the EM field for which it is intended.
2.3.3. Sensitivity The sensitivity corresponds to the minimum detectable level of the measured quantity. This quantity is directly related to the antenna factor of the probe and the noise level of the measuring instrument. It is necessary that the probe has a good sensitivity to allow the measurement of even lower values of the electric or magnetic field. The magnitude is proportionally related to the geometric dimensions of the probe. On the other hand, the spatial resolution is inversely proportional to the size of the probe. Therefor a compromise between the sensitivity and spatial resolution must be taken into consideration during the design of the probe.
2.3.4. Spatial resolution The spatial resolution represents the minimum distance between two adjacent sources that a measurement probe can dissociate. This parameter is related to the size of the sensor and the distance to the device under test. It is necessary that the sensor has the smallest possible dimensions to have a good spatial resolution. The lower this parameter for a probe, the more accurately it will be able to accurately distinguish the fields emitted at measurement points, but this is affects the probe sensitivity.
36 Design & Characterization of Tri-Axial Magnetic Field Probes
2.3.5. Frequency band The frequency band reflects the frequency range in which the probe must be functional [fmin-fmax]. The value of the minimum frequency fmin is determined by the sensitivity of the measurement probe while the value of the maximum frequency fmax is strongly related to the selectivity. These different criteria are somewhat contradictory, which sometimes makes it difficult to make a compromise in order to obtain an acceptable solution allowing the realization of measurement probes appropriate to the desired application.
2.4. DEVELOPMENT OF THE 3D NEAR-FIELD SCANNING SYSTEM
The near-field test bench is intended to be an EMC diagnostic tool for printed circuits, integrated circuits, and also for small antennas near field characterization. It must be able to selectively measure the different components of the electric or magnetic fields with a good sensitivity and a good spatial resolution. The displacement of the probe above the device under test is provided by scanning system that should be able to make a 3D scan in the region of interest. In this work we developed 2 scanning systems for different applications depending on the physical size of the DUT, one dedicated to small devices and the other for large ones.
The first, is a delta robot from ALIRA [3] having three translation axes x, y and z. This Robot features a single arduino-based electronic board megatronics version 2.0, which has a powerful ATmega2560 processor with 256kB of memory, clocked at 16 MHz. It is programmable from the standard arduino integrated development environment (IDE). The system is designed by the company for 3D printing applications,butwe‘dmodified it to fit our application in near field scanning. The useful area is 18 cm for the x- and y-axes and 20 cm for the z-axis. The mechanical resolutionofthesystemis100μmforallaxes.Oneachaxisthereisasensorallows returning to the initial position of the robot as well as limiting the moving block, indicating if one of the axes exceeds the allowed margin. The robot arm then offers the possibility of mapping a surface of 18×18 cm2. Its mechanical resolution of 100 μm offers the possibility of good field mapping resolution depending on the used probe.
Design & Characterization of Tri-Axial Magnetic Field Probes 37
y-axis
ymax. Scanned Area
Scanning Path ymin. x-axis x x min. max. Figure 2-5: Mechanism of the robot scanning in the plane above the DUT
The scanning is done in a row-wise manner; i.e. scanning along x-axis in the positive direction then moving to the next row in y-axis, as shown in Figure 2-5. At each point the coordinate of the current position is recorded with the measured data then the robot arm moves to the next point and so on until the whole plane is scanned. If a 3D scan is needed the probe attached to the robot arm moves in z-axis by a predefined step and the plane scanning is repeated. At the end, the probe is returned to the initial position i.e. (0, 0, 200). A software interface based on a Labview program was developed in order to automatically control both the mechanical movement of the robot and data collection from the measuring instrument. The measuring instrument, that is the WaveRunner oscilloscope in our case, is connected internally to a PC via GPIB while the scanning robot is connected to the PC via USB type B serial com port. The developed program is integrated with Labview run-time engine; hence it can be run on any PC without having the Labview software. The program was done such that at each point in the scanned grid, it sends a run command to the Visual Basic Script (VBScript) on the WaveRunner oscilloscope to perform the corresponding data acquisition. The used measurement instrument has various functions that could be assigned on the measured signal, in both time and frequency domains. Up to 8 functions could be applied at a time. These functions are
38 Design & Characterization of Tri-Axial Magnetic Field Probes
programmed in a VBScript with the specific commands defined by the LeCroy automation manual [4]. The software interface has 2 main windows, positioning and scanning configuration. In the positioning window; one can assign manually the point of interest and click on the corresponding bottom that leads to the movement of the robot arm to the desired position and the needed measurement is done. The measured data is then saved in a .txt file in the selected directory. The scan window, shown in Figure 2-6, contains the geometrical parameters for the 3D scanning configuration. The coordinates of the boundary points on each axis can be assigned; on x-axis from Xmin to XMax, y-axis from Ymin to YMax, and on z-axis
(the height) from heightmin to heightMax. Also the step size is assigned in the corresponding box (Pas), and it is the same for the three axes. After filling these parameters, one can start the measurement by clicking start the measurement bottom, then the scanning starts from the point (Xmin, Ymin, heightmin) and it continues in the row-wise manner as described in the previous section.
Figure 2-6: Scanning configuration window
Design & Characterization of Tri-Axial Magnetic Field Probes 39
The second scanning system is developed to be used for large size devices under test; it is a 2D scanner that consists of 2 roads each one is controlled by a stepper motor. The motors are controlled by a microprocessor based on Arduino kit. This scanner has a work area of 2×1.5 m which makes it possible to scan the space near
large systems, and having mechanical resolution of 1 mm for the two translation axis.
Robot arm
axis
- y
x-axis
Figure 2-7: 2D Scanner for large devices
2.5. MEASUREMENT CHAIN
The near-field test bench will then be as shown in Figure 1 -7. A probe is connected to a measuring instrument (network analyzer, spectrum analyzer or oscilloscope) and placed in the environment close to the device to be characterized. Mapping of the electromagnetic field is achieved by scanning the probe over the device on a predefined surface by the user. A computer controls the movement of the probe and ensures the acquisition of data from the measuring instrument. The vector network analyzer (VNA) is a measuring instrument that is used to recover both the amplitude and the phase of the electromagnetic field, which gives the possibility to develop models of radiated emissions and in the post-processing of the near-field data [5]–[8].
40 Design & Characterization of Tri-Axial Magnetic Field Probes
For passive circuits, the use of VNA allows simultaneous measurement of the amplitude and phase of the electromagnetic field. The excitation of the DUT is injected from one port of the VNA and the other port is connected to the used probe, hence the transmission coefficient S21 will then be used to extract the desired electromagnetic field.
VNA
Port 1 Port 2
Probe
Passive Device Figure 2-8: Measuring Near-field of passive devices via VNA
On the other hand, the use of the spectrum analyzer (SA) is limited mainly to the acquisition of the amplitude of the electromagnetic field. If it is desired to measure the radiation of active devices, it becomes difficult to inject a power since the latter are powered externally to generate their own signals. In this case, only the amplitude of the field can be measured. A low noise amplifier (LNA) can be inserted into the measurement chain to improve the dynamics of the system.
Spectrum Analyzer
External Probe Source
Active Device Figure 2-9: Measuring Near-field of active devices via SA
In this work we propose a new measuring instrument to be used instead of the VNA and the SA. We will use Teledyne LeCroy WaveRunner 640Zi oscilloscope. It is an oscilloscope that has the option to measure in frequency domain as if it is a spectrum analyzer, and it has 4 input channels [9].
Design & Characterization of Tri-Axial Magnetic Field Probes 41
Table 2-1: Oscilloscope technical specifications [10] WaveRunner 640Zi oscilloscope Specs Bandwidth 4 GHz Number of Channels 4 simultaneously Channel-Channel Isolation DC -2.5 GHz: 40 dB Sampling Rate 20 GS/s on 4 Ch 40 GS/s on 2 Ch Memory Length 16M on 4 Ch 32M on 2/1 Ch Maximum Input Voltage 50Ω:5VRMS±10Vpeak 1MΩ:400Vmax. Time/Division Range 20 ps/div - 1.6 ks/div with standard memory
The developed scanning system consists of a 3D scanning robot, Teledyne LeCroy WaveRunner 640Zi oscilloscope, PC and the corresponding electromagnetic field probe (Figure 2-10).
Tri-axial H-field Probe 3D Scanning Robot
WaveRunner 640Zi DUT Oscilloscope
Signal Generator
Figure 2-10: Measurement system setup
The 3D scanning robot is connected with the oscilloscope to the PC where the Labview program automatically controls both the mechanical movement and data
42 Design & Characterization of Tri-Axial Magnetic Field Probes
collection. As presented before, the 3D scanner has the following travelling ranges oneachofthethreeaxes:Δx=180mm,Δy=180 mm, Δz=200 mm. The movement of the robot arm can be precisely adjusted to 0.1 mm/step by a 3D movable controller system. The probes are attached to the robot arm which moves them over the device under test to perform the needed scan. The PC acquires data measured by the oscilloscope. These data (voltages) are converted into electric or magnetic fields using the specific antenna factor of each probe.
2.6. EM-FIELD PROBES CALIBRATION TECHNIQUES
Electromagnetic field probes are indirect sensors, the measured quantity at the probe terminals is an intermediate quantity that represents the field measured. In order to extract the correct value of the corresponding field a probe factor should be used. Calibration is the process done to find the probe performance factor which is also called the antenna factor (AF) or calibration factor. It is a transfer function between the measured voltage across the loop / dipole and the magnetic / electric field in the scanned geometry. For calibrating electromagnetic field probes we need to generate a reference field whose intensity and spatial configuration are known with good accuracy inside a certain volume where the calibrated probe is placed.
Several probe calibration techniques are available in literature. For example, for low and medium frequency bands (9 kHz – 10 MHz), Helmholtz coil can be used, but it is not a preferred method. However for large size probes or high field levels this method is used [11]. For a frequency band of 200 MHz – 450 MHz, an open- ended wave guide is used, and for higher frequencies (450 MHz up to 40 GHz) the pyramidal horn antennas, and conical transmission lines are used [12]. Transverse Electromagnetic (TEM) and Gigahertz TEM (GTEM) cells are also used for probes calibration with a moderate frequency band (9 kHz – 1 GHz) that is common in different EMC applications [13]. The fact that TEM cells are fully shielded makes it good for probes calibration as there is no interference between the calibrated probes and the nearby devices. As well as it features a long term stability of the calibrating system. The TEM cell is a section of a strip line that has a middle conductor (called septum) between two plates
Design & Characterization of Tri-Axial Magnetic Field Probes 43
that are tapered from the ends. Normally the septum is located in the midway between the top and the bottom conductor plates, and the wave propagates in a TEM mode in the area in between. The dimensions of the section are chosen to have characteristic impedance of 50 Ω along the cell.
Outer Shield Port 2
Septum h Port 1
Figure 2-11: Typical TEM cell
In this work FCC-TEM-JM1 cell was used to produce a calculable electric and magnetic field strengths. The corresponding specifications of this TEM cell are shown in Table 2-2. Table 2-2: FCC-TEM-JM1 TEM cell specifications Specifications FCC-TEM-JM1 Frequency Range DC-1GHz Height 44 mm Max. Input Power 500 W Dimensions (L×W×H) 33.8×15.2×9.9 cm RF-connector Type-N50Ω Cell Impedance 50Ω Wave Impedance 377Ω
The field strength can be theoretically calculated from the geometrical parameters of the cell, its impedance at the measurement plane, and the input power as shown in the equations below [14]:
ZP. [watt] EV[ / m] c in hm[] ( 2-5 )
44 Design & Characterization of Tri-Axial Magnetic Field Probes
E[/] V m H[/] A m [] ( 2-6 )
Where Z is the input impedance of the measuring instrument in ohms c P is the input power in watts in h is the distance in meters between the septum and the outer conductor of the TEM cell is the intrinsic impedance of the free space in ohm
0 120 377 ( 2-7 ) 0
The antenna factor then can be calculated by placing the probe inside the TEM cell and measuring the induced voltage across the loop terminals, according to the following equation:
A/ m H [A/ m] AF[dB ] 20log ( 2-8 ) VVVind.[]
where V is the induced voltage in the loop (in volts) ind.
Depending on the measuring instrument Vind can be calculated. When using a
VNA, we measure the transmission coefficient (S21), while if we use a spectrum analyzer the power is then measured. These two quantities can be easily converted into voltages by using the following expressions respectively:
2 jS.arg(21 ) Vind. 2. Z c . S 21 . P in [ watt ]. e ( 2-9 )
VZPind. c. SA[]watt ( 2-10 )
Design & Characterization of Tri-Axial Magnetic Field Probes 45
VNA Port 1 Port 2 EM-field Probe
TEM Cell 50 Ω Load
Figure 2-12: Typical probe calibration setup using TEM cell
In the center of the TEM cell between the septum and the upper (or lower) conductor, the electric field is vertically polarized while the magnetic field is horizontally polarized and orthogonal to the electric one and both are quit uniform in this region. If the dimension of the calibrated probe is less than on third of the height h, the effect of the probe insertion on the perturbation of the field is less than 10% [15], [16]. A numerical simulation of the used TEM cell was performed on HFSS in order to check the field uniformity. The first port of the cell was excited with a 10 dBm signal, while the other one was terminated with a 50 Ω impedance. The simulated model is shown in Figure 2-13.
Figure 2-13: Simulation model of the used TEM cell
The electric and magnetic field distribution was then extracted in both the transversal and longitudinal cross sections. The results show clearly the uniformity of the field inside the cell and in the area that corresponds for the placement of the probe.
46 Design & Characterization of Tri-Axial Magnetic Field Probes
(a)
(b) Figure 2-14: (a) Transversal and (b) Lonigtuidinal cross-sections of the TEM cell showing the E-field distribution inside the cell
(a)
(b) Figure 2-15: (a) Transversal and (b) Lonigtuidinal cross-sections of the TEM cell showing the H-field distribution inside the cell
Design & Characterization of Tri-Axial Magnetic Field Probes 47
To test the validity of the proposed calibration method we choose 2 commercial probes that we already know their antenna factor values along the corresponding frequency range, and we recalibrate them with the proposed method and we compare the extracted AF with pre-known one. The chosen probes are 11941A and 11940A, shown in Figure 2-16. They are close field probes from Agilent operating in frequency range of 9 kHz – 30 MHz and 30 MHz – 1 GHz respectively [17].
Figure 2-16: Commercial magnetic close-field probes from Agilent
Each probe was placed in the TEM cell in the same setup as shown in
Figure 2-12 then S21 was extracted using the VNA. After that the AF was calculated using equations ( 2-8 ) and ( 2-9 ). The results are in good agreement with the given values in the data sheet, as shown in Figure 2-17 and Figure 2 -18.
Figure 2-17: HP1141A magnetic field probe antenna factor
48 Design & Characterization of Tri-Axial Magnetic Field Probes
Figure 2-18: HP1140A magnetic field probe antenna factor
2.7. SINGLE MAGNETIC FIELD PROBES
As stated in section 2.2 , the magnetic field probe is formed from a loop that it could be made from a simple wire or fabricated on a PCB. In this section we will address two configurations of magnetic field probes, the first one is a circular loop of a simple wire and the second one is a square loop designed on an FR4 PCB. These proposed probes was designed and simulated on HFSS, then validated using a standard circuit.
2.7.1. Probes Description
The first probe, named P1xy consists of a circular loop with 3-turns of 0.5 mm wire with a spacing of 0.5 mm between each turn and having a total diameter of 6 mm. The induced voltage across the loop terminals increases as the number of turns increases, which enhance the probe sensitivity. The second probe, named P2xy have a square loop with the dimensions of 3.2×3.2 mm fabricated on an FR4 substrate with 3.2 mm thickness and having copper trace of 0.5 mm. In order to form the loop, top and bottom conductors are connected through a cylindrical via having a diameter of 500 µm. The structural and geometrical parameters of these probes are shown in Figure 2-19 and the main parameters are summarized in Table 2-3. The calibration and the performance of these 2 probes were studied in details.
Design & Characterization of Tri-Axial Magnetic Field Probes 49
Table 2-3: Conventional magnetic probe and PCB probe main parameters
Classical Probe P1xy PCB Probe P2xy Loop type Circular Rectangular Number of turns 3 turns 1 turn Loop surface area Pi×(3mm)3 3.2×3.2mm2
Isolation Air (εr =1) FR4 (εr=4.4)
Ø 0.5 mm
Ø 0.5 mm
3.2 mm
0.5 mm
Figure 2-19: Geometrical parameters of the magnetic field probes; P1xy (left) P2xy (right)
The connection of these probes to the measuring instrument will be via a coaxial cable that it is connected to the top the probe (loop terminals as shown in Figure 2-19). To measure the z-component of the magnetic field the loop surface should be orthogonal to the z-axis, for this purpose we propose the probes P1z and
P2z which are a modified version of the probes P1xy and P2xy. P1z have the same o geometrical parameters as P1xy just bended 90 . P2z is a square small loop with the dimensions of 3.2×3.2 mm in a planar structure on the bottom layer of the substrate and connected to the pads at the top layer by 2 via with a diameter of 500 µm. Ø 0.5 mm Ø 0.5 mm
3.2 mm
0.5 mm
Figure 2-20: magnetic field probes; P1z (left) P2z (right)
50 Design & Characterization of Tri-Axial Magnetic Field Probes
The equivalent circuit for a simple circular loop, with a radius r1, can be as shown in Figure 2 -21. Where L is the self-inductance of the loop, C is the self- capacitance of the loop, and R is the radiation resistance given by:
8r1 Lr01ln 2 ( 2-11 ) r2
2 r C 01 1 ( 2-12 )
r1 0 R ( 2-13 ) r2 2
where the loop wire has a radius r2 andconductivityσ.
Assuming that a plane wave hits the loop having the electric field along the loop surface while magnetic field is perpendicular to it; this configuration corresponds to the maximum interaction between the loop and the EM wave. In this case only first-
order electric response VE and ideally pure magnetic response VH are considered, with:
VE r1 E ( 2-14 )
2 VH j01 r H ( 2-15 )
R
r 1 C L V Z ~ m Vm Z
VE VH r2
Figure 2-21: Simplified equivalent circuit model of a loop
Design & Characterization of Tri-Axial Magnetic Field Probes 51
The voltages across the receiver are correspondingly VmE and VmH and the ratio of the H-field response to the E-field response is approximated by equation ( 2 -16 ) [18][19].
VmH c V4 r f ( 2-16 ) mE 1 where c is the speed of light. Equation ( 2 -16 ) indicates that the E-field response increases rapidly as the radius of the loop and the frequency increases, and thus the H-field measurements are affected.
2.7.2. Probes Calibration
These two probes were calibrated using the TEM cell as presented in the previous section. ZVL vector network analyzer (VNA), from Rohde & Schwarz, was used to measure the S-parameters [20]. The induced voltage Vind. is then extracted from the transmission coefficient S21 using the equation ( 2-9 ). After that the AF of each probe is calculated using the formula ( 2-8 ).
VNA
Probe (Port 2) Input (Port 1)
50 Ω TEM Cell
Figure 2-22: Calibration Setup
52 Design & Characterization of Tri-Axial Magnetic Field Probes
AF can be calculated theoretically for the studied probes, depending on the geometrical parameters of each one. The corresponding formula for a loop probes is given by: 1 AF theoretical j NS ( 2-17 )
Where ω is the angular frequency given by ω=2πf µ is the permeability of the medium S is the area of the loop surface N is the number of turns
Figure 2-23: Antenna factor of P1xy probe
Figure 2-24: Antenna factor of P2xy probe
Design & Characterization of Tri-Axial Magnetic Field Probes 53
The results are in good agreement compared to the theoretical calculations, however at high frequencies the AF for P1xy is little bit different and this is expected due to the fact that the wavelength at these frequencies starts to be comparable to the probe size, and the effect of the 3 turns of the loop in the probe.
2.7.3. Selectivity
As we stated before, the selectivity of a probe is the ability of this probe to distinguish between the main component measured by the probe and the other field components. In other words, in the case of a magnetic field probe, the selectivity is how can it differentiate between the normal component of the magnetic field, that is perpendicular to the loop surface, from the tangential components.
z
y
Figure 2-25: Probe selectivity measurement configuration
54 Design & Characterization of Tri-Axial Magnetic Field Probes
To test the selectivity of the proposed probes we use the TEM cell. We know that the wave propagating in the cell is a TEM wave. The dominant electric field component is the vertical one (Ez) while the dominant magnetic field is the horizontal one (Hy). We place the studied probes in the cell in two orientations and we measure the transmission scattering parameter S21 in both cases. The first orientation is to place the probe such that the loop surface is orthogonal to the y-axis, and the other one we rotate the probe by 90o as shown in Figure 2-25.
The measured S21 for the first probe P1xy shows that the probe selectivity is acceptable below 300 MHz, in which it is more than 10 dB, but at higher frequencies it starts to decrease to reach about 3 dB at 1 GHz. This is expected as the probe has 3 turns which leads to such problems at high frequencies.
Figure 2-26: P1xy Probe selectivity
Increasing the number of turns enhances the probe sensitivity but on the other hand it degrades the selectivity. This appears clearly in the analysis for the second probe P2xy that shows good selectivity, and this is normal as it consists of a single turn loop. The selectivity of this probe is about 30 dB below 100 MHz and it decreases as the frequency increases but it stays more than 10 dB at 1 GHz.
Design & Characterization of Tri-Axial Magnetic Field Probes 55
Figure 2-27: P2xy probe selectivity
2.7.4. Validation
In order to test the performance of the proposed probes, we want to use a standard circuit that we can previously know the radiated magnetic field from the theoretical calculations, and then we compare the experimental measurements using the probes with these values. We choose a reference circuit that consists of a cylindrical conductor above a ground plane as a DUT. The cylindrical conductor is placed at a height h above the ground plane and it is terminated by 50 Ω load at one end and excited by a constant power Pin from a signal generator at the other end (Figure 2-28-a). The electromagnetic field components above this circuit depend on the geometrical parameters (Figure 2-28-b) and the injected power level.
50 Ω Pin
z y
x
(a)
56 Design & Characterization of Tri-Axial Magnetic Field Probes
probe Cylindrical conductor d Ground Plane (100mm 80mm) z
y h=2.05mm a=1.5mm
(b) Figure 2-28: (a) Reference Circuit; (b) Cross section, showing the geometrical parameters
The electromagnetic field above the reference circuit was computed theoretically using the expressions of the transverse electric and magnetic field components that are obtained by electrostatic calculation with the use of image theory [21][22], and they are given by: yzn EKy 8 2 2 2 2 ( 2-18 ) (y ( z n ) )( y ( z n ) )
n() y2 z 2 n 2 EKz 4 2 2 2 2 ( 2-19 ) (y ( z n ) )( y ( z n ) )
1 HEyz ( 2-20 )
1 HE zy ( 2-21 )
2PZ 22 in c 0 where n h a ; K and hn ln 0 hn
The propagating waves along the cylindrical conductor in the circuit are characterized by a quasi-TEM mode. So we will only measure the transverse components of the magnetic field (Hy and Hz).
Design & Characterization of Tri-Axial Magnetic Field Probes 57
The near-field scanning system presented in section 2.4 was used to perform the validation setup. The reference circuit was placed such that the cylindrical conductor is along x-axis, then the scan is done along y-axis from -40 mm to 40 mm, with a step size of 1 mm. Different signals was generated from SMIQ 06B, each having a specific frequency. The corresponding probe was attached to the robot arm and connected to channel 1 of the oscilloscope, and it was placed at a height d according to Table 2-4. The height is measured from the top of the conductor to the center of the probe loop as shown in Figure 2-29.
Table 2-4: Distance between the probes and the reference circuit Probe Height (d)
P1xy 4 mm
P1z 4 mm
P2xy 3 mm
P2z 2 mm
z x y 50 Ω Probe Vin.
Cylindrical Input Conductor z Vin. x Input d 50 Ω
Figure 2-29: Validation setup using P1xy probe
The validation was performed at 6 different frequencies; 1, 10, 50, 100, 500
MHz, and 1 GHz. The y-component of the magnetic field Hy measured with P1xy is almost in good agreement with the theoretical values as shown in Figure 2-30. However, at high frequencies (500 MHz and 1 GHz) the 2 valleys that are clear in
58 Design & Characterization of Tri-Axial Magnetic Field Probes
the theoretical plot are not detected by the probe, and this is due to the small wave length at these frequencies. Also at low frequencies the sensitivity of the probe is not good but the peak of the field is still detectable. Adding a LNA between the probe and the measuring instrument can solve the problem of sensitivity.
Figure 2-30: Theoretical values compared to measurement results of Hy using P1xy at different frequencies
Figure 2-31: Theoretical values compared to measurement results of Hz using P1z at different frequencies
Design & Characterization of Tri-Axial Magnetic Field Probes 59
The z-component of the magnetic field Hz measured with P1z seems to be good until hundreds of MHz, but at higher frequencies the sharp valley is not detected. This is due to the small wavelength at these frequencies if compared to the probe size and on the other hand the measurement is affected by the bend made in this probe for detecting the z-component as presented in 2.7.1 .
On the other hand, measurements with the PCB probe P2xy are better than those measured using P1xy, this can be clearly seem in Figure 2-32. At 1 MHz the probe is more sensitive and at high frequencies the measured field is in good agreement with the theoretical one.
For z-component measurements using P2z, at low frequencies the results are quite good but it seems that the probe is little bit sensitive, while until hundreds of MHz the measured values are very good if compared to the theoretical values as shown in Figure 2-33. However at high frequencies the probe is affected by the DUT itself, as it is clear that one peak is detected while the other is not and this is due to the coupling between the cylindrical line and the probe.
Figure 2-32: Theoretical values compared to measurement results of Hy using P2xy at different frequencies
60 Design & Characterization of Tri-Axial Magnetic Field Probes
Figure 2-33: Theoretical values compared to measurement results of Hz using P2z at different frequencies
Design & Characterization of Tri-Axial Magnetic Field Probes 61
2.8. DEVELOPMENT OF TRI-AXIAL MAGNETIC FIELD PROBES
In near-field characterization it is necessary to have all the three components of magnetic or electric field. This requires a large number of scans if using single probes, which measures one component of the field at a time, and therefore large scanning time in order to complete one surface near the DUT. In order to minimize the scanning time we propose a three axis probes which measures the three components of the magnetic field at the same time. In this section we present the design and the calibration of these probes. They are based on the single probes presented in the previous section.
2.8.1. Design
10 10 mm
z
x y
Figure 2-34: Fabricated 3D H-field probe P1
The first tri-axial probe P1 was designed based on the classical circular loops
P1xy and P1z. It consists of 2 P1xy probes for (Hx , Hy) detection and one P1z probe 3 for Hz component. They are combined in a 1×1.2×1.3 cm plastic cube such that the loop surfaces are orthogonal and having their centers at the same plane (Figure 2-34), as mentioned before, each loop has 3 turns with a radius of 3 mm. x and y probe elements was placed at the edges of the plastic cube whereas the z element is placed in the corner facing the previous edges, this minimizes the coupling to the minimum. The difference between the loops centers is solved in post processing as if we have concentric elements. The terminals of each element are connected to a coaxial cable
62 Design & Characterization of Tri-Axial Magnetic Field Probes
with an SMA connector at the other end to be connected to the measuring instrument. Also a 10 cm plastic road holds the probe and it is used to be attached to the scanning robot arm, in addition the probe and the road was covered with a plastic shield that makes it possible to perform measurement inside liquids. For better spatial resolution as well as suffering from fabricating the probe manually (that is the case in P1), we propose another tri-axial probe P2 that is made in PCB technology. The printed circuit probe P2 consists of 2 P2xy probes and one
P2z probe, the 3 loops oriented in 3 different directions x, y, and z. As stated before, each square loop have a dimension of 3.2 mm×3.2 mm and it is fabricated on both sides of an FR4 substrate with a thickness of 3.2 mm. The probe has an overall dimension of 9×9×3.2 mm3. The connecting pads for all individual loops are provided at the top surface in order to be able to connect it to the coaxial cable. The volume of this probe is only 16 % of the previous tri-axial probe P1. Compared to the conventional circular loop probes printed circuit probes are easy to miniaturize and less susceptible to damage. Also, these probes have the advantage of being low cost, compact and easy to handle.
3.2 mm
z
x y
Figure 2-35: Fabricated 3D H-field probe P2
The electrical connection of the loop terminals in the probes is critical especially in near-field measurements. The field profile can be distorted due to a lack in this connection. Also another loop can be formed due to this connection. For this reason we optimize the connection to reduce the coupling effect in the measured medium as
Design & Characterization of Tri-Axial Magnetic Field Probes 63
much as possible. The terminals of each loop are connected to a 2.5 mm (diameter) coaxial cable.
2.8.2. Coupling Analysis Coupling analysis between the loop elements of each tri-axial probe was done with the help of the VNA. Each two loop elements of the tri-axial probes was connected to the two ports of the VNA whereas the third element is terminated by 50 ohm impedance, then the transmission coefficient is measured. Figure 2-36 shows the coupling between the loops of the probe P1, the coupling is less than -30 dB until 700 MHz and it increases at higher frequencies. For P2 the coupling is less than -40 dB till 350 MHz and less than -30 dB until 600 MHz as shown in Figure 2-37. This is expectable as in P1 the loop elements have 3 turns while in P2 there is just one turn.
Figure 2-36: Coupling between P1 loop elements
Figure 2-37: Coupling between P2 loop elements
64 Design & Characterization of Tri-Axial Magnetic Field Probes
2.8.3. Calibration
The calibration setup for these tri-axial probes is the same used in section 2.7.2 . Both probes were placed inside the FCC-TEM-JM1 TEM cell in the usable area, which is 6×6×1 cm3, whereas the cables of the probe are outside the cell. P1 have a height of 1 cm so it can fit in the specified usable volume of the cell. The field inside the cell is calculated as stated in 2.6 . The measured antenna factor as a function of frequency of the two probes P1 & P2, and for a frequency range of [10MHz –1GHz] is shown in Figure 2 -38 and Figure 2 -39 respectively.
Figure 2-38: Measured antenna factor versus frequency for the probe P1
Figure 2-39: Measured antenna factor versus frequency for the probe P2
Design & Characterization of Tri-Axial Magnetic Field Probes 65
It is obvious that the antenna factor for probe P1 is smaller than probe P2 because of its larger dimension compared to the printed circuit probe P2. The AF for probe P1 is 46 dB[A/(m.V)] at 10 MHz and reduces by 20 dB for every 10 times increase in frequency. In the case of probe P2 the AF is 58 dB[A/(m.V)] at 10 MHz (for x and y elements) and decreases by 20 dB for every 10 times increase in frequency. In Figure 2-38 the AF of each loop in the probe P1 is plotted along with the theoretical AF of a single loop. Compared to the theoretical AF, the antenna factor of each loop in the tri-axial probe has small fluctuations above 300 MHz in probe P1; these are due to the coupling between the loops probe. Similarly, in the case of printed probe P2, these coupling effects are increased above 300 MHz.
2.8.4. Validation
To validate these probes we use the same validation test bench used in 2.7.4 . The magnetic near-field components of the reference circuit are measured and compared to theoretical ones. The measuring instrument which is the WaveRunner Z640i Oscilloscope contains 4 channels; three of them were used in this setup. The plastic arm of the robot holds the probe above the DUT as shown in Figure 2-40. The tri- axial probe terminals are connected to the oscilloscope, via coaxial cable, such that each channel corresponds for one component of the magnetic field as shown in the diagram below: Ch###1 Ch 2 Ch 3
HHHx y z
A reference probe can be connected to the fourth channel of the oscilloscope if measuring in time domain; hence the phase can be also measured. As the 3 loops in the tri-axial probe don‘t have the same center, the difference between the loop centers was taken into account in post processing. Knowing that in P1 the centers are in the same plane. And in P2, the centers of the loops corresponding to Hx and Hy are in the same plane and the difference of Hz component was taken into account in calculating the theoretical Hz value.
66 Design & Characterization of Tri-Axial Magnetic Field Probes
Tri-axial H-field Probe P1 Input Power
50 Ω Ground
Figure 2-40: Standard validation circuit with the tri-axial probe P1
The z-axis is normal to the scanned plane. Measurements were done at three different heights (4, 6, and 8 mm from the ground), and the scan was along y-axis with a step of 1 mm. The computed theoretical field values are compared with the measured ones at 13 MHz and they are presented in Figure 2-41 and Figure 2-42.
(a)
Design & Characterization of Tri-Axial Magnetic Field Probes 67
(b)
Figure 2-41: Tangential H-field component Hy for (a) Probe P1 (b) Probe P2
(a)
68 Design & Characterization of Tri-Axial Magnetic Field Probes
(b)
Figure 2-42: Normal H-field component Hz at for (a) Probe P1 (b) Probe P2
Figure 2-41 shows the tangential magnetic field component Hy, while
Figure 2-42 shows the normal one Hz. The comparison shows that the field measurements are in good agreement with the theoretical calculations for both probes. Also good resolution is obtained for the probes in measuring the magnetic field. However, measurements with P2 are more precise than those using P1. We can notice that by comparing Figure 2-41 (a) and (b) graphs as well as Figure 2-42 (a) and (b) graphs.
Design & Characterization of Tri-Axial Magnetic Field Probes 69
2.9. CONCLUSION
In this chapter we have presented the development of a new near-field scanning system. It consists of two types of scanners, a 3D one for small size applications and a 2D scanner for larger applications. In addition, the measuring instrument in the proposed system is an oscilloscope that can perform both time and frequency domain measurements. This system is dedicated for both EMC measurements and dosimetric applications.
In the second part of the chapter, a new tri-axial magnetic field probes has been proposed. They can be used to measure the three components of the magnetic field separately and at the same time. The calibration of these probes was performed using a TEM cell where the antenna factors are extracted. After that the coupling analysis between the probe elements was studied and they were validated showing a good performance.
The developed scanning system including the new probes was used for the near- field characterization and exposure analysis near radiating sources as demonstrated in the next chapters.
70 Design & Characterization of Tri-Axial Magnetic Field Probes
2.10. REFERENCES
[1] E.L.Nativel,―Imagerieélectromagnétiquehyperfréquence en champ proche :duconceptdesondelocaleauxapplicationsindustrielles,‖Thèsededoctorat en Électronique, optronique et systèmes, Université de Montpellier II, 2002.
[2] C. A. Balanis, Antenna theory: analysis and design, 3rd ed. Hoboken, NJ: John Wiley, 2005.
[3] ―Alira.‖[Online].Available: http://www.alira.fr/.
[4] ―AutomationManualForWaveExpertSeriesOscilloscope.‖LeCroy Corporation, Mar-2009.
[5] A.Ramanujan,Z.Riah,A.Louis,andB.Mazari.,―Modelingthe Electromagnetic Radiation of Passive Microwave Components Using a Near-Field ScanningMethod,‖IEEE Trans. Electromagn. Compat., vol. 52, no. 4, pp. 1056– 1059, Nov. 2010.
[6] Y. Vives-Gilabert, C. Arcambal, A. Louis, F. de Daran, P. Eudeline, and B. Mazari,―ModelingMagneticRadiations of Electronic Circuits Using Near-Field ScanningMethod,‖IEEE Trans. Electromagn. Compat., vol. 49, no. 2, pp. 391–400, May 2007.
[7] H.Shall,Z.Riah,andM.Kadi,―Predictionof3D-near field coupling between a toroïdal inductor and a transmissionline,‖2013,pp.651–656.
[8] A.TaagholandT.K.Sarkar,―Near-field to near/far-field transformation for arbitrary near-fieldgeometry,utilizinganequivalentmagneticcurrent,‖IEEE Trans. Electromagn. Compat., vol. 38, no. 3, pp. 536–542, Aug. 1996.
[9] ―WaveRunner6ZiHROOscilloscopesOperator‘sManual.‖Teledyne LeCroy, Sep-2015.
[10] ―WaveRunner6ZiOscilloscopes- Datasheet.‖TeledyneLeCroy.
[11] R.A.Pahl,J.L.Rovey,andD.J.Pommerenke,―Calibrationofmagnetic field probes atrelevantmagnitudes,‖2013,pp.1–6.
[12] IEEE Electromagnetic Compatibility Society, Standards Development Committee, Institute of Electrical and Electronics Engineers, and IEEE-SA Standards Board, IEEE standard for calibration of electromagnetic field sensors and probes (excluding antennas) from 9 kHz to 40 GHz. 2013.
[13] M.Crawford,J.Workman,andC.Thomas,―ExpandingtheBandwidthof TEMCellsforEMCMeasurements,‖IEEE Trans. Electromagn. Compat., vol. EMC-20, no. 3, pp. 368–375, Aug. 1978.
Design & Characterization of Tri-Axial Magnetic Field Probes 71
[14] J.P.Karst,C.Groh,andH.Garbe,―CalculablefieldgenerationusingTEM cells applied to the calibration of a novel E-fieldprobe,‖IEEE Trans. Electromagn. Compat., vol. 44, no. 1, pp. 59–71, Feb. 2002.
[15] E.D.Mantiply,―AnAutomatedTEMCellCalibrationSystem,‖US Environmental Protection Agency, Las Vegas, EPA 520/1-84-024, Oct. 1984.
[16] M.L.Crawford,―GenerationofStandardEMFieldsUsingTEM TransmissionCells,‖IEEE Trans. Electromagn. Compat., vol. EMC-16, no. 4, pp. 189–195, Nov. 1974.
[17] ―11945AClose-FieldProbeSet,9kHzto1GHz,‖Keysight Technologies. [Online]. Available: www.keysight.com.
[18] C.F.M.Carobbi,L.M.Millanta,andL.Chiosi,―Thehigh-frequency behavior of the shield in the magnetic-fieldprobes,‖IEEEInt.Symp. Electromagn. Compat. Symp. Rec. (Cat. No.00CH37016), vol. 1, pp. 35–40, 2000.
[19] Y.T.ChouandH.C.Lu,―Magneticnear-field probes with high-pass and notch filters for electricfieldsuppression,‖IEEETrans.Microw.TheoryTech.,vol. 61, no. 6, pp. 2460– 2470, 2013.
[20] ―R&S®ZVLVectorNetworkAnalyzers,‖Rohde & Schwarz. [Online]. Available: https://www.rohde-schwarz.com/us/product/zvl-productstartpage_63493- 9014.html.
[21] D. Baudry, C. Arcambal, A. Louis, Bé. Mazari, and P. Eudeline, ―Applications of the Near-FieldTechniquesinEMCInvestigations,‖IEEE Trans. Electromagn. Compat., vol. 49, no. 3, pp. 485–493, Aug. 2007.
[22] D.Baudry,A.Louis,andB.Mazari,―CHARACTERIZATIONOFTHE OPEN-ENDED COAXIAL PROBE USED FOR NEAR-FIELD MEASUREMENTS INEMCAPPLICATIONS,‖Prog. Electromagn. Res., vol. 60, pp. 311–333, 2006.
72 Design & Characterization of Tri-Axial Magnetic Field Probes
Chapter 3: Radiated EMF Prediction Method Based on PWS Algorithm
Contents
Chapter 3: Radiated EMF Prediction Method Based on PWS Algorithm ... 73 3.1. Introduction ...... 74 3.2. Electromagnetic wave propagation ...... 75 3.3. Principle of the Plane Wave Spectrum Algorithm ...... 76 3.4. Algorithm Implementation ...... 80 3.4.1. F(,) k k Calculation at z=0 ...... 80 h x y
3.4.2. Fh(,) k x k y Calculation at z=d ...... 81 3.4.3. H(,,) x y d Calculation ...... 82 3.5. Validation of the Algorithm ...... 83 3.5.1. Loop Antenna ...... 83 3.5.2. PIFA Antenna ...... 87 3.6. Conclusion ...... 89 3.7. References ...... 90
Radiated EMF Prediction Method Based on PWS Algorithm 73
3.1. INTRODUCTION
Near field characterization is a challenging process in electromagnetic compatibility studies. Several approaches have been proposed to achieve the required scanning of the radiating source throughout the space. Radiated emission and immunity tests of electronic circuits are important tasks of system electromagnetic compatibility analysis and they have become growing concerns in prediction of system reliability [1]–[3]. In addition, dosimetric analysis shows the necessity of having three dimensional (3D) field mapping in the vicinity of the radiating source. For this kind of applications, several scans of the near fields should be done within different planes in order to build the 3D field mapping. Being a challenging process in electromagnetic compatibility studies, near field characterization is being treated by several algorithms that propose different approaches to achieve the required scanning on the radiating source, where great efforts have been made in recent years in the scientific community to develop numerical methods to solve temporally and spatiallyMaxwell‘sequations.
In this context, several near-field scanning systems were introduced [4], [5]. In the previous chapter, we introduced a 3D scanning system with a tri-axial magnetic field probes. Having such probes allow the simultaneous measure of the three components of the magnetic field based on a single planner scan (scan in one plane only) above the DUT. However, to perform multi-planes scan is time consuming and it would be better to find an analytical solution that reduces the number of scans needed and hence the time requirements. For this reason it is better to find an analytical solution that reduces the number of scans needed. Here comes the Plane Wave Spectrum (PWS) method which has been found useful for determining the far field from near field measurements [6], [7]. The reconstruction algorithm implemented by PWS requires just the 2D scan of the tangential components, in an initial plane, to reconstruct the 3D magnetic field distribution.
In this chapter we present the near field characterization near two different antennas. The first one is a square loop antenna operating in HF band, while the second one is a PIFA (planar inverted-F antenna) antenna that operates at UHF band. The PWS method is applied to extract the three components of the magnetic field in
74 Radiated EMF Prediction Method Based on PWS Algorithm
the near field region, at different planes above the antenna, from the tangential components at an initial plane near the radiating source.
3.2. ELECTROMAGNETIC WAVE PROPAGATION
The propagation of an electromagnetic wave in a given medium of permittivity ε [F/m], conductivity σ [S/m], and permeability µ [H/m] can be described by Maxwell‘sequations,whichcanbewrittenintheirdifferentialorintegralformsas follows:
Differential form Integral form
D HJ D t H... dl dS J dS ―Ampere‘slaw‖ ( 3-1 ) LSSt
B E B ―Faraday‘slawof t E.. dl dS ( 3-2 ) LSt induction‖
.0B B.0 dS ―Gauss‘slawfor ( 3-3 ) S magnetism‖
.D D. dS dV ―Gauss‘slaw‖ ( 3-4 ) SV
Where H is the magnetic field in [A/m] E is the electric field in [V/m] B is the magnetic induction in [T] D is the electric flux density in [C/m2]
J is the electric current density in [A/m2]
and is the electric charge density [C/m3]
In order to completely model the electromagnetic phenomena, it is necessary to add the following equations, for isotropic and homogenous materials, the relation between the field quantities and flux densities is given by the following relations:
BH ( 3-5 )
Radiated EMF Prediction Method Based on PWS Algorithm 75
DE ( 3-6 )
JE ―Ohm‘slaw‖ ( 3-7 )
Relative permittivity εr and permeability µr are normally used for simplicity, they are normalized to their values in vacuum (ε0 and µ0), i.e. r 0 and r 0 . All non- magnetic material has their relative permeability µr close to 1. For loss-less materials ε and µ are real numbers however most of materials exhibit dielectric losses called loss-tangent tan and that can be defined in terms of the complex permittivity j as follows: tan ( 3-8 ) whereωistheangularfrequency.
3.3. PRINCIPLE OF THE PLANE WAVE SPECTRUM ALGORITHM
Electromagnetic fields can be represented as plane waves propagating in the space in all directions and with different amplitudes. PWS transformation allows the decomposition of the electromagnetic field components mapped in a plan as being the sum of 2D harmonic spectrums of the propagating plane waves. Hence the knowledge of the transverse field components over a plane near a radiating antenna can be used to extract the plane wave spectrum of the radiated electromagnetic fields. In harmonic regime, plane waves satisfy Helmholtz equation given by:
22 A k A 0 ( 3-9 ) where A is the field quantity vector, which can be either electric or magnetic field, and k is the wavenumber.
A general solution of ( 3-9 ) can be:
76 Radiated EMF Prediction Method Based on PWS Algorithm
jkr A()() r FA k e ( 3-10 ) where ⃗ is the wave vector, given by: k k u k u k u x x y y z z
222 ( 3-11 ) k kx k y k z
r is the position vector, expressed by: r xu yu zu x y z ( 3-12 )
and FA is the plane wave spectrum vector of the field A .
From the wave vector relation one can notice that knowing the frequency and two of the components it is possible to calculate the third one. Assuming that the direction of propagation is along uz direction, the transverse field components will be in the xy plane, so we can obtain the following:
k2 k 2 k 2 if k 2 k 2 k 2 x y x y k z ( 3-13 ) i k2 k 2 k 2 other xy
In the case of the non-evanescent wave (kz is real) we could have chosen the opposite sign for kz. This arbitrary choice is dictated by the fact that we implicitly assume that the source of the field is "down", that is to say in the half-plane z < z0, and thus the waves propagate "upwards". With regard to the evanescent wave (kz is imaginary), it is desired that the wave amplitude vanishes for z → + ∞, which explains the minus sign.
In this work, we will focus on the magnetic field quantity. At a given frequency, any solution of the Helmholtz equation ( 3-9 ) could be represented by a superposition of plane waves and can be determined by Fourier transform that leads to a solution expressed in terms of the plane wave spectrum [8]:
Radiated EMF Prediction Method Based on PWS Algorithm 77
1 j() k x k y k z Hxyz(,,)(,) Fkkex y z dkdk 2 h x y x y 4 ( 3-14 ) 1 j() k x k y F(,) k k e jkz z exy dk dk 2 h x y x y 4
F(,) k k e jkz z h x y ( 3-15 )
where ℱ denotes the Fourier transform with respect to the variables kx and ky.
Similarly, the function Fkh () can be represented by inverse Fourier transform of H
with respect to x and y. In particular, for z = z0 we have:
Fkk(,)(,,)(,,) Hxyzej() kxy x k y dxdy1 Hxyz h x y 00 z0 ( 3-16 )
By applying ( 3-16 ) to the magnetic field components Hx and Hy at a plane P0
we can obtain the plane wave spectrum components Fhx and Fhy. Moreover, by substituting ( 3-10 ) inGuess‘s lawformagnetismfromMaxwell‘sequations ( 3-3 ), we can find a relation between the three spectral components of the magnetic field:
kF.0 h ( 3-17 )
k F k F k F 0 x hx y hy z hz ( 3-18 )
k F k F F x hx y hy hz k ( 3-19 ) z
The magnetic field component Hz can then be reconstructed from ( 3-19 ) by using the inverse of the plane wave spectrum (IPWS) method:
1 j() kxy x k y Hz F(,) k k e dk dk 2 hz x y x y ( 3-20 ) 4
78 Radiated EMF Prediction Method Based on PWS Algorithm
Moreover, the spectral components of the magnetic field at a high z0 above the plane P0 can be obtained from the spectral components at z0 by the following relationship:
F(,)(,) k k F k k ejkz (z z0 ) h x y h x y ( 3-21 ) zz0
The magnetic field components at height d can then be calculated by IPWS method as in ( 3-20 ). In the near field region the plane P0 at a height z0 above the DUT should satisfy the following relation: z 0 2 ( 3-22 ) where λ is the wavelength of the signal at the operating frequency f. Figure 3-1summarizes the whole PWS algorithm and its inverse showing the spatial and spectral components.
Spatial domain Spectral domain
(,,)x y z (,,)kx k y k z PWS FF, HH, hx00 hy xy00
F H hz0 z0 IPWS FFF,, HHHx,, y z hx hy hz
Figure 3-1: PWS method for magnetic field extraction
Radiated EMF Prediction Method Based on PWS Algorithm 79
3.4. ALGORITHM IMPLEMENTATION The corresponding formulations of the presented algorithm were developed via MATLAB and they are presented below.
3.4.1. F(,) k k Calculation at z=0 h x y
Based on ( 3 -16 ) the calculation of the Fourier transform was done by an approximation using the two-dimensional FFT. At the initial plane P0, with z=z0, we have:
Fkk(,)(,,) Hxyzej() kxy x k y dxdy h x y 0 z0 ( 3-23 ) j() kx x m k y y n H(,,) xmn y z0 e x y mn
z
y
y x Ly z0 Lx x
Figure 3-2: Spatial domain parameters representing the plane P0 at z=z0
Suppose that we know the values of the magnetic field on Nx × Ny equally spaced grid on the rectangle [0; Lx] × [0; Ly], such that:
Lx Lx x ; xmm Nx Nx
Ly Ly y ; ynn N y N y
The goal is then to find a way to approach equation ( 3 -26 ) with a two- dimensional IFFT. For this purpose, we impose the following equality:
80 Radiated EMF Prediction Method Based on PWS Algorithm
LL i m i n jkxky jkxx mk yy n 2 j x m y m x x ( 3-24 ) NNNNx y x y
2 2 ki ; ki xx yy ( 3-25 ) Lx Ly then: 2 j ( kx x m k y y n ) Fiixy H( xmn , y ,0) e x y mn
im iny 2 jx 1 NNxy L L Hmn e xyNN ( 3-26 ) xymn imx iny N 1 N y 1 2 j x 1 NNxy LL Hemn xyLLIFFTH . mn x y x y NNxymn00 where 22 Lx Ly Fii F i, i Hmn H m, n xy h x y ; LLxy NNxy
3.4.2. Fh(,) k x k y Calculation at z=d
In order to obtain the value of Fh(,) k x k y , we must apply the transformation zd in ( 3 -13 ).Wecould applythis formulaas itis, but theresultwouldn‘t begood.
Indeed, if iNxx0; 1 , iNyy0; 1 according to ( 3 -25 ) the values of kx and ky would necessarily be positive. To solve this problem, let us observe that in ( 3 -26 ) the result is not modified if we replace ix by ix±Nx, because:
i N m i m i m 2jxx 2 jxx 2 j eNNNx e x e2 jm e x ( 3-27 )
And the same argument holds for iy i y N y
NN NN So we can take i xx; and i yy; . x 22 y 22
This amounts to replacing ( 3 -25 ) with the following equation:
Radiated EMF Prediction Method Based on PWS Algorithm 81
2 2 i if i N /2 ix if i x N x /2 y y y Lx Ly k ; k x y ( 3-28 ) 2 2 ixx N other iyy N other L L x y
By calculating kx and ky with this last equation, then kz with ( 3 -13 ) we can
calculate the plane wave spectrum Fdiixy()on any plane z = d, with d ≥ z0.
This last condition d ≥ z0 can be somewhat relaxed, as long as the plane z = d remains in the vacuum, otherwise the Helmholtz equation must be modified to take into account the discontinuities of ε and μ.
3.4.3. H(,,) x y d Calculation
We could repeat the same calculations that led us to ( 3 -26 ), but it is much simpler to observe that for d = z0 we must obtain the starting field. So, we get: 11 Hmn...() FFT L L IFFT H mn FFT F i i z xy xy 0 ( 3-29 ) LLLLx y x y so for any value of d we will have: 1 Hmn().() d FFT F i i d xy ( 3-30 ) LLxy
Indeed, the direct calculation gives the same result:
L L j kx m ky n k d L x y z Lx y 1 NNxy Hmn (),,(,) d H m n d F k k e dk dk NN 4 2 h x y x y xy 22Lx Ly N 1 N y 1 j i m i n k d x x y z 1LNLNx x y y 2 2 Fii (). d e 2 xy 4 iixy00 LLxy ( 3-31 ) 2ix 2iy Nx 1 N y 1 j m n NN 1 jkz d xy Feii e xy LLxyiixy00 1 .()FFT Fii d xy LLxy
82 Radiated EMF Prediction Method Based on PWS Algorithm
3.5. VALIDATION OF THE ALGORITHM
To validate the presented algorithm, we carried out 2 tests on different antenna types operating at different frequencies. The first one is a rectangular loop antenna operating at 13.56 MHz, and the second is a PIFA antenna operating at 840 MHz. The calculations of the magnetic field profile at different planes above the antenna were performed and compared to the simulated ones.
3.5.1. Loop Antenna These antennas are made of a loop with a conductor that may have various shapes: circular, rectangular, triangular, elliptical, etc. They are widely used in communication links up to the microwave bands and also as electromagnetic field probes. W
t L
Feeding
port Figure 3-3: Single turn rectangular loop antenna with the corresponding radiation pattern diagram
Loop antennas are usually classified as electrically small and electrically large loop antennas. Electrically small loop antennas are those who have their circumference C less than tenth of the wavelength (C <0.1λ),whileinelectrically large antennas the circumference is near the wavelength (C ≈λ). Small loop antennas are equivalent to a magnetic dipole and they have a far-field radiation pattern very similar to that of a small electric dipole normal to the plane of the loop as shown in Figure 3-3 [9]. In this work, a rectangular small loop antenna operating at 13.56 MHz was modeled on MATLAB and simulated on HFSS in order to test the PWS algorithm.
Radiated EMF Prediction Method Based on PWS Algorithm 83
This antenna is a typical RFID reader antenna used in library RFID systems [10]. The antenna was modeled on MATLAB based on the theoretical formulations loop antennas given in [9]. The loop has a length L=140mm, width W=140mm and a copper trace thickness t=5mm. The feeding point is at the center of one of the sides. The antenna is excited by a time-harmonic voltage signal with the amplitude of 1 V and a frequency of 13.56 MHz at the feeding port. The HFSS designed structure is shown in Figure 3-4 including three different planes above the antenna at heights of
1, 5, and 10 centimeters and with 2D dimensions of Lx = 40cm and Ly = 40cm. The three magnetic field components Hx, Hy, and Hz are exported at these planes with a grid size of 10mm. In order to obtain high accuracy in the exported data, the planes are meshed by a length based operation with a maximum length of 10mm.
z
x y 0 150 300(mm)
Figure 3-4: RFID Loop Antenna structure
The numerical execution of the PWS algorithm is performed on the data obtained from the simulated structure and it is then compared to the theoretical ones. The initial input parameters for the numerical calculations are:
the tangential H-field components, Hx & Hy
the spatial parameters: Lx, Ly, Δx, and Δy the wavelength λ that is λ= c / f
The plane P0 is at height z0=1cm above the DUT and have the dimensions of
Lx×Ly with Lx = Ly = 40cm. The plane is discretized in both x & y axis with an equal
84 Radiated EMF Prediction Method Based on PWS Algorithm
step size of Δx = Δy = 10mm. The discrete indices along these two axes are nx & ny respectively: L L n x ; n y x x y y The discretized tangential wave vector components used in the numerical computation are calculated using the following formulas: 2 2 kx ; ky Lx Ly The spatial and discretization parameters should be chosen according to the 2 following condition: Max kxy, k
The first step in the proposed algorithm is to obtain the normal magnetic field Hz from the tangential components. As introduced in section 3.3, this occurs by calculating the plane wave spectrum of the tangential components using ( 3-16 ), and then by applying ( 3-19 ) we obtain the normal spectral component of the magnetic field Fhz, after that Hz is reconstructed with the inverse of PWS as stated in ( 3-20 ). The results are shown in Figure 3-5; it shows the theoretical, simulated, and the calculated magnitudes of Hz. It is clear that the reconstructed filed is in good agreement with the theoretical one. |H | z theoretical A/m |Hz| HFSS A/m
|Hz| PWS A/m
A/m
Figure 3-5: Comparison between theoretical, simulated, and calculated values of the Hz magnetic field
component at P0 (units of x & y axis is m)
Radiated EMF Prediction Method Based on PWS Algorithm 85
|Hx| |Hy| |Hz|
theoretical
HFSS
PWS
Figure 3-6: Comparison between theoretical, simulated, and calculated magnetic field profiles at z=5cm above the DUT (units: colorbar in A/m, x & y axis in m)
The other part of the PWS method is to extract the field components at different planes in the space above the DUT just from the field quantities in a single plane near the radiation source. By applying ( 3-21 ) we can obtain the spectral components of the magnetic field at the needed height z. Then by performing IPWS we can have the magnetic field at this height. In RFID near-field characterization, we aim to study the field levels in the near-field region in order to see the dosimetric levels of the radiated magnetic field. For this reason we would chose 2 planes that relay in the normal region of operation of the RFID systems. We chose the plane P1 and P2 that are respectively at 5 and 10 centimeters above the reader antenna. Then the reconstruction algorithm is applied based on the magnetic field quantities at P0. The results are shown in Figure 3-6 and Figure 3-7, good agreement can also be found by comparing the theoretical, simulated, and extracted magnetic field profiles at the chosen planes P1 and P2. We can observe a little noisy profile in the normal extracted magnetic field Hz at the edges of the computed area; this could be solved by adding zero padding to the initial data at P0.
86 Radiated EMF Prediction Method Based on PWS Algorithm
|H | |H | |H |
x y z
theoretical
HFSS
PWS
Figure 3-7: Comparison between theoretical, simulated, and calculated magnetic field profiles at z=10cm above the DUT (units: colorbar in A/m, x & y axis in m)
3.5.2. PIFA Antenna
0 40 80mm Figure 3-8: PIFA antenna design structure
Radiated EMF Prediction Method Based on PWS Algorithm 87
PIFA antennas are widely used in mobile devices and modern RF electronics. This is due to their space saving properties and their compact size that can be integrated in the device. In this section we present the validation of the PWS algorithm applied on this type of antennas operating at UHF band, in particular at 840 MHz. The antenna was designed on HFSS, it has an overall dimensions of
100mm×70mm with a PVC substrate (εr=3.3) that have a thickness of 10mm. The patch element is 16mm×40mm shorted with a strip having a width of 5mm. The ground plane on the bottom side has the dimensions of 70mm×55mm. The antenna design is shown in Figure 3 -8 with the corresponding geometrical parameters.
Figure 3-9: Simulation and calculated values of the Hz magnetic field component at P1 above the PIFA antenna (colorbar unit: A/m)
The tangential H-field components were extracted in the initial plane P1 at z0=5mm above the antenna surface. P1 have the dimensions of Lx×Ly with Lx = Ly = 138mm. The plane is discretized in both x & y axis with an equal step size of Δx =
Δy = 2mm, and thus having Nx = Ny = 70. With the same procedure described in the previous section, the Hz component at P1 was calculated based on the tangential components using the PWS algorithm and it is compared to the simulation values as shown in Figure 3 -9. Moreover, all the magnetic field components were extracted in a plane P2 at z=10mm above the antenna surface and the results are also compared to simulations and are presented in Figure 3 -10.
88 Radiated EMF Prediction Method Based on PWS Algorithm
Figure 3-10: Simulation and calculated values of the magnetic field components at P2 above the PIFA antenna (colorbar unit: A/m)
3.6. CONCLUSION
From a dosimetric point of view, the knowledge of the electromagnetic field distribution in the area near the radiating element/device is essential. In this context, we have presented in this chapter the PWS method applied on the near-filed data above a radiating device. Two antenna types were chosen, each operating at a different frequency addressing by that two frequency bands, HF and UHF. The algorithm was computed to extract the 3D magnetic field components from the tangential ones at an initial plane above the antenna under test (AUT), as well as the extraction of the magnetic field components at different heights above the AUT. The calculations of the magnetic field were performed via MATLAB and the results are compared to the simulation, showing a good agreement.
Radiated EMF Prediction Method Based on PWS Algorithm 89
3.7. REFERENCES
[1] W.-J. Zhao et al.,―AnEffectiveandEfficientApproachforRadiated Emission Prediction Based on Amplitude-Only Near-FieldMeasurements,‖IEEE Trans. Electromagn. Compat., vol. 54, no. 5, pp. 1186–1189, Oct. 2012.
[2] W.-J. Zhao et al.,―Adifferentialevolutionbasedequivalentsourceapproach for predicting electromagnetic emissions using near-fieldscanning,‖in2013 IEEE International Symposium on Electromagnetic Compatibility, Denver, CO, USA, 2013, pp. 182–186.
[3] J.J.H.Wang,―Anexaminationofthetheoryandpracticesofplanarnear- fieldmeasurement,‖IEEE Trans. Antennas Propag., vol. 36, no. 6, pp. 746–753, Jun. 1988.
[4] T.Ordas,M.Lisart,E.Sicard,P.Maurine,andL.Torres,―Near-Field Mapping System to Scan in Time Domain the Magnetic Emissions of Integrated Circuits,‖inIntegrated Circuit and System Design. Power and Timing Modeling, Optimization and Simulation, vol. 5349, L. Svensson and J. Monteiro, Eds. Berlin, Heidelberg: Springer Berlin Heidelberg, 2009, pp. 229–236.
[5] T.Stadtler,L.Eifler,andJ.L.terHaseborg,―Doubleprobenearfield scanner,anewdeviceformeasurementsintimedomain,‖in2003 IEEE Symposium on Electromagnetic Compatibility. Symposium Record (Cat. No.03CH37446), Boston, MA, USA, 2003, vol. 1, pp. 86–90.
[6] R.Rudduck,D.C.Wu,andM.Intihar,―Near-field analysis by the plane- wavespectrumapproach,‖IEEE Trans. Antennas Propag., vol. 21, no. 2, pp. 231– 234, Mar. 1973.
[7] P. C. Clemmow, The Plane Wave Spectrum Representation of Electromagnetic Fields. Wiley-IEEE Press, 1997.
[8] BurdetteE.,―PlaneWaveAnalysisofNear-zone Fields Using Wavenumber Band-passFiltering,‖GeorgiaInstituteofTechnology,1976.
[9] C. A. Balanis, Antenna theory: analysis and design, 3rd ed. Hoboken, NJ: John Wiley, 2005.
[10] G.Viziteu,B.Florean,G.M.Moraru,andA.Pintea,―AnoverviewofRFID technologyusedinlibrary,‖in2012 International Conference and Exposition on Electrical and Power Engineering, Iasi, 2012, pp. 108–111.
90 Radiated EMF Prediction Method Based on PWS Algorithm
Chapter 4: RFID Near-field Characterization
Contents
Chapter 4: RFID Near-field Characterization ...... 91 4.1. Introduction ...... 92 4.2. RFID Systems ...... 93 4.2.1. RFID Readers ...... 95 4.2.2. RFID Tags ...... 96 4.3. RFID Reader Antennas ...... 98 4.4. In Situ Measurements ...... 100 4.4.1. Instrumentation Used ...... 100 4.4.2. Measurementsat―VaiseLibrary‖ ...... 102 4.4.3. Conclusion ...... 106 4.5. In-Lab Experimental Analysis ...... 106 4.5.1. RFID 1-turn loop antenna ...... 107 4.5.2. Designed RFID 2-turns loop antenna ...... 110 4.5.3. Exposure Analysis near the 3M RFID Pad - Model P12 ...... 115 4.6. Conclusion ...... 123 4.7. References ...... 124
RFID Near-field Characterization 91
4.1. INTRODUCTION
The everyday use of devices and systems emitting radio frequency electromagnetic fields is continuously increasing, especially in internet of things (IoT) technologies [1], [2]. Some of these devices are operating in the vicinity of human body, in which people are in the near-field region of the radiating source, and they are exposed to electromagnetic fields. Radio Frequency Identification (RFID) systems are used today in wide variety aspects of everyday life. RFID refers to technologies that are intended to recognize or identify remotely labels placed on an object, animal or person using radio waves. The corresponding applications are numerous and widespread such as: cashless payments, transportation ticketing systems, residential door keys, antitheft protection systems, library systems (Figure 4-1) [3] etc.
Figure 4-1: RFID self-check system used in libraries
This chapter presents the study of RFID systems from a dosimetric point of view. RFID systems used in libraries are mainly addressed under the project Expo@Lyon funded by ANSES (Agence Nationale de Sécurité Sanitaire de l'alimentation). This project proposes to carry out a study of the professional exposure to electromagnetic
92 RFID Near-field Characterization
fields in libraries. As a result, we have performed some measurements in some libraries in Lyon. The ANFR / DR 15-3 measurement protocol adopted by the ANFR (National Frequency Agency), which follows the standard EN 50492, is the basic standard for measuring the electromagnetic field on site. Measurements in site was done using commercial EM-field probes while for in lab experiments the designed probes, presented in chapter 2, was used for more detailed analysis.
4.2. RFID SYSTEMS The term RFID is the acronym of the expression RadioFrequency IDentification that corresponds for a broad set of applications for identifying objects remotely by means of radio waves. The term RF covers all types of wireless or contactless links made using electromagnetic waves, ranging from very low frequencies to infra-red and visible light.
The most common identification technique is the storage of a serial number in a chip to which is attached a transmitting / receiving antenna. The chip-antenna assembly is generally called RFID tag. An interrogator sends an identification request to the labels, which in turn communicate the information contained in the chip. The information can then be processed by a computer system to meet a wide variety of application needs [4].
Computer
RFID Reader
Data Base Antenna RFID Tag
Figure 4-2: Main blocks of an RFID system
RFID Near-field Characterization 93
An RFID system consists of:
Interrogator: or sometimes called RFID reader which can be fixed or movable (handheld).
Reader antenna: that is responsible for converting the electrical signal into a wireless radio wave signal.
Tag: deported elements that can be named differently depending on the markets and applications. We can talk about contactless smart cards, electronic tags or electronic access badges, for example. The general term tag is frequently used.
Transmission medium: it is the surrounding environment around the system (e.g. air, water, or a combination of air and water, glass, metal, etc).
Digital link: that links the RFID reader to the database through a computer system.
RFID uses different radio frequencies depending on the technology used. There are mainly two major classes of RFID technologies:
Systems that operate by magnetic coupling (also called inductive coupling): the operation is in the near-field region at 125-148 kHz and 13.56 MHz frequency bands. These systems are for short-range applications (few cm to 50 cm), such as standard labels or contactless smart cards which are mostly passive tags. The bidirectional transfer of the digital data is performed by the energy emitted by the interrogator and modulated by the tag.
Systems that operate by electrical coupling: The operation is in the far-field at 434 MHz, 860 MHz, 2.45 GHz and 5.8 GHz bands. The operating distances can reach several meters. The transfer of the data from the label takes place this time through the back propagation of the signal transmitted by the reader, and if necessary using a transmitter in the label. Tags in these systems are often active labels.
We can also differentiate between two types of RFID systems: passive and active systems, depending on the way in which the communication between the label and the interrogator is assured.
94 RFID Near-field Characterization
In passive RFID systems the tag consists of an electronic chip that stores the digital identification data and an antenna that transmits this recorded information. The RFID interrogator emits an electromagnetic wave that induces a current in the antenna of the tag. The RFID tag then transmits back a fixed alphanumeric sequence serving as an identifier to the tagged object. While in active RFID systems the tag returns information to the interrogator by producing itself an electromagnetic wave (it therefore integrates a transmitter).
4.2.1. RFID Readers An RFID reader is a device that is used to interrogate the RFID tag. The reader acts as transmitter and receiver. The reader has an antenna that emits radio waves so the tag responds by returning its data. The reader uses its antenna to collect the data received back from the tag. It then transmits this data to a computer for processing. The RFID system communication is based on the Master-Slave relationship, where the RFID reader acts as the master and the tag as the slave [4].
RFID readers can take many shapes and sizes, and can offer a wide range of applications. Currently, many applications rely on fixed devices. The readers may be stationary in a shops or factories, or integrated in electronic devices and in vehicles. RFID readers can also be integrated into handheld mobile devices.
(a) (b)
(c)
Figure 4-3: ZK-RFID406 RFID reader (a), ASR-R250G handheld RFID reader (b), and ZK-RFID101 UHF RFID reader (c)
RFID Near-field Characterization 95
Figure 4-3 presents different types of RFID readers, in (a) ZK-RFID406 RFID reader is shown, it has 4 channels and RF output power up to 30 dBi, it is a fixed RFID system that can be used in different types of applications [5]. Figure 4-3 shows a handheld RFID system that can be linked to a smart phone and can display the locations of tagged items in 3D from a range of about 10 meters, and it has an RF power of 30 dBm that can be lowered manually [6]. In Figure 4-3 we can see ZK- RFID101 system, it is an UHF RFID system with an built-in 8dBi circular polarization antenna. It has an output power up to 30 dBm with a reading range of 5 m [7]. The choice of the RFID reader is mainly done according to the desired reading range. RFID readers with reading distances of a few centimeters are referred to near- field RFID readers. While those whose ranges to several hundred meters are referred to as long-range readers. To increase the range of the reader (up to 20 m), the power needed to reach the tag must be greater. A number of factors can affect the reading range: the frequency used, antenna gain, orientation and polarization of the reader and tag antennas, as well as tag placement on the object to be identified
The exchange of data between the RFID reader and the tag can use different types of modulation and coding. The reader uses carrier modulation to send information to one or more tags. The most commonly used modulations are: Amplitude Shift Keying (ASK) which is the most widely used [8], Phase Shift Keying (PSK) and Frequency Shift Keying (FSK).
4.2.2. RFID Tags The RFID tag consists of an integrated circuit, also called RFID chip, connected to an antenna in a compact and robust housing. The package is structured to allow the RFID tag to be attached to the object to be tracked [9].
Mainly there are two types of RFID tags:
Chip-less RFID tags
These are low-cost RFID tags without the use of any chip inside, commonly known by chip-less tags [10]. These tags are usually used in electronic article surveillance (EAS) applications. EAS tags, shown in Figure 4-4, are usually found in stores as an anti-theft system. They are often referred to as "1-bit" RFID tags. The reason for this is simply that they are only designed to communicate a bit of
96 RFID Near-field Characterization
information. If the RFID tag is present and active, then it means that the item was not at checkout. These tags are rarely nowadays due to several disadvantages in this technology.
Figure 4-4: EAS RFID tags
RFID tags with chip
These tags are divided into three categories: active, semi passive, passive.
Activetagscontainaninternalbatteryanddon‘tdependonanexternalsignalto generate a response. The power source is used to activate the chip and to broadcast a signal to the reader. As a result, active tags can have higher reading ranges, with distances up to 100 meters. Active Tags can be read-only or read/write, allowing the reader to modify the tag data.
The semi-passive tags use a battery to communicate with a reader using back scatter power. The battery used to power the chip that allows more energy from the reader antenna to be reflected back, thereby giving a longer reading range than ordinary passive tags. Active and semi-passive tags are useful for tracking valuable goods that need to be digitized over long distances, such as the railroad, but they cost more than passive tags.
Passive RFID tags are very similar to semi-passive tags but without having any power source. Instead, they are powered by the reader antenna, which sends electromagnetic waves that induce a current in the tag antenna. Passive tags are the most used today, an example of the HF tags (13.56 MHz) is shown in Figure 4-5 and another one for the UHF tags (860-960 MHz) is shown in Figure 4-5.
RFID Near-field Characterization 97
Chip Substrate
Antenna
(a) (b)
Figure 4-5: RFID tags, for HF bands (a) and UHF bands (b)
4.3. RFID READER ANTENNAS
For near-field RFID systems, the inductive coupling is preferable for most applications, since most of the reactive energy is stored in the magnetic field. The inductively coupling RFID systems are able to operate in close proximity to metals and liquids but are affected by objects of high magnetic permeability. For many years, the conventional loop antennas, sensitive in reception to the magnetic field, have been used as antennas for RFID pads in LF and HF bands because these so-called magnetic framework antennas are capable of producing an intense and uniform magnetic field in the area around the antenna. Loop antennas feature simplicity, low cost and versatility. They are made of a loop of a conductor that may have various shapes: circular, triangular, square, elliptical, etc. They are widely used in communication links up to the microwave bands (up to 3 GHz). They are also used as electromagnetic field probes.
Loop antennas are usually classified as electrically small and electrically large antennas as described in 3.5.1. Small loop antennas of a single turn have very small radiation resistance. Their radiation resistance can be substantially improved by adding more turns. Multi-turn loops have better radiation resistance although their efficiency is still poor. Because of that they are poor radiators and usually used as receiving antennas or for short range applications. Also small loop antennas have a far-field radiation pattern similar to an electric dipole as shown in Figure 4-6 supposing the plane of the loop is xy-plane and it is centered at the origin [11].
98 RFID Near-field Characterization
z C=0.1λ
y x Figure 4-6: 3D radiation pattern of a small loop antenna (plane of the loop is xy plane) [11]
However, when the operating frequency of the system rises to the UHF band, the perimeter of the loop antenna becomes comparable to the wavelength. So the antenna cannot produce a uniform magnetic field anymore, because the current flowing along the loop has phase inversion and the current vanishes along the circumference. As a result, the antenna produces a relatively weak magnetic field in certain regions of the loop antenna which degrades the RFID tag detection reliability. The antenna design challenge for the near-field UHF reader is to ensure that the current is in phase and almost of the same magnitude along the loop antenna, and to produce a strong magnetic field distribution and uniform in the near field region.
By segmenting the loop antenna, the perimeter of the antenna becomes large and greater than the wavelength. Segmented antennas have been constructed to operate at the desired frequency while ensuring good magnetic coupling [12], the authors in [13] present electrically wide frame antenna design, with an overall size of 160mm×180mm at the UHF band with a uniform magnetic field distribution. The segmented coupling line sections provide a very low phase delay between adjacent sections so that the current flowing along the segmented lines is kept in a single direction. In addition, knowing that the magnetic field strength of the antenna decreases as the size of the antenna increases, the antenna presented in [14], with an overall size of 175mm×180mm, has demonstrated the ability to produce a strong magnetic field in the near field region, even though the perimeter of the antenna is greater than twice the wavelength of the antenna. Another types were also developed, for example in [15] a fractal antenna that operates at 897 MHz was developed. This antenna gives a reading distance of 9.5 cm for an input power of 13 dBm.
RFID Near-field Characterization 99
Some RFID applications require near-field and far-field communication. Although several works has been done on near-field RFID antennas, we hardly find publications on the design of the reader's antenna for near and far-field operations at the same frequency and at the same time. As the antenna requirements for the near-field RFID systems are different from those for the far-field systems. In [16] authors propose a patch antenna included inside a segmented loop operating simultaneously in near and far field. The segmented loop technique is implemented for near-field operation, while the patch antenna is included within the loop for far-field operation. The reading capabilities of the antenna are up to 9 cm in the near field and 6 m in the far field. In [17], the authors made a metal plate-supported segmented loop antenna that showed a uniform magnetic field distribution in the near-field region, and acceptable far-field radiation characteristics. Also in [18], authors proposed a compact dipole antenna folded with a Split Ring Resonator (SRR) for mobile RFID applications. This antenna has a compact size of 31mm31mm and it works in near-field and in far-field regions. It has a strong surface current distribution with a gain of -2.0 dBi in the far field. The near-field reading range can be up to 48 mm with a transmission power level of 20 dBm.
4.4. IN SITU MEASUREMENTS As mentioned in the introduction of this chapter, our work is focused on the analysis of RFID systems used in libraries. For this reason we performed several measurements at different libraries located in Lyon-France where different RFID systems are addressed. In this section we will present the results obtained, and in the following section we will present a more accurate analysis that was done in lab. In particular, this section presents the results of the real measurements made in ―Vaise Library‖ that is equipped with 3M RFID systems, while you can find other measurements done in different libraries in the Appendix.
4.4.1. Instrumentation Used The measurements was done using the probe EHP-200A from Narda [19]. It is a magnetic and electric field probe that measures accurately the magnetic field ranging between 6 mA/m and 300 A/m, as well as the electric field from 0.02 up to 1000 V/m in a frequency band of 9 kHz to 30 MHz. The corresponding sensors and the electronic circuits are all assembled in a plastic cube having the dimensions of 92×92×109 mm (Figure 4-7). The magnetic field sensor system is composed of three
100 RFID Near-field Characterization
orthogonal loops whereas the electric field sensor system is composed of three orthogonal parallel capacitors installed on opposite side of the magnetic loops.
Figure 4-7: EHP-200A electric and magnetic field probe
The probe is connected to the PC via an optical fiber where an FO to USB adapter is used to convert the optical signal into a digital one to be processed. The PC software is used to visualize the results, where the frequency spectrum analysis is displayed with frequency ranges that can be adjusted within the band of operation of this probe. Each component of the electric or magnetic fields can be measured separately as well as the total field (peak and average values). The RMS value, over a duration of 6 minutes, of the field was measured based on the European directive on the exposure of workers [20] that is based on ICNIRP guidelines [21]. The field measurements are carried out according to the recommendation of the ICNIRP guidelines and the directive on the exposure of workers (2013/35/Eu): RMS values, over a duration of 6 minutes. EHP-200A Probe
Optical fiber
Tripod USB-OC converter
PC Figure 4-8: Measurement setup using EHP-200A probe
RFID Near-field Characterization 101
4.4.2. Measurements at “Vaise Library”
There are four different types of RFID pads used in this library: 3M™MiniRFIDPadModel210(BluePad) 3M™PadStaffWorkstationModel800(BlackPad) 3M™Self-Check System Model 8420 Automated Material Handling (AMH) System
The magnetic field strength measurements of each type were done and the results are tabulated as shown below, comparing the measured values with the guidelines action levels (AL). The EHP-200A probe was placed at the center of each pad, and the RMS value of both E and H fields was recorded over 6 minutes.
4.4.2.1. 3M™ Mini RFID Pad Model 210 (Blue Pad) In this section we present the measurements that was done near two similar 3M mini RFID (model 210) pads. They are normally used by the workers at the library, where they can be found on almost all the library staff tables. The pad has the dimensions of 18.4cm × 16.5cm with a read range up to 15.2cm. The reader and the electronic system is dedicated in the pad itself and it is connected directly to the PC via a USB cable with 5V power supply. The following figure shows one of these pads in the presence of the used probe.
Figure 4-9: Mini RFID pad (model 210) with the presence of the EHP-200A probe
102 RFID Near-field Characterization
Table 4-1: Measured RMS field values near the mini RFID Pads, compared to Action Levels Measured RMS Value Percentage Limitation Percentage with Limitation (over 6 minutes) with respect to Action Level respect to the Action Electric field in [V/m] the reference for RF pulses reference level Level (AL) Magnetic field in [A/m] level (32×AL) for RF pulses 10.68 V/m 61 V/m 17.51 % 1952 V/m 0.55 % Pad #1 0.66 A/m 0.16 A/m 412.5 % 5.12 A/m 12.89 %
11 V/m 61 V/m 18.03 % 1952 V/m 0.56 % Pad #2 0.64 A/m 0.16 A/m 400 % 5.12 A/m 12.5 % Note: For RF pulses, the reference level is 32 times the Action level given in the guidelines
4.4.2.2. 3M™ Pad Staff Workstation Model 895 (Black Pad) In this section we present the measurements done near three similar 3M pad staff workstations (model 895) dedicated for library staff to program the tags before attaching them into the corresponding library materials (reports, books, CDs…). This system consists of a reader (model 810), an RFID Pad (model P12), and the conversion and workstation software [22]. The RFID pad is a loop antenna that has the dimensions of 28.9cm×28.9cm. It is excited by a coaxial cable connected to the RFID reader and the system has a reading range starting from 20 cm. Figure 4-10 shows one of the pads under test in the presence of the measuring probe.
Figure 4-10: 3M RFID pad (model P12) with the presence of the EHP-200A probe
RFID Near-field Characterization 103
Table 4-2: Measured RMS field values near the 3M RFID pad (model P12), compared to Action Levels Limitation Percentage Measured RMS Value Percentage Limitation Action Level with respect to (over 6 minutes) with respect to Action for RF the reference Electric field in [V/m] the reference Level (AL) pulses level for RF Magnetic field in [A/m] level (32×AL) pulses 45.9 V/m 61 V/m 75.25 % 1952 V/m 2.35 % Pad #1 2.75 A/m 0.16 A/m 1718.75 % 5.12 A/m 53.71 %
32.4 V/m 61 V/m 53.11 % 1952 V/m 1.66 % Pad #2 2.99 A/m 0.16 A/m 1868.75 % 5.12 A/m 58.4 %
9.32 V/m 61 V/m 15.28 % 1952 V/m 0.48 % Pad #3 0.83 A/m 0.16 A/m 518.75 % 5.12 A/m 16.21 %
4.4.2.3. 3M™ Self-Check System Model 8420 In this section measurements were done near two similar 3M Self-Check systems (model 8420, kiosk) used by public users [23]. Figure 4-11 shows the kiosk in the presence of the measuring probe. It consists of a touch screen, a bar code scanner, and an RFID pad that is at height of 90 cm.
Figure 4-11: 3M self-check system (model 8420)
104 RFID Near-field Characterization
Table 4-3: Measured RMS field values near the 3M self-check system (model 8420), compared to Action Levels Measured RMS Value Limitation Percentage with (over 6 minutes) Limitation Percentage with Action Level respect to the Electric field in [V/m] Action respect to the for RF pulses reference level Magnetic field in Level (AL) reference level (32×AL) for RF pulses [A/m] 7.89 V/m 61 V/m 12.93 % 1952 V/m 0.4 % Table #1 2.26 A/m 0.16 A/m 1412.5 % 5.12 A/m 44.14 %
10.05 V/m 61 V/m 16.48 % 1952 V/m 0.51 % Table #2 2.46 A/m 0.16 A/m 1537.5 % 5.12 A/m 48.05 %
4.4.2.4. Automated Material Handling (AMH) System This RFID system shows different structure than the previous ones, the RFID pad is hidden inside a metallic cover that is not near the user. It consists of conveyor and sorting systems that can check-in the library materials and sort them by category into separate bins. The measurement near this system was done in two places, the first in the area dedicated for users and the second one in the workers place. Figure 4-12 shows the probe position in these two places.
Figure 4-12: AMH Library RFID system. Area dedicated for users (left), Area dedicated for workers (right)
RFID Near-field Characterization 105
Table 4-4: RMS measured values near the AMH RFID system, compared to Action Levels Percentage Measured RMS Value Percentage Limitation Limitation with respect to (over 6 minutes) with respect to Action Level Action the reference Electric field in [V/m] the reference for RF pulses Level (AL) level for RF Magnetic field in [A/m] level (32×AL) pulses 0.29 V/m 61 V/m 0.47 % 1952 V/m 0.015 % Outside 0.0045 A/m 0.16 A/m 2.81 % 5.12 A/m 0.088 %
3.22 V/m 61 V/m 5.28 % 1952 V/m 0.16 % Inside 0.016 A/m 0.16 A/m 10 % 5.12 A/m 0.31
Notes: Outside: Measurements in the area dedicated for users (Figure 4 -12, left) Inside: Measurements in the area dedicated for workers (Figure 4 -12, right)
4.4.3. Conclusion The measurements near the different RFID systems show a great variability in terms of the electric field and a relatively small variability in terms of the magnetic field. Near the different RFID pads and kiosks, no overshoot is observed in terms of the E-field, while the values of the H-field have exceeded the ALs sometimes for several orders of magnitude. Concerning the anti-theft gates, at certain positions the values of E and/or H field exceeds the ALs sometimes. However, given the pulse character of the signal, no overshot was observed in all RFID systems if the measured values are compared to 32 times the standard AL value. Finally, for the AHM RFID system, both the electric and magnetic fields are very weak and blew the reference levels. For better understanding of the field distribution near the RFID systems we performed more detailed analysis in the Lab that will be presented in the next section.
4.5. IN-LAB EXPERIMENTAL ANALYSIS
In the previous section we presented the measurements that were done at different libraries in Lyon using the EHP-200 electromagnetic field probe. These measurements were performed in the free space by placing the probe, that have the dimensions of 92×92×109 mm, in the center of the corresponding RFID pad and by
106 RFID Near-field Characterization
measuring the RMS value of the field strength over a duration of 6 minutes. In order to have more precise analysis and higher resolution field mapping we performed a detailed study on a typical RFID system (i.e. 3M Pad Staff Workstation - Model 895) that is used normally by library staff. The near-field characterization of this system was done using the near-field test bench presented in chapter 2 with the designed probes P1 and P2. First, we validated the test bench by designing an RFID loop antenna where the simulated magnetic field was compared to the measured ones.
4.5.1. RFID 1-turn loop antenna As stated in4.3 most RFID antennas at HF frequency band are loop antennas. In this section we present the design of a rectangular loop antenna operating at 13 MHz, it has the dimensions of 140 mm×140 mm with a copper trace width of 5 mm. In order to match the antenna to the 50Ω feed line at the operating frequency, an impedance matching circuit was added at the input of the antenna. Loop antennas have inductive input impedance; hence a parallel RC circuit should be used as a matching circuit. The latter was designed using SMD (surface mounted device) lumped elements and it consists of two capacitors connected to the loop in series with a resistor and all in parallel to another capacitor, where the values of these elements was optimized to meet the design requirements. The circuit is shown in Figure 4-13 with the equivalent circuit of the loop antenna. The reflection coefficient measurement shows that the antenna resonant frequency is exactly at 13 MHz and it is in good agreement with the simulation as shown in Figure 4-14.
Loop Antenna Signal C2 Generator R R C1 rad
Rloss
C3 L
Matching Circuit Figure 4-13: Impedance matching circuit with the loop antenna equivalent circuit
RFID Near-field Characterization 107
S11 of the designed loop antenna 0 Simulation Measurement -10
-20 S11(dB)
-30 10 11 12 13 14 15 16 17 18 Frequency [MHz] Figure 4-14: Simulated and measured reflection coefficient of the antenna
The antenna prototype was then fabricated on a dielectric substrate FR4 (εr=4.4, tan(δ)=0.025) with the dimensions of 150 mm×150 mm and having a thickness of 1.6 mm. The antenna was fed by a coaxial cable with a power of 10 dBm from the signal generator.
Matching Network
RF input y-axis Signal
Matching Network
Figure 4-15: Fabricated 1-turn RFID loop antenna (top) with the HFSS model (bottom)
The 3D scanning system, presented in chapter 2, 2.4, was used to map and characterize the magnetic near-field radiated by the designed RFID reader antenna. The scanning measurement system consists of the 3D scanning robot, Teledyne
108 RFID Near-field Characterization
LeCroy WaveRunner 640Zi oscilloscope, signal generator, and the tri-axial H-field probes P1 and P2. The plastic arm of the robot holds the corresponding probe where the desired scan above the antenna is performed within the pre-defined margins.
Tri-axial H-field 3D Scanning Robot Probe
Designed RFID WaveRunner 640Zi Reader Antenna Oscilloscope
Signal Generator
Figure 4-16: Near-field mapping system with the designed RFID reader antenna
The field mapping was done along y-axis (shown in Figure 4-15-top) with Δy=160mm, a step size of 1 mm, and a heights h=5 cm above the designed RFID reader loop antenna.
Figure 4-17 shows the plots of the 3 components of magnetic field (Hx, Hy, and
Hz) using probe P1 whereas Figure 4-18 shows the plot of the measured field components using probe P2. The measured components are very close to the simulation, however the small difference in the Hx component is due to the coupling between the probe elements and its effect is somehow negligible as the level of the field is too small compared to the other components.
RFID Near-field Characterization 109
Magnetic field above the RFID antenna along y-axis at h=5cm 0.1 simulations 0.05 measurements (P1)
Hx [A/m] Hx 0 -80 -60 -40 -20 0 20 40 60 80 y axis [mm] 0.1
0.05
Hy [A/m] Hy 0 -80 -60 -40 -20 0 20 40 60 80 y axis [mm] 0.1
0.05
Hz [A/m]Hz 0 -80 -60 -40 -20 0 20 40 60 80 y axis [mm]
Figure 4-17: Magnetic field components above the RFID reader antenna measured with probe P1
Magnetic field above the RFID antenna along y-axis at h=5cm 0.1 simulations 0.05 measurements (P2)
Hx [A/m] Hx 0 -80 -60 -40 -20 0 20 40 60 80 y axis [mm] 0.1
0.05
Hy [A/m] Hy 0 -80 -60 -40 -20 0 20 40 60 80 y axis [mm] 0.1
0.05
Hz [A/m]Hz 0 -80 -60 -40 -20 0 20 40 60 80 y axis [mm] Figure 4-18: Magnetic field components above the RFID reader antenna measured with probe P2
4.5.2. Designed RFID 2-turns loop antenna
Multi-turn loop antennas have better radiation resistance, for this reason another 2- turns rectangular loop antenna operating at 13.56 MHz was designed. It is the typical RFID reader antenna used in library RFID systems. The antenna prototype was also fabricated on a dielectric substrate FR4 with the dimensions of 170 mm × 170 mm and 1.6 mm thickness as shown in Figure 4-19. The first loop has the dimensions of
110 RFID Near-field Characterization
160 mm × 160 mm whereas the second one has the dimensions of 80 mm × 80 mm, and the copper trace width is 6 mm. The matching circuit has the same schematic of that shown in Figure 4-13 with different component values.
Matching Network
RF input Signal
z y x
Figure 4-19: Fabricated 2-turns RFID loop antenna
The radiation effects are analyzed in the near-field region as the RFID loop antenna requires usage in the near-field. At the operating frequency the wavelength is about 22 m, so the tags and the RFID reader are very close to each other compared to the wavelength. In this analysis, the RFID loop antenna was excited with two signal types, CW signal and a modulated signal, and the radiated H-field was studied in both cases. The CW signal was generated at 13.56 MHz with a source power of 20 dBm. The modulated signal, shown in Figure 4-20, is a pulsed RF signal with pulse width PW=32ms, pulse repetition period PRT=250ms, and a duty cycle D = PW / PRT = 0.128. This modulated signal is usually used in RFID library systems.
RFID Near-field Characterization 111
f=13.56 MHz 30 20 10 0 -10 -20 -30 -2 -1 0 1 2 x 10-7
30
20 PRT 10 0 -10 -20 PW
Amplitude [V] Amplitude -30 -0.4 -0.2 0.0 0.2 0.4 Time [s] Figure 4-20: Pulsed modulated RFID reader signal
The field mapping was done along y-axis with Δy=180mm and at two different heights (h1=1 cm & h2=5 cm) above the RFID reader loop antenna. These heights were chosen since they are the typical heights for the human exposure to library RFID systems. Then the measured magnetic field with probe P1 is compared with the recommended exposure limit values given in ICNIRP guidelines.
The magnetic field strengths of the three components (Hx, Hy, and Hz) at the two different heights above the RFID reader antenna are shown in Figure 4-21 for CW signal excitation. At h1 the Hz component has a maximum level of 0.7A/m while Hy has a maximum level of 0.5 A/m where Hx has a level below 0.2 A/m. At h2 the Hz component has a maximum level of 0.25 A/m while Hy has a maximum level of 0.11
A/m where Hx has a level below 0.05 A/m.
According to ICNIRP Guidelines, and for the operating frequency of 13.56 MHz, the reference levels for a human exposure scenario, for both general public and occupational, are as listed in Table 4-5 [21].
112 RFID Near-field Characterization
H-field along y-axis at h=1cm | CW Signal 1
0.5
Hx [A/m] Hx 0 -100 -50 0 50 100 y axis [mm] 1
0.5
Hy [A/m] Hy 0 -100 -50 0 50 100 y axis [mm] 1
0.5
Hz [A/m]Hz 0 -100 -50 0 50 100 y axis [mm]
(a)
H-field along y-axis at h=5cm | CW Signal 0.2 0.1
Hx [A/m] Hx 0 -100 -50 0 50 100 y axis [mm] 0.2 0.1
Hy [A/m] Hy 0 -100 -50 0 50 100 y axis [mm] 0.4 0.2
Hz [A/m]Hz 0 -100 -50 0 50 100 y axis [mm]
(b)
Figure 4-21: Magnetic field strength above the RFID reader antenna with CW signal, at heights 1cm (a) and 5cm (b)
Table 4-5: Recommended Exposure Limit Values at 13.56 MHz [20] General Public Occupational
Reference Magnetic field [A/m] 0.073 0.16
Maximum Recommended Magnetic Field for RF pulses 2.336 5.12 [A/m]
RFID Near-field Characterization 113
The maximum recommended magnetic field value for RF pulses is 32 times higher than the reference magnetic field values [20]. At our operating frequency the reference level in the Guidelines is equal to 0.073A/m for general public and 0.16A/m for occupational exposure. We notice that the previous measured H-field values exceed these limits. Normally this limit value refers to full body exposure for 6 minutes period. Considering the RFID reader used in library systems, there is a potential arm and hand exposure which lasts just for few seconds. Moreover, in some configurations, the RFID reader antenna in Library systems is placed under a table so there is about 2cm between the antenna and the table surface.
For the modulated signal, the magnetic field strengths of the three components
(Hx, Hy, and Hz) above the RFID reader antenna are shown in Figure 4-22. The H- field here is extracted from the RMS measured value, since the signal is an RF pulses.
In this case, at h=1cm the Hz component has a maximum level of 2.3 A/m while Hy is below 2 A/m where Hx has a maximum level of 0.6 A/m. At h=5 cm the Hz component has a maximum level of 0.8 A/m while Hy has a maximum level of 0.4
A/m where Hx has a level below 0.25A/m.
H-field along y-axis at h=1cm | Mod. Signal 1
0.5
Hx [A/m] Hx 0 -100 -50 0 50 100 y axis [mm] 2
1
Hy [A/m] Hy 0 -100 -50 0 50 100 y axis [mm] 4
2
Hz [A/m]Hz 0 -100 -50 0 50 100 y axis [mm]
(a)
114 RFID Near-field Characterization
H-field along y-axis at h=5cm | Mod. Signal 0.5
Hx [A/m] Hx 0 -100 -50 0 50 100 y axis [mm] 0.5
Hy [A/m] Hy 0 -100 -50 0 50 100 y axis [mm] 1
0.5
Hz [A/m]Hz 0 -100 -50 0 50 100 y axis [mm]
(b)
Figure 4-22: Magnetic field strength above the RFID reader antenna with modulated signal, at heights 1cm (a) and 5cm (b)
In this case, by comparing these measured results with the maximum recommended magnetic field for RF pulses (2.336A/m for general public & 5.12A/m for occupational exposure), the limits are not exceeded if we consider the special limits for RF pulse signal.
4.5.3. Exposure Analysis near the 3M RFID Pad - Model P12
RFID pad P12 is the corresponding reader antenna of the 3M staff workstation - model 895. It is a 2-turns loop antenna with total dimensions of 28.9cm×28.9cm. This work station is intended to be used by library staff for programming the corresponding RFID tags attached to library materials. We have studied the exposure to this pad in terms of the magnetic field strength as it is the dominant field due to the loop antenna type of the pad. Figure 4-23 shows the measurement setup including the 3D scanner and the tri-axial probe that was used to measure the radiated field above the RFID pad P12. The latter was excited via a coaxial cable by a signal generated from the RFID reader (model 810); it is a pulsed signal as shown in Figure 4-24.
RFID Near-field Characterization 115
3D Scanning Robot Tri-axial H-field Probe
WaveRunner 640Zi Oscilloscope 3M RFID Pad Model P12
3M RFID Reader Model 810
Figure 4-23: Measurement setup for H-field characterization above 3M RFID pad P12
The blue signal corresponds for that generated in normal operation of the pad and without the presence of any tag near it, it has a PW = 32ms and a D = 12.8%. The pulse width of the signal increases as the number of tags near the antenna increases, due to the communication between the 2 terminals. The red signal in in Figure 4-24 corresponds to the signal generated by the reader in the presence of 8 tags near the pad, and it has a PW = 140ms and D = 56%. As we can notice that the duty cycle of this signal is about 4 times more than the previous one, and this will obviously lead to an increase in the strength of the radiated field.
116 RFID Near-field Characterization
Figure 4-24: 3M RFID reader (model 810) signals: without any tag near the antenna (blue signal) and in the presence of 8 tags (red signal)
The field mapping was done along y-axis (dotted line in Figure 4-23) in the interval [-90mm 90mm] where the loop center is at the origin, and at two different heights (h1=1cm, h2=5cm) with a step size of 1mm. P1 and P2 tri-axial probes was used for this measurements, the antenna factors of these probes at 13.56 MHz are tabulated in Table 4-6. Table 4-6: Antenna factor of P1 and P2 at 13.56 MHz H-field Probe AF in dB[A/(m.V)] P1 42 P2 (x & y components) 54 P2 (z component) 52
The RMS value of the signal measured by the oscilloscope was recorded then it is converted to the corresponding magnetic field component. First, the near field scan was performed above the RFID pad without having any tag. The dominant H-field component is Hz as the pad is a loop antenna and it has a maximum value of 2.8 A/m at h1 and 1 A/m at h2. Hy component has a maximum value of 2 A/m at h1 and 0.48
A/m at h2, whereas the third component Hx has strength less than 0.5 A/m and 0.2
A/m at h1 and h2 respectively. We then performed the scan by adding 1-tag to the region of the RFID pad; we noticed that the H-field strength has increased. The field level increases as the number of tags near the pad increases, in the presence of 8-tags
RFID Near-field Characterization 117
the Hz component reaches 4.7 A/m at h1 and 1.5 A/m at h2 whereas Hy component has the values of 3.2 A/m and 0.7 A/m at h1 and h2 respectively, with a slight difference in the Hx component.
(a)
(b) Figure 4-25: Magnetic field components measured with P1 above the RFID pad P12 along y-axis
at h1=1cm (a) and at h2=5cm (b)
The H-field measurements in the presence of different numbers of tags near the
RFID pad are shown in Figure 4-25 at the heights h1 & h2. The same near field scans at the same heights but using the tri-axial H-field probe P2 was also done and the results are shown in Figure 4-26, where they are identical to those measured with P1 except a little difference in the Hx component but it has negligible effect on the total H-field level.
118 RFID Near-field Characterization
(a)
(b) Figure 4-26: Magnetic field components measured with P2 above the RFID pad P12 along y-axis
at h2=1cm (a) and at h2=5cm (b)
Moreover, a 2D scan was performed above the RFID pad at the same heights h1
& h2. The scanned area is a square in the center of the pad with the dimensions of 126mm×126mm (-63mm 63mm along both x- and y-axis) and a step size of 6 mm. The corresponding field mapping of each component are shown in Figure 4-27 whereas the total H-filed cartography is shown in Figure 4-28. The exposure limit values in terms of H-field at 13.56 MHz, presented in Table 4-5, indicates that the maximum permissible level is 2.336 A/m for general public and 5.12 A/m as the occupational ALs. By comparing the results obtained in the several scans that were done in different scenarios, we noticed that the
RFID Near-field Characterization 119
occupational action levels were respected. However the field strength at the height h1 exceeds the ALs of the general public.
Hx at 1cm A/m Hx at 5cm A/m
axis [mm] axis
- y
Hy at 1cm Hy at 5cm
axis [mm] axis
- y
Hz at 1cm Hz at 5cm
axis [mm] axis
- y
x-axis [mm] x-axis [mm] Figure 4-27: 2D cartography of the H-field components above RFID pad P12
Ht at 1cm A/m Ht at 5cm A/m
axis [mm] axis
- y
x-axis [mm] x-axis [mm]
Figure 4-28: Total H-field mapping above the RFID pad P12 at h1 & h2
120 RFID Near-field Characterization
Tri-axial H-field Cylindrical Probe P1 Phantom
z y x
3M RFID Pad Model P12
Figure 4-29: Measurement setup in the presence of the cylindrical phantom with TSL
In the previous analysis we were measuring the radiated magnetic field in the free space above the RFID pad. In order to check field levels in the presence of a human body, another scan in a cylindrical phantom containing TSL (tissue simulating liquid) was performed. The TSL was prepared in the lab by adding 2.5 g/L of sodium chloride (NaCl) to pure water. The electrical properties of this liquid was extracted using a coaxial fixture and by applying the marital properties extraction technique presented in [25].
Figure 4-30: Electrical properties of the TSL used to fill the cylindrical phantom
RFID Near-field Characterization 121
The phantom was placed in the center of the pad as shown in Figure 4-29 and P1 was used for the measurements. H-field components were measured in a vertical plan with the dimensions of 78mm×70mm along y- and z-axis respectively and with a step size of 3 mm. The measurements were done in the same plane in free space then in the phantom.
Hx - free space A/m Hx - TSL A/m
axis [mm] axis
- z
Hy - free space Hy - TSL
axis [mm] axis
- z
H - free space Hz - TSL
z
axis [mm] axis
- z
y-axis [mm] y-axis [mm] Figure 4-31: H-field components measured in a vertical plane above the RFID pad in free space (left column) and inside the cylindrical phantom (right column)
Ht - free space A/m Ht - TSL A/m
axis[mm]
- z
y-axis [mm] y-axis [mm] Figure 4-32: Total H-field measured above the RFID pad in free space (left) and inside the phantom (right)
122 RFID Near-field Characterization
By comparing the measured field levels in the two cases, we can notice that the field inside the TSL is slightly less than that is the free space. Figure 4-31 shows the comparison between the three H-field components separately while Figure 4-32 shows the comparison of the total H-field mapping.
4.6. CONCLUSION
In this chapter we have presented the analysis of the radiated electromagnetic field near RFID loop antennas that are used in Libraries. The exposure assessment in terms of the H-field was studied near different RFID systems. In-situ measurements were done at different libraries in Lyon where a commercial EMF probe (EHP 200A) from Narda STS was used. More detailed study was performed in lab using the developed near field scanning system with the designed tri-axial probes presented in the previous chapter. Two loop antennas was fabricated, the first one that is a simple 1-turn loop antenna was used to validate the test bench by comparing the measurements to a simulated HFSS model. The near field characterization was then performed near the second 2-turns loop antenna and near a commercial RFID pad P12 from 3M Company. The field levels were compared to the action levels stated in the international and European guidelines. Even though they exceed the ALs given in the directives, but from the performed measurements we see the strong decrease in the level of the field (between 1 and 5 cm above the antenna). This clarifies that the general public and users of such systems are not exposed to the high field levels; however workers should be aware not to be too close to the RFID antennas during their job.
RFID Near-field Characterization 123
4.7. REFERENCES
[1] H.HadaandJ.Mitsugi,―EPCbasedinternetofthingsarchitecture,‖in2011 IEEE International Conference on RFID-Technologies and Applications, Sitges, Spain, 2011, pp. 527–532.
[2] V.C.GungorandG.P.Hancke,―IndustrialWirelessSensorNetworks: Challenges, Design Principles, and Technical Approaches,‖IEEE Trans. Ind. Electron., vol. 56, no. 10, pp. 4258–4265, Oct. 2009.
[3] G.Viziteu,B.Florean,G.M.Moraru,andA.Pintea,―AnoverviewofRFID technologyusedinlibrary,‖in2012 International Conference and Exposition on Electrical and Power Engineering, Iasi, 2012, pp. 108–111.
[4] K. Finkenzeller, RFID Handbook: Fundamentals and Applications in Contactless Smart Cards, Radio Frequency Identification and near-Field Communication. Chichester, UK: John Wiley & Sons, Ltd, 2010.
[5] Impinj,―SchoolAttendanceLongRangeRFIDReader,4PortUHFRFID Reader.‖[Online].Available:http://www.uhfrfid-reader.com.
[6] AsReaderCompany,―AsReader- NewHandheldRFIDReader.‖[Online]. Available: http://www.rfidjournalevents.com/live2017/news/product- announcements/15877.
[7] ShenZhenZKHYRFIDTechnologyCo.,Ltd.,―HighIdentificationRateMid RangeRFIDReaderForLivestockManagement.‖[Online].Available: http://www.uhfrfid-reader.com.
[8] N.Pillin,C.Dehollain,andM.J.Declercq,―Readrange limitation in IF- based far-fieldRFIDusingASKbackscattermodulation,‖in2009 Ph.D. Research in Microelectronics and Electronics, Cork, Ireland, 2009, pp. 348–351.
[9] CharlesA.Walton,―Portableradiofrequencyemittingidentifier,‖ US4384288A, 17-May-1983.
[10] A.Vena,E.Perret,andS.Tedjini,―ChiplessRFIDTagUsingHybridCoding Technique,‖IEEE Trans. Microw. Theory Tech., vol. 59, no. 12, pp. 3356–3364, Dec. 2011.
[11] C. A. Balanis, Antenna theory: analysis and design, 3. ed. Hoboken, NJ: Wiley-Interscience, 2005.
[12] YongSimOng,XianmingQing,CheanKhanGoh,andZhiNingChen,―A segmented loop antenna for UHF near-fieldRFID,‖in2010 IEEE Antennas and Propagation Society International Symposium, Toronto, ON, 2010, pp. 1–4.
124 RFID Near-field Characterization
[13] X. Qing,C.K.Goh,andZ.N.Chen,―SegmentedloopantennaforUHFnear- fieldRFIDapplications,‖Electron. Lett., vol. 45, no. 17, p. 872, 2009.
[14] X.Qing,C.K.Goh,andZ.N.Chen,―ABroadbandUHFNear-Field RFID Antenna,‖IEEE Trans. Antennas Propag., vol. 58, no. 12, pp. 3829–3838, Dec. 2010.
[15] J. Kizhekke Pakkathillam, M. Kanagasabai, C. Varadhan, and P. Sakthivel, ―ANovelFractalAntennaforUHFNear-FieldRFIDReaders,‖IEEE Antennas Wirel. Propag. Lett., vol. 12, pp. 1141–1144, 2013.
[16] B. Shrestha,A.Elsherbeni,andL.Ukkonen,―UHFRFIDReaderAntennafor Near-Field and Far-FieldOperations,‖IEEE Antennas Wirel. Propag. Lett., vol. 10, pp. 1274–1277, 2011.
[17] X.Qing,Z.N.Chen,andC.K.Goh,―AUHFnear-field/far-field RFID metamaterial-inspiredloopantenna,‖inProceedings of the 2012 IEEE International Symposium on Antennas and Propagation, Chicago, IL, USA, 2012, pp. 1–2.
[18] L.Xiaozheng,X.Zeming,andC.Xuanliang,―ACompactRFIDReader Antenna for UHF Near-Field and Far-FieldOperations,‖Int. J. Antennas Propag., vol. 2013, pp. 1–5, 2013.
[19] ―EHP-200ACompactEandHfieldmeter,‖narda Safty Test Solutions. [Online]. Available: https://www.narda-sts.com/en/selective-emf/ehp-200a/.
[20] ―Directive2013/35/EUoftheEuropean Parliament and of the Council of 26 June 2013 on the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (electromagnetic fields) (20th individual Directive within the meaning of Article 16(1) of Directive 89/391/EEC) andrepealingDirective2004/40/EC.‖,26-Jun-2013.
[21] ―ICNIRPSTATEMENTONTHE‗GUIDELINESFORLIMITING EXPOSURE TO TIME-VARYING ELECTRIC, MAGNETIC, AND ELECTROMAGNETICFIELDS(UPTO300GHz)‘:,‖Health Phys., vol. 97, no. 3, pp. 257–258, Sep. 2009.
[22] 3MScienceAppliedtoLife,―3MTM RFID Pad Staff Workstation (Model 895).‖[Online].Available:www.3m.com.au.
[23] 3M,―3MTM SelfCheck System R-SeriesModels8420and8422.‖2011.
[24] NedapLibrarySolutions,―PG45Stylishand discreet protection of your librarycollection.‖[Online]. Available: https://www.nedaplibrary.com.
[25] F. Moukanda. "Contribution A La Caractérisation des Matériaux Utilises en Microélectronique Radiofréquence" Mémoire de Thèse Université Joseph-Fourier - Grenoble I, 2008. Français.
RFID Near-field Characterization 125
Chapter 5: Industrial RF Welding Machines EMF Characterization
Contents
Chapter 5: Industrial RF Welding Machines EMF Characterization ...... 127 5.1. Introduction ...... 128 5.2. Industrial RF Welding Machines ...... 129 5.3. Numerical Analysis ...... 133 5.3.1. Effects of human tissues dielectric parameters...... 133 5.3.2. RF welding machine numerical model ...... 138 5.4. Experimental Analysis ...... 139 5.4.1. Preliminary measurements using EHP200A ...... 139 5.4.2. Magnetic field measurements ...... 142 5.4.3. Electric field measurements ...... 147 5.5. Conclusion ...... 149 5.6. References ...... 150
Industrial RF Welding Machines EMF Characterization 127
5.1. INTRODUCTION
The French national research and safety institute for the prevention of occupational accidents and diseases INRS (INstitut national de Recherche et de Sécurité), estimates around 100000 RF industrial machines in France, which generate a high electromagnetic field [1]. Due to this reason and in order to develop an effective tool for easily obtaining relevant information on workers exposure to RF electromagnetic fields and, if necessary, to propose appropriate measures to reduce exposure, EXTI project was lunched. EXTI (EXposition des Travailleurs aux champs électromagnétiques Industriels) is a scientific project (PNREST 2015/2 RF/002 - ANSES) that aims to study the exposure to RF electromagnetic fields of workers in industrial environments and to implement a new method of exposure characterization by numerical dosimetry. For mobile radio waves that have been studied a lot, the sources are well identified and their radiation can be characterized. On the other hand, in the case of industrial machines generally using RF microwave category (heating for dielectric welding, induction heating ...), it is sometimes difficult to calculate the exposure levels of workers in the work environment, due to the fact that: The machines are being large (radiation sources are often distributed) The posture of the operator is variable
The focus of this chapter will be on a particular type of industrial systems that is the RF welding machines, which radiates a strong electromagnetic field at 27.12 MHz. The exposure to these machines takes place in the near field region, where the spatial distribution of the electromagnetic field is very complicated. We have an industrial RF welding machine on the site of INRS (in Nancy-France), in which we carried out three measurement campaigns, which allowed us to establish the cartography of the electric and magnetic fields around the machine. In the present work we also carried out a numerical modeling of the machine from a realistic CAD. This model allows us to make a comparison with the experimental measurements, and on the other hand to perform simulated measurements, which would be too long and complicated to perform experimentally. Moreover, the effect of a human body immersed in an RF electromagnetic field was studied by simulating a virtual phantom having different material layers. The
128 Industrial RF Welding Machines EMF Characterization
measurements obtained will be also used to feed the numerical FEM formulation that is under development on ONELab software that aims to extract the EM field inside a phantom from the measurements in the free space near a radiating source.
5.2. INDUSTRIAL RF WELDING MACHINES
Machines using high-frequency (HF) dielectric heating can weld, mark, bend or glue different insulating materials (wood, plastics, etc.). In this process, the energy expended by the HF alternating electric field polarizes the molecules within the materials. The intermolecular friction develops a heat allowing the needed process (welding, marking, bending or gluing, etc.) of these materials according to the application. For example, the microwave oven is a device used mainly for the rapid heating of products by stirring the water molecules they contain under the effect of a microwave radiation (f = 2.45 GHz). However, the dielectric loss welding process is used for instant sealing of thermoplastics [2]. The molecules in the product are subjected to a high frequency electric field with a certain pressure. This leads to vibration that generates, by the friction of the molecules, an internal heating sufficient to cause the welding between the molecules of a material [3],[4]. This method has the advantage of requiring no added material like glues or other fillers. However, an electromagnetic radiation is emitted by these machines in particular by the RF applicator, commonly called the electrode. Figure 5-1 shows a basic schematic for an RF welding machine where the equivalent electrical circuit is shown in Figure 5 -2. The material to be welded is placed between the electrode and the bed plate, under the pressure of the hydraulic press, before the RF energy being applied. Signals emitted by industrial equipments can extend over a wide frequency band, from few MHz up to several GHz. These bands are known internationally as ISM (industrial, scientific and medical) radio bands and they are dedicated for applications that use RF energy in industrial, scientific and medical applications other than telecommunications. In the case of the RF welding machines, the signals are generally at one of these frequencies: 6.78, 13.56, 27.12, or 40.68 MHz [4]. RF welding machines generates a strong electromagnetic field in which the workers using such appliances are daily exposed to this field, thus it is important to quantify their exposure.
Industrial RF Welding Machines EMF Characterization 129
Rigid Hydraulic Coaxial Line Press Mainframe Electrode Holder RF Insulator Electrode Material RF Generator Bed Plate
: RF Current Path
Figure 5-1: Schematic of an RF welding machine
Oscillator Inductor Chord Capacitor
Oscillator RF energy supply Capacitor Foil Inductor
Capacitor parallel (Electrode- to electrode material- plate) Capacitor
Machine Ground Figure 5-2: RF welding machine equivalent electrical circuit
Some welding systems are automated and there is no presence of workers near the machines, however other systems need manual operations where it is necessary to have the presence of one or two workers assigned to the machine as shown in Figure 5 -4. The electromagnetic field emitted by the machine is essentially due to the intense electrical field created by the electrode. The heat energy generated is
130 Industrial RF Welding Machines EMF Characterization
proportional to the square of the intensity of the electric field. The RF current circulating in the metal parts of the machine is considered as another source of the EMF radiation. The field emitted by welding machines dealing with small-sized products can be considerably reduced by shielding. This can be done by enclosing the radiating parts of the machine by a metallic shield, precisely enclosing the volume containing both the electrode and the product to be welded as shown in Figure 5 -3 [5]. However, this solution is not feasible for machines dealing with large products. For this reason another solution was suggested that is based on adding a ground plate to the welding system, which leads the current back to the machine and therefor decreasing the radiated energy. Recent welding machines design, have this ground plate as an alternative field reduction solution. It consists of a metal plate made of aluminum or copper that is arranged parallel to the electrode of the welder and having the same length of the electrode with a width of about 15 to 20 cm. These dimensions are determined so the plate-table electrical capacitance large enough to prevent heating of the material under it. The thickness of the plate is not important electromagnetically. The plate can be entirely rigid or be constituted by a flexible foil itself fixed on a rigid support. Figure 5 -4 shows a new welding machine equipped with the ground plate.
(a) (b) Figure 5-3: Small RF welding machine: with no shielding (a), with shielding (b)
Industrial RF Welding Machines EMF Characterization 131
Ground Plate Electrode
Figure 5-4: RF welding machine with the ground plate
EMF exposure assessment carried out by INRS for new machines equipped with the ground plate shows that the ALs are respected and no longer exceeded [6]. For example, measurements carried out near a machine from Matrelec, having a nominal power of 20 kW, shows that the maximum electric field intensity is 104 V/m. By applying the 6-minute averaging factor, the electric field strength becomes 29V/m, which is less than the 61V/m value given in the directives. However, field strength of several hundreds of V/m are still commonly found near old welding machines that have no ground plates, who are still exists in different industrial sectors, even though there is some work on adding the ground plates to these machines. For this reason the aim of this work is to characterize the emitted RF energy near these machines and to perform the corresponding numerical and experimental analysis in order to have more details about the workers exposure. The gooseneck welding machine used by S.E.F. Company in France, which exists at the site of INRS, was selected. It works at 27.12 MHz (26.957 MHz – 27.283 MHz) and has a power of 7.5 kW.
Table 5-1: Gooseneck welding machine specifications Generator Type GS7500 Number 3079-1 Sector Voltage 3×220 V Frequency 27.12 MHz Sector Current 45 A Power 7500 W
132 Industrial RF Welding Machines EMF Characterization
Asthismachineisanoldone,itdoesn‘thaveagroundplate.However, a detailed work was done by INRS [6] for adding a ground plate in order to decrease the radiating field. But still there is a fear of having high radiated EMF. A 3-dimensional mapping of the field radiated by this machine was performed in the space in front of it. The measurement system and calculations that was used are generic and can be applied in the industrial environment having such RF energy radiators. In the framework of this work we focused on a particular machine, in which it represents fairly a wide range of industrial machines. Given the frequency of operation of these machines that have a wavelength of about 11 m in vacuum, and due to the fact that the worker is in the vicinity of the machine, the exposure takes place in the reactive near-field region. This is the most delicate situation because the electric and magnetic fields are coupled and have a complex spatial distribution [7].
Table 5-2: Action levels at 27.12 MHz E-field Magnetic flux density, B Frequency [MHz] H-field [A/m] [V/m] [µT] 27.12 61 0.2 0.16
5.3. NUMERICAL ANALYSIS
5.3.1. Effects of human tissues dielectric parameters
The presence of the worker‘s body in front a radiating source may have a significant influence on the radiation. To check that an RF model implemented for finite element simulations in ANSYS HFSS was developed. The model, shown in Figure 5 -5, was built from a simple electrode above a ground plane and having a
PVC material (εr=2.7) in between, in the addition of a realistic multilayer anthropomorphic computational phantom. The latter is an anatomical voxel body model Duke from the virtual family, it is 70kg male that has a high of 1.74m with a voxel size of 5mm [8]. For living tissues, the dielectric properties from IT’IS Foundation were used [9]. The source is simulated as a lumped port between the electrode and the ground plane with a power of 3 kW and at 27.12 MHz. The
Industrial RF Welding Machines EMF Characterization 133
different human body models configurations are described in Table 5 -3. The simulations are performed with HFSS by using adaptive mesh iterative solver of The 4 layers phantom in front of the electrodes of the welding machine (only the electrode is modeled). second order.
Electrode Model
174 cm 10 cm
86 cm
(a) (b) Figure 5-5: Duke Phantom in front of the electrode model of the welding
Table 5-3: Different human models material configurations (units: length in mm, permittivity (ɛ) in F/m, and conductivity (ζ) sigma in S/m) Model Corresponding material configuration Number (1) No phantom (2) Skin(thickness=10,ɛ= 166,σ=0.33),blood(ɛ=127,σ=1.16) Skin(thickness=10,ɛ=166,σ=0.33),fat(thickness=30,ɛ=18,σ= (3) 0.061)blood(thickness=10,ɛ=127,σ=1.16),muscle(ɛ=95,σ=0.65) (4) PVC(thickness=10,ɛ=2.7,σ=0),blood(ɛ=127,σ=1.16) PVC(thickness=10,ɛ=2.7,σ=0),blood(thickness=30,ɛ=127, (5) σ=1.16),PVC(thickness=10),vacuum(ɛ=1,σ=0)
The simulated configurations are presented in Figure 5 -6. It is found that the presence of the phantom (models number: 2, 3, 4, and 5) modifies to a large extent
134 Industrial RF Welding Machines EMF Characterization
the electric field, with respect of the case where the phantom is absent (model number 1). However, the internal structure of the phantom seems to play a quite minor role. E-field [V/m]
(1) (2) (3) (4) (5) Figure 5-6: E-field distribution for different models (defined in Table 5-3) at 27.12 MHz
Moreover, in the presence of any phantom it is observed that the electric field approximately satisfies quasi-static hypotheses outside the worker's body (in particular, the field is almost orthogonal to the body surface), but not at all inside it (Figure 5 -7). This can be easily explained by the high permittivity and conductivity of the phantom with respect of air (Table 5 -3). E-field [V/m]
Figure 5-7: E-field vectors on the surface of the body #2 E-vector field on the surface of the body. nxE is quite null.
Industrial RF Welding Machines EMF Characterization 135
#5
1 (1) 0.8 (2) (3) 0.6 (4)
(5) F(x) 0.4
0.2
0 0 100 200 300 400 500 600 |E| [V/m] Figure 5-8: Cumulative distribution function of the electric field for the 5 different models
In terms of the magnetic field, the numerical simulations confirm that the magnetic field is modified by the presence of the human body, difference between model number 1 and the other models as shown in Figure 5 -9, while the internal structure of the phantom have little effect. In the model having just blood inside (model number 2) the H-field seems to be little bit higher than the other models. The latter are almost having the same H-field levels. In addition, the H-field seems to be
continuous on the body surface (µ = µ0 is constant) as shown in Figure 5 -10. H-field [A/m]
(1) (2) (3) (4) (5) Figure 5-9: H-field distribution for different models (defined in Table 5-3) at 27.12 MH
136 Industrial RF Welding Machines EMF Characterization
H-field [A/m]
Figure 5-10: Continuity of the magnetic field at 27.12 MHz
#2 the continuity of magH at 27 MHz On the base of these preliminary simulations, we can build an instrumented phantom composed of PVC filled (at least partially) with a tissue simulated liquid and check the compliance with basic restrictions. The electric field must be measured in the presence of a phantom which mimics the human body, in order to obtain realistic data. Probes for measuring the normal component of the electric field En should be placed on the surface of the phantom. The knowledge of En, measured in #1 realistic#2 situations, allow computing numerically the electric field inside the body, so #3 #4 as to characterize#5 the exposure of workers.
1
0.8 (1) (2) 0.6 (3)
(4) F(x) 0.4 (5)
0.2
0 0 0.02 0.04 0.06 0.08 0.1 |H| [A/m] Figure 5-11: Cumulative distribution function of the magnetic field for the 5 different models
Industrial RF Welding Machines EMF Characterization 137
5.3.2. RF welding machine numerical model
As stated in 5.2 , the exposure analysis will be performed near the industrial welding machine that exists at the site of INRS. A numerical model of this machine was designed on HFSS, based on a realistic CAD from INRS, as it is shown in Figure 5 -12. Themodelwasbuiltwiththeadditionofthegroundplate,thatdoesn‘t existintheinitialCAD.ThisgroundplatewasbuiltonthebasisofKlein‘sdesignas stated in [10]. A 2.7 mm PVC layer, simulating the plastic material to be welded in the real case, was added between the electrode and the machine table (bed plate in Figure 5 -1). The source is simulated as a lumped port between the electrode and the bed plate with a power of 4 kW at 27.12 MHz.
Electrode
PVC
Ground plate
0 500 1000 mm Figure 5-12: RF welding machine HFSS model
The simulation is sensitive to many parameters including the different layers of ground system, copper connections, and the dielectric layer on the welding table (PVC layer). The radiated electromagnetic field in the space in front of this model was extracted at different planes in which it will be compared to the real measurements.
138 Industrial RF Welding Machines EMF Characterization
5.4. EXPERIMENTAL ANALYSIS
The RF energy emitted by an industrial welding machine was characterized in terms of the magnetic and electric fields. We carried out three different measurement campaigns near the Gooseneck machine located at INRS site in Nancy (presented in 5.2 ). This allowed us to make the mapping of the radiated magnetic and electric fields in the place where the operator is normally located in front of the machine. Measurements were done without the presence of the operator in order to know the cartography of the radiated field as it is strongly distorted by the presence of a worker. The distance between the machine and the nearest wall was greater than 2m. The evaluation of the exposure to electric and/or magnetic fields is carried out at different planes in front of the machine taking into account that the average height of an operator is about 1.75 m. In the first campaign the commercial EHP200A probe from Narda was used, the purpose was to have an idea about the spectrum of the radiated field and to have an initial plot of the distribution of the EMF around the machine. After that we used the tri-axial magnetic field probe P2, presented in Chapter 2, to measure the H-field in front of the machine. And in the final campaign we measured the E-field using electro-optical probes (eoProbe) from Kapteos.
5.4.1. Preliminary measurements using EHP200A
The commercial electromagnetic field probe EHP200A (presented in 4.4.1 ) was used to determine the frequency spectrum of the radiated field, and to have an initial idea about its distribution around the welding machine. The probe can measure both the electrical and magnetic field components in a frequency range from 9 kHz to 30 MHz. The spectrum of the radiated electromagnetic field was measured with the probe, having a span of 10 MHz and RBW of 100 kHz in the probe‘s software configuration. We notice that the signal is around 27 MHz, and it depends on the configuration of the machine. The latter has a ground plate that can be used or not. When it is not used, we observe that the spectrum has two peaks at 26.9 and 28.8 MHz, as we can see in Figure 5 -13. On the other hand, when the ground plate used, we observe the presence of just one peak at 27.3 MHz (Figure 5 -13) with much
Industrial RF Welding Machines EMF Characterization 139
lower E-field level than the previous case and this is really the role of the ground plate, i.e. to reduce the radiated RF energy.
f=28.8 MHz f=27.3 MHz |E|=472 V/m |E|=73 V/m
f=26.9 MHz |E|=462 V/m
Figure 5-13: Spectrum of the radiated signal in front of the RF welding machine, measured using EHP200A probe. The machine with the ground plate (left) and without the ground plate (right)
The electric field was measured in a horizontal plane in front of the machine, at a height of 80 cm and with the dimensions of 160cm × 180cm along x- and y-axis respectively. At each point, the signal was recorded twice depending on the machine‘sconfiguration,i.e.withandwithoutthegroundplate.Theexposureindex (EI) is then calculated, using equation ((5 -1), from the E-field values, knowing that the reference action level given in the 2013/35/EU directive is 61 V/m at 27.12 MHz.
E EI fi (5-1) i E AL, fi
140 Industrial RF Welding Machines EMF Characterization
Exposure index – without ground plate
RFWM
axis [cm] axis
- y
CP Distance along Distance
Distance along x-axis [cm] Figure 5-14: Exposure index calculated from the measurements made without ground plate (RFWM = RF welding machine position, CP = control panel position)
Exposure index – with ground plate
RFWM
axis [cm] axis
- y
CP Distance along Distance
Distance along x-axis [cm]
Figure 5-15: Exposure index calculated from the measurements made with ground plate (RFWM = RF welding machine position, CP = control panel position)
Where Efi is the measured electric field at the frequency fi and EAL,fi is the corresponding action level given in the directive. Unsurprisingly, the intensity of the electromagnetic field decreases as we move away from the machine as we can see in the plots. Figure 5 -14 corresponds for the exposure index distribution for the field radiated in the case without using the ground plate, the place of the welding machine is shown as well as that of the control panel. On the other hand Figure 5 -15 shows
Industrial RF Welding Machines EMF Characterization 141
the exposure index for the other configuration in which the ground plate in the machine is used. The observed asymmetry in the distribution of the field is probably due to the asymmetry in the machine itself as the RF generator cabinet is not centered, which leads to more reflections. The results shows that the ALs are exceeded in the vicinity of the machine in both cases (with and without the ground plate), while the place of the control panel seems to be in a safe region just in the case if using the ground plate.
5.4.2. Magnetic field measurements For more precision and higher resolution field mapping, we performed the measurements in vertical planes in front of the machine using the tri-axial magnetic field probe P2, presented in details in 0 in chapter 2, where the three components of H-field was recorded. The measurement setup (Figure 5 -16) consists of the WaveRunner 640Zi oscilloscope that was used to measure the RF signals, in addition to a 3D robotic system which will allow more measurement points to be taken in less time and with greater precision.
Ref. Probe 3D Scanning Robot RF Welding Position Machine Control Panel
Tri-axial Probe P2
WaveRunner PC Oscilloscope 32
Figure 5-16: Measurement setup for H-field measurements in front of the RF welding machine using P2 probe
A reference probe (P2z) near the electrode of the machine was added in order to be able to extract the phase of the signal. Hence, for each point in the space we have
142 Industrial RF Welding Machines EMF Characterization
recorded four signals, three of them are for H-field components and the fourth is for the reference probe signal. Channels 1, 2, and 3 of the oscilloscope were used for Hx,
Hy, and Hz field components respectively, whereas the fourth channel is used for the reference probe. The fourth channel signal was used as a triggering signal as it corresponds to the fixed reference probe. The waveforms were recorded in time domain for duration of 20ms. As the frequency of the signal is around 27.12 MHz, a sampling rate of 100 MS/s was chosen for data acquisition. Figure 5 -17 shows an example of the recorded signals; the measured voltage levels were converted to H- field values using the AF of the probe P2 at 27.12MHz (Figure 2-39).
Figure 5-17: Example of the recorded waveforms for each measured point
In the post processing, all the references signals were aligned by interpolating each point using FFT methods (interpolation using the Fourier transform of the temporal signal) to resample at 1 GHz. Then we chose one point as a reference of the reference points then we align the other points to this point with a cross-correlation function. The signal that is generated by the RF welding machine is a modulated signal and has the shape of an AM signal. The maximum amplitude level extracted from the measured signals (VMS,max) should be corrected before converting it to the corresponding magnetic field. The correction factor given in equation (5 -2) was used. V 2 CF CW, rms (5-2) 2 VMS, max
Industrial RF Welding Machines EMF Characterization 143
Where VCW,rms is the RMS value to be used for magnetic field extraction and VMS,max is the maximum amplitude of the modulated signal measured by the oscilloscope. The CF was calculated with a signal generated at 27.12 MHz at different power levels, with and without modulation, and it was found to be 0.027.
The measurements were done at 4 different vertical planes in the free space in front of the machine. The dimension of each plane is 50cm × 50cm with a step size
of 10cm; therefore we have 36 points at each plane. The first plane, P0, was at 15cm
away from the machine table, whereas the other planes P1, P2, and P3 were respectively at 25cm, 35cm, and 45cm from the table. The corresponding configuration is shown in Figure 5 -18.
P3 P2 P1 P0
2cm
36cm
15cm Reference
10cm 50cm 10cm Probe Position 50cm z z y x
Figure 5-18: Measurement planes configurations in front of the RF welding machine
144 Industrial RF Welding Machines EMF Characterization
Each component of the magnetic field was measured separately, and then the total value was calculated by the root sum square of all the field components.
P3 P2 P1 P0
25 25 25 25 1.4 1.1 0.9 0.7
0.8 0.5 0 0.8 0 0 0.5 0
0.2 0.4 0.2 0.2 -25 -25 -25 -25- 25 0 25 -25 0 25 -25 0 25 -25 0 25
25 25 25 25 0.22 0.07 0.09 0.13
0.14 0 0.05 0 0.06 0 0.08 0
0.03 0.03 0.03 0.04 -25 -25 -25 -25 -25 0 25 -25 0 25 -25 0 25 -25 0 25 Figure 5-19: Magnetic field plots in the planes in front of the welding machine. Measurements (top row) and simulations (bottom row)
The magnetic field distribution in the same planes was extracted from the HFSS model (shown in Figure 5 -12). The measured and simulated magnetic field values in
plane P0 have the same distribution while the values are quite different. The H-field in the upper section of the scanned plane shows a level of about 0.7 A/m which is much higher than the ALs (0.16 A/m at 27.12 MHz) given in the guidelines, hence workers must avoid being in this area while the machine is in operation. In the other planes, the measurement shows some hot spots in the cartographies (dotted circles in Figure 5 -19)thatweren‘tfound in the simulation. This is probably due to the robotic metallic arm, even though the tri-axial probe P2 was attached to a plastic bar having a length of 40cm away from the robot arm, but this didn‘t prevent enough the reflected RF waves. Especially at certain positions, when the end of the robot metallicarmcoincideswiththemachine‘stableedge,andonthesidewheretheRF generator exists, i.e. to the right side of the machine. Other measurements were made in the presence of a phantom represented by a cylindrical glass, filled with 2.5L of TSL, which has a diameter of 15cm and a height of 21cm. The prepared liquid was water plus NaCl salt (2.5 g/L of Sodium Chloride).
Industrial RF Welding Machines EMF Characterization 145
The electrical properties of this TSL were extracted in the frequency range of [10- 100] MHz and it is shown in Figure 4 -30.
Scanning was done in 3 vertical planes (Pc, Pt1, and Pt2) along x- and z-axis as shown in Figure 5 -20. Pc is in the center of the phantom with the dimensions of 12cm ×12cm, and the other two are tangential planes that are tangent to the surface of the phantom with the dimensions of 18cm ×18cm with a step size of 3 cm. The measurements and simulations results, shown in Figure 5 -21, have almost the same distribution however the levels are not the same, where the measured values are much higher. This analysis shows that the magnetic field is strongly influenced by the presence of the phantom, and the ALs are exceeded.
Cylindrical Phantom Machine’s Table
Pt1 20cm Pc
Pt2 z y x x
Worker’s Place Figure 5-20: Measurement planes configurations in front of the RF welding machine in the presence of the cylindrical phantom
146 Industrial RF Welding Machines EMF Characterization
Pt1 Pc Pt2 20 1.5 0.6 20 14 1.2
10 12 1 0.45 12 0.9
0.35 4 0.6 4 4 0.6 54 62 70 56 62 68 54 62 70
20 0.12 14 0.095 20 0.14
12 0.09 10 0.07 12 0.1
4 0.05 4 0.045 4 0.05 54 62 70 56 62 68 54 62 70 Figure 5-21: Magnetic field plots in the planes in front of the welding machine in the presence of the cylindrical phantom. Measurements (top row) and simulations (bottom row)
5.4.3. Electric field measurements
The measurements of the electric field radiated from the RF welding machine were performed using an electro-optical probe (eoProbe) from Kapteos (shown in Figure 5 -22). It is an ultra-wide band sensor that measures one component of E-field with high precision [11], [12]. The probe must be used with the eoSense instrument. The latter converts the optical signal transmitted by the eoProbe into an electrical signal that can be analyzed with another data acquisition instrument like an oscilloscope or a spectrum analyzer [13].
Figure 5-22: Electro-optical probe used for the E-field measurements [11]
Industrial RF Welding Machines EMF Characterization 147
The scan was performed in three vertical planes with dimensions of 80cm × 120cm along z- and y-axis respectively, with a step size of 10cm. The three planes
P0, P1, and P2 wererespectivelyat37cm,46cm,and56cmawayfromthemachine‘s electrode. Each plane was scanned 3 times, each time having different orientation of the eoProbe, so that the 3 components of the electric field Ex, Ey, and Ez are measured. The antenna factor of the eoProbe is given by the eoSense and it was noted at the beginning of each mapping, and adjusted at the end, according to the variations observed. A spectrum analyzer (Anritsu MS2724B) was used to measure the electrical signal at the output of the eoSense. The signal was at 27.3 MHz and the measured power is converted to electric field using the following formula:
P SA 3 (5-3) E AF 50 10 10 ii
Where Ei is the corresponding E-field component (i = x, y, or z), AFi istheprobe‘s antenna factor, and PSA is the power measured by the spectrum analyzer. Then the total electric field was calculated.
EEEE222 (5-4) x y z
Robot Plastic-arm
z
eoProbe y
Figure 5-23: E-field eoProbe measurement configuration
In this measurement campaign, the metallic arm of the displacement system was replaced by a plastic-armtoavoidanyreflections.Themachine‘sconfigurationwas with the ground plate when the measurements were done. The measurement results are shown in Figure 5 -24, the dashed lines shows the place of the electrode and the
148 Industrial RF Welding Machines EMF Characterization
weldingmachine‘stable,andthereddotsrepresentsthemeasuredpoints.Thelevel
of the E-field ranges between 60 V/m and up to 300 V/m as it is clear in plane P0, these levels exceeds the ALs given in the directives. The observed asymmetry in the field plots is explainedbythefact thatthemachine‘s RFgenerator,locatedat the back, is being shifted to the right.
P2 P1 P0
80 130 80 150 80
300 [cm]
40 100 40 100 40 200
axis
- z
100 0 0 60 0 50 0 60 120 0 60 120 0 60 120 y-axis [cm] y-axis [cm] y-axis [cm] Figure 5-24: Electric field plots in the planes in front of the RF welding machine
5.5. CONCLUSION
In this chapter we presented the analysis and characterization of the electromagnetic fields emitted by industrial machines, in particular RF welding machines. These machines radiate a strong electromagnetic field that may causes hazardous problems to the exposed workers. Numerical simulations show that the presence of human body significantly alters the electric and magnetic fields, as they are strongly modified by the presence of the body. However the internal structure of the human body has a lite effect on the field. Measurements were performed on a real machine that exists at INRS; this welding machine operates at 27.12 MHz with a power of 7.5 kW. Magnetic field was measured using the tri-axial probe P2 (presented in Chapter 2) while for the electric field an electro-optic probe was used. Good precision cartographies were obtained (to be used for the numerical formulations) showing that the intensities of the measured quantities are extremely dependent on the environment near the machine. Even though the ground plate in the machine is used, the field levels still high, and ALs are exceeded in some places, hence workers should avoid being in these places while the machine is in operation.
Industrial RF Welding Machines EMF Characterization 149
5.6. REFERENCES
[1] P.DémaretandP.Donati,―Intégrerlerisque‗rayonnements électromagnétiques‘dansledocumentuniqued‘évaluation des risques professionnels.‖INRS-Hygiène et Sécurité du Travail, 2011.
[2] J. Wilén et al.,―ElectromagneticfieldexposureandhealthamongRFplastic sealeroperators:RFOperatorsandHealth,‖Bioelectromagnetics, vol. 25, no. 1, pp. 5–15, Jan. 2004.
[3] A.TOURAINE,―Chauffageparpertesdiélectriques.‖TechniquesDe L‘ingenieur,10-Mar-1994.
[4] C.Bissieriexetal.,―Lespressesutilisantlechauffageparpertes diélectriques.‖INRS- Champs électromagnétiques ED 4205, Aug-2015.
[5] Lambert LalibertéandLaurentGratton,―Leblindagecommemoyende contrôle des radiofréquences des machines industrielles chauffant par perte diélectrique.‖l‘InstitutderechercheRobert-Sauvé en santé et en sécurité du travail (IRSST), 01-May-1995.
[6] R. KLEIN,―Réductiondurayonnementélectromagnétiquedessoudeuses hautefréquencedeconceptionancienneàl‘aided‘unpatindemasse.‖INRS- Hygiène et sécurité du travail ND 2360, Jun-2012.
[7] K. Hansson Mild et al.,―Exposureofworkerstoelectromagneticfields. A reviewofopenquestionsonexposureassessmenttechniques,‖Int. J. Occup. Saf. Ergon., vol. 15, no. 1, pp. 3–33, 2009.
[8] A. Christ et al.,―TheVirtualFamily—development of surface-based anatomical models of two adults and two children for dosimetricsimulations,‖Phys. Med. Biol., vol. 55, no. 2, pp. N23–N38, Jan. 2010.
[9] ―TissueProperties- Database,‖IT’IS Foundation. [Online]. Available: www.itis.ethz.ch/virtual-population/tissue-properties/database/dielectric-properties/.
[10] R. KLEIN andP.Donati,―Réalisationetinstallationd‘unpatindemassesur lessoudeuseshautefréquencedeconceptionancienne.‖NoteScientifiqueet Techniquedel‘INRS,NS301,2013.
[11] Kapteos,―eoProbe.‖[Online].Available:www.kapteos.com/en/products- portfolio/emf-probe/.
[12] L.DuvillaretandG.Gaborit,―Electroopticprobeformeasuringtemperature andelectromagneticfield,‖US12305068,15-Jun-2007.
[13] Kapteos,―eoSense.‖[Online].Available:www.kapteos.com/en/products- portfolio/eosense-instrument/.
150 Industrial RF Welding Machines EMF Characterization
Conclusion and Perspectives
This thesis presents the development of a new near-field scanning system for electromagnetic near-field characterization, in particular for occupational exposure and dosimetric analysis. As well as the dosimetric assessments near two applications used by professionals.
In the first chapter, a general review on the propagation of electromagnetic fields and the corresponding interactions with matter has been presented. An overview of RF dosimetry is presented stating the different parameters used to quantify the electromagnetic phenomenon, in addition of introducing numerical and experimental dosimetry. At the end of this chapter, the main standards and guidelines that have been established to manage the exposure of people to electromagnetic fields are presented.
The existing electromagnetic field probes for exposure analysis are either designed to measure one component of the field or their size is somehow large. These limitations lead to designing new compact low coast tri-axial probes, in particular for magnetic field measurements. Two versions were presented; the first is based on conventional loop elements while the other is based on printed circuit board technology. The design of these probes was presented in chapter two, as well as the corresponding calibration. The probes was validated and then used in characterizing different radiating sources. In addition, a near-field scanning system was developed including two scanners foe different types of applications. The developed system can perform near-field measurements in both time and frequency domains.
Using the designed probes the scanning time near a radiating source is reduced by a factor of three. However, for some applications the 3D field mapping is needed. In this context, a computational algorithm has been reported. It is based on the decomposition of the electromagnetic field components and computing Maxwell‘s equations. Passing from spatial to spectral domain, the tangential magnetic field components are used to extract the third component and then from the near-field scan in a single plan above the DUT, the 3D magnetic field distribution is constructed.
Conclusion and Perspectives 151
The performance of this algorithm has been realized in the third chapter, where it was validated near two applications; a loop and PIFA antennas.
In chapter four, the exposure assessment near RFID reader antennas was performed. A detailed study was presented on the radiated electromagnetic field from RFID systems used in libraries which operates at 13.56 MHz. Electromagnetic field measurements, both in situ and in the laboratory, made it possible to evaluate the field levels to which users of these systems can be subjected. The strength of the field degrades rapidly as we go far from the antenna, whereas in the vicinity area the levels were found high, but it still within the reference levels when it is averaged.
In the fifth chapter, the exposure of workers to the strong energy emitted by RF welding machines is studies. A numerical simulation was performed at the beginning to check the effect of the presence of an operator on the field distribution near the machine. Then, both electric and magnetic fields were measured near a real machine in the place of the presence of workers while it‘s in operation. It has been demonstrated that the presence of any metallic element near the machine strongly affects the field distribution. The recorded field levels have shown high values near themachine‘selectrodethatexceedsthereferencelevels.
Concerning the presented measurement system, the further enhancement that can be done in terms of the scanning time is the development of an array of probes. Some applications have large sizes and there is a need of having the filed map in different planes near these devices. The presented tri-axial probes in this thesis reduce the scanning time by a factor of 3. For more optimization in the measurement time we can go forward and developed an array of these probes. This helps in making the measurement at different locations in the space at the same time. We have already started in fabricating a linear array of 8 elements form the P2 tri-axial probe, in which they are connected to 3 RF switches (HMC321) so each switch corresponds to one field component. The figure below clarifies the system under development that is under validation progress.
152 Conclusion and Perspectives
PC
Controller Measuring Instrument
RF Switch 3DP2 Probe Element
Probe Array
DUT
Moreover, the presented prediction algorithm has several points to be improved among which: Extraction of E-field components from the tangential H-field ones Extraction of SAR from the obtained E-field values, to be validated in TSL.
Finally, we are developing another prediction algorithm which aims for extracting the electromagnetic field distribution in a phantom present near a radiating device starting from the magnetic field values measured in the same area without having the presence of the phantom. The necessary FEM formulations of this method are developed using ONELab and preliminary simulation results show the validity of his method. The performed measurements in this thesis will be used to feed the developed algorithm and to validate it on real radiating applications.
Conclusion and Perspectives 153
Appendix
A.1. La Duchère Library
In this library there is just one type of RFID systems that is used, in addition to an anti-theaft detection system based on RFID technology. Both systems are from Nedap Library Solutions. The radiated near field is measured using EHP-200A and the results are tabulated, comparing the measured values with the guidelines levels.
A.1.1. Nedap Flex-desk Station This section shows the measurement that was done near two similar self-check RFID systems from Nedap (type: C-O Station Flex B). The probe was placed in the center of the table where the RFID pad is found. The figure below shows one of these systems in the presence of the probe, followed by the results table.
Figure A-1: Nedap flex-desk station
155
Table A-1: RMS measured values above the RFID pad in the Nedap flex-desk station, compared to Action Levels Measured RMS Value Limitation Percentage with (over 6 minutes) Limitation Percentage with Action Level respect to the Electric field in [V/m] Action respect to the for RF pulses reference level Magnetic field in Level (AL) reference level (32×AL) for RF pulses [A/m] 22.34 V/m 61 V/m 36.62 % 1952 V/m 1.14 % Table #1 1.35 A/m 0.16 A/m 843.75 % 5.12 A/m 26.37 %
24.45 V/m 61 V/m 40.08 % 1952 V/m 1.25 % Table #2 1.14 A/m 0.16 A/m 712.5 % 5.12 A/m 22.27 %
A.1.2. Nedap PG45 detection system In this section the measurement was done at different positions near the anti-theft detection system (PG45-4W-PCC12-3) from Nedap. It has the dimensions of 480mm×1665mm, with a gate opening of one meter. The measurement positions are sown below. Pos. 2
Pos. 1 Pos. 3
Pos. 5 Pos. 4 Pos. 6 1665 mm Pos. 7
428 mm Figure A-2: Nedap PG45 Detection System indicating the different measurement positions
156
Table A-2: RMS measured values near the Nedap PG45 Detection System, compared to Action Levels Measured RMS Value Limitation Percentage with (over 6 minutes) Limitation Percentage with Action Level respect to the Electric field in [V/m] Action respect to the for RF pulses reference level Magnetic field in Level (AL) reference level (32×AL) for RF pulses [A/m] Pos 1 11.5 V/m 61 V/m 18.85 % 1952 V/m 0.59 % (h=150 cm) 0.16 A/m 0.16 A/m 100 % 5.12 A/m 3.13 %
Pos 2 40.9 V/m 61 V/m 67.05 % 1952 V/m 2.1 % (h=150 cm) 1.9 A/m 0.16 A/m 1187.5 % 5.12 A/m 37.11 %
Pos 3 87.56 V/m 61 V/m 143.54 % 1952 V/m 4.49 % (h=110 cm) 1.83 A/m 0.16 A/m 1143.75 % 5.12 A/m 35.74 %
Pos 4 102.5 V/m 61 V/m 168.03 % 1952 V/m 5.25 % (h=75 cm) 2.74 A/m 0.16 A/m 1712.5 % 5.12 A/m 53.52 %
Pos 5 59.52 V/m 61 V/m 97.57 % 1952 V/m 3.05 % (h=75 cm) 2.01 A/m 0.16 A/m 1256.25 % 5.12 A/m 39.26 %
Pos 6 43.88 V/m 61 V/m 71.93 % 1952 V/m 2.25 % (h=75 cm) 0.169 A/m 0.16 A/m 105.63 % 5.12 A/m 3.35 %
1 Pos 7 54.91 V/m 61 V/m 90.02 % 1952 V/m 2.81 % (h=40 cm) 2.26 A/m 0.16 A/m 1412.5 % 5.12 A/m 44.14 %
A.2. Bachut Library In this library the measurements were done near two types of RFID Systems and an anti-theft detection system, and all of them are from 3M Company. The near fields are measured and the results are tabulated as in the previous sections. The used probe was placed at the center of the RFID pads and at different positions in the detection system, and the RMS over 6 minutes was measured.
A.2.1. 3M™ Pad Staff Workstation Model 800 Measurements was done near this Black pad, used by the workers at the library, at two situations with and without a presence of a tag. The following figure shows this pad in the presence of the used probe.
157
Figure A-3: 3M RFID pad (model P12) used by the library staff
Table A-3: RMS measured values near the RFID P12 Pad, compared to Action Levels Measured RMS Value Limitation Percentage with Limitation Percentage with (over 6 minutes) Action Level respect to the Action respect to the Electric field in [V/m] for RF pulses reference level Level (AL) reference level Magnetic field in [A/m] (32×AL) for RF pulses The Pad in 21.2 V/m 61 V/m 34.75 % 1952 V/m 1.09 % normal 3.12 A/m 0.16 A/m 1950 % 5.12 A/m 60.94 % operation
The Pad in 28.7 V/m 61 V/m 47.05 % 1952 V/m 1.47 % the presence 3.86 A/m 0.16 A/m 2412.5 % 5.12 A/m 75.4 % of a tag
A.2.2. 3M Self-check System (Classic) In this section measurements were done near two similar 3M self-check systems (R-series classic) used by public users. The following figure shows one of these systems in the presence of the measuring probe placed in the center of the RFID pad.
158
Figure A-4: 3M self-check system (classic R-series) used by public users
Table A-4: RMS measured values above the RFID pad in the self-check system, compared to Action Levels
Measured RMS Value Limitation Percentage with Limitation Percentage with (over 6 minutes) Action Level respect to the Action respect to the Electric field in [V/m] for RF pulses reference level Level (AL) reference level Magnetic field in [A/m] (32×AL) for RF pulses
30.04 V/m 61 V/m 49.25 % 1952 V/m 1.54 % Table 1 1.92 A/m 0.16 A/m 1200 % 5.12 A/m 37.5 %
14.4 V/m 61 V/m 23.6 % 1952 V/m 0.74 % Table 2 1.27 A/m 0.16 A/m 793.75 % 5.12 A/m 24.8 %
A.2.3. 3M Detection System (Model 9100 BC) In this section the measurements was done at different positions near the 3M anti- theft detection system (model 9100 BC). It has gate width of 91.4 cm. The corresponding results are tabulated below.
159
Pos. 1
Pos. 2 Pos. 3
Figure A-5: 3M Detection System (model 9100 BC) indicating the different measurement positions
Table A-5: RMS measured values near the 3M Detection System, compared to Action Levels Measured RMS Value Limitation Percentage with (over 6 minutes) Limitation Percentage with Action Level respect to the Electric field in [V/m] Action respect to the for RF pulses reference level Magnetic field in Level (AL) reference level (32×AL) for RF pulses [A/m]
43.64 V/m 61 V/m 71.54 % 1952 V/m 2.24 % Pos 1 0.84 A/m 0.16 A/m 525 % 5.12 A/m 16.4 %
151.7 V/m 61 V/m 248.69 % 1952 V/m 7.77 % Pos 2 0.75 A/m 0.16 A/m 468.75 % 5.12 A/m 14.65 %
25.34 V/m 61 V/m 41.54 % 1952 V/m 1.3 % Pos 3 0.13 A/m 0.16 A/m 81.25 % 5.12 A/m 2.54 %
160
List of Publications
Journal papers:
Kassem Jomaa, Fabien Ndagijimana, Hussam Ayad, Majida Fadlallah, Jalal Jomaah,―RFIDAntenna Near-field Characterization Using New 3D Magnetic Field Probe‖AdvancesinScience,TechnologyandEngineeringSystemsJournal- 2017
Mazeh F, Ayad H, Fadlallah M, Jomaa K,JomaahJ&NdagijimanaF,―Even- odd mode of a double-Lorentz metamaterial and its application to a tri-band branch- linecoupler‖EPJAppliedMetamaterialsJournal- 2016
International Conferences:
Kassem Jomaa, Fabien Ndagijimana, Houssam Ayad, Majida Fadlallah, Jalal Jomaah,―PlaneWaveSpectrumMethodAppliedonRadiatedMagneticField from RFIDReaderAntenna‖InternationalSymposiumandExhibitiononElectromagnetic Compatibility EMC Europe 2018
Kassem Jomaa, Nimisha Sivaraman, Fabien Ndagijimana, Hussam Ayad, Majida Fadlallah, Jalal Jomaah, ―Development of a 3D Scanning System for Magnetic Near-fieldCharacterization‖InternationalSymposiumonElectromagnetic Compatibility, Signal and Power Integrity 2017
Kassem Jomaa, Fabien Ndagijimana, Houssam Ayad, Majida Fadlallah, Jalal Jomaah,―Near-Field Characterization for 13.56 MHz RFIDAntenna‖International Symposium and Exhibition on Electromagnetic Compatibility EMC Europe 2017
Kassem Jomaa, Fabien Ndagijimana, Houssam Ayad, Majida Fadlallah, Jalal Jomaah, ―Exposure to Electromagnetic Near-Fields Radiated by an RFID Reader Antenna‖ IEEE Applied Electromagnetics Conference (AEMC-2017)
Kassem Jomaa,FabienNdagijimana,JalalJomaah,HoussamAyad,―Near-field Measurement System with 3D Magnetic-field Probe Design for Dosimetric Applications‖IEEEMiddleEastConferenceonAnt.and Propagation MECAP 2016
Giacomo DI BENEDETTO, Riccardo SCORRETTI, Arnaud BREARD, Damien VOYER, Fabien NDAGIJIMANA , Kassem JOMAA,MelinaBOULDI,―Exposure of workers to the electromagnetic field of a RF welding machine‖ The 18th International Symposium on Applied Electromagnetics and Mechanics ISEM-2017
Nimisha Sivaraman, Kassem Jomaa, Fabien Ndagijimana,―Three Dimensional Scanning System for Near-field Measurements‖ 11th European Conference on Antennas and Propagation EUCAP-2017
161
Bilal Hammoud, Fatima Mazeh, Kassem Jomaa, Hussam Ayad, Fabien Ndagijimana, Ghaleb Faour, and Jalal Jomaah ―Dual-Frequency Oil Spill Detection Algorithm‖ComputingandElectromagnetics International Workshop CEM-2017
Bilal Hammoud, Fatima Mazeh, Kassem Jomaa, Hussam Ayad, Fabien Ndagijimana, Ghaleb Faour, Majida Fadlallah, and Jalal Jomaah, ―Multi-Frequency ApproachforOilSpillRemoteSensingDetection‖ International Conference on High Performance Computing & Simulation HPCS-2017
Mohamad.H.Haroun, Hussam Ayad, Jalal Jomaah, Majida Fadlallah, Kassem Jomaa, Marta Cabedo Fabres, Miguel Ferrando Bataller, ―SampledAntennaArray Digital Beamforming for LTE-Advanced‖ International Conference on High Performance Computing & Simulation HPCS-2017
F. Mazeh, H. Ayad, M. Haroun, K. Jomaa, J. Jomaah and F. Ndagijimana, ―Even-Odd Mode of a Tri-band Double-Lorentz Transmission Line Metamaterial‖ International Workshop on Metamaterials-by-Design: Theory, Methods, and Applications to Communications and Sensing IWMbD-2015
National Conferences:
Kassem JOMAA, Giacomo DI BENEDETTO, Melina BOULDI, Fabien NDAGIJIMANA, Patrice DONATI, Riccardo SCORRETTI, ―Caractérisation de l‘expositiondetravailleursauchamp radiofréquencegénéréparunepresseHF‖, Journée Scientifique SFRP 2018
Nimisha Sivaraman, Kassem Jomaa,YanFu,FabienNdagijimana,―Systèmede balayage tridimensionnel pour les mesures à champ proche‖ 20èmes Journées Nationales Micro-Ondes JNM-20 2017
162
Abstract
The analysis of radiated emissions from a source throughout the space, is very essential for both dosimetric and electromagnetic interference analysis. The concerns about the latter are growing because of the need to have prediction of the system reliability of the electronic circuits. Moreover the everyday use of devices and systems emitting radio frequency electromagnetic fields is continuously increasing. Some of these devices are operating in the vicinity of human body, and operators are in the near-field region of the radiating source, and they are exposed to electromagnetic fields. For this reason, dosimetric analysis, that shows the necessity of having three dimensional (3D) field mapping in the vicinity of the radiating source, should be performed. For this kind of applications, several scans of the near fields should be done within different planes in order to build the 3D field mapping. Being a challenging process in electromagnetic compatibility studies, near field characterization is being treated by several algorithms that propose different approaches to achieve the required scanning on the radiating source. In this work, we introduce a 3D scanning system with a low cost three axis magnetic field probes. Having such probes allow the simultaneous measure of the three components of the magnetic field based on a single planner scan above the device under test. The designed probes consist of three orthogonal loops combined together; the first probe contains three conventional loops joined in a plastic cube with a total dimension of 10×12×13 mm3, whereas the second probe is a PCB probe printed on an FR4 substrate of 3.2 mm with a reduced dimension of 9×9×3.2 mm3. The designed probes were calibrated with a TEM cell and the corresponding antenna factors were extracted. The presented scanning system uses an oscilloscope as a measuring instrument that gives the possibility of both time and frequency domain measurements. The second part of this thesis presents a reconstruction algorithm based on plane wave spectrum method. In order to reduce the number of scans and hence the time requirements, the presented algorithm requires just a 2D near field scan of the field components, to reconstruct the 3D magnetic field distribution above the radiating device. The third part of the thesis is devoted for the dosimetric analysis of the radiated electromagnetic fields near RFID systems and RF-welding machines. The near-field exposure assessment of the radiated fields from RFID reader antennas operating at 13.56 MHz and used in Libraries was performed. The measurements of the magnetic field near the antenna were established using the designed probes. The results are then analyzed and compared to the regulations in European Directives and ICNIRP Guidelines. Moreover, the exposure to RF electromagnetic fields of workers near RF-welding machines in industrial environment is studied. These machines, operating at 27.12 MHz, emit strong radiation and the exposure takes place in the near-field region. The spatial distribution of the electromagnetic fields in this region is studied in both numerical simulations and measurements.
Keywords: Near-field scanning, magnetic field probe, dosimetry, exposure, RFID, RF-welding machine, prediction algorithm
163
Résume
L'étude des émissions rayonnées d'une source dans tout l'espace, est essentielle pour l'analyse dosimétriques etl‘analyse des interférences électromagnétiques. L'importance du dernier augmente en raison de la nécessité d'avoir une prédiction de la fiabilité des circuits électroniques. De plus, l'utilisation quotidienne des dispositifs et des systèmes émettant des champs électromagnétiques radiofréquences ne cesse d'augmenter. Certains de ces dispositifs fonctionnent à proximité du corps humain, et les opérateurs se trouvent dans la région des champs proches de la source rayonnante, et ils sont exposés à des niveaux de champs électromagnétiques pouvant être élevés. Pour cette raison, l'analyse dosimétrique, qui passe par une cartographie tridimensionnelle (3D) au voisinage de la source rayonnante, doit être effectuée. Pour ce type d'applications, plusieurs scans des champs proches doivent être effectués dans différents plans afin de construire la cartographie de champs 3D. Étant un processus difficile dans les études de compatibilité électromagnétique, la caractérisation en champ proche est traitée par plusieurs algorithmes qui proposent différentes approches pour réaliser le scanning requis au voisinage de la source rayonnante. Dans ce travail, nous introduisons un système de scanning 3D avec des sondes de champ magnétique à trois axes à faible coût. Le fait de disposer de telles sondes permet la mesure simultanée des trois composantes du champ magnétique sur la base d'un seul scan au voisinage du dispositif testé. Les sondes conçues se composent de trois boucles orthogonales combinées ensemble; la première sonde contient trois boucles conventionnelles réunies dans un cube en plastique d'une dimension totale de 10×12×13 mm3, tandis que la deuxième sonde est une sonde PCB imprimée sur un substrat FR4 de 3,2 mm avec une dimension réduite de 9×9×3,2 mm3. Les sondes conçues ont été étalonnées avec une cellule TEM et les facteurs d'antenne correspondants ont été extraits. Le système de scanning présenté utilise comme un instrument de mesure un oscilloscope RF- 4 canaux, qui donne la possibilité de mesurer à la fois dans le domaine temporel et dans le domaine fréquentiel. La deuxième partie de cette thèse présente un algorithme de reconstruction basé sur la méthode du spectre d'ondes planes. Afin de réduire le nombre des scans et donc les exigences de temps, l'algorithme présenté nécessite juste un scan en champ proche 2D des composantes de champ, pour reconstruire la distribution du champ magnétique 3D au-dessus du dispositif rayonnant. La troisième partie de la thèse est consacrée à l'analyse dosimétrique des champs électromagnétiques rayonnés à proximité des systèmes RFID et des machines de soudage RF. L'évaluation de l'exposition en champ proche des champs rayonnés des antennes de lecture RFID, fonctionnant à 13,56 MHz et utilisées dans les bibliothèques, a été réalisée. Les mesures du champ magnétique près de l'antenne ont été établies à l'aide des sondes conçues. Les résultats sont ensuite analysés et comparés aux réglementations des normes européennes et des lignes directrices de l'ICNIRP. En outre, l'exposition aux champs électromagnétiques RF des travailleurs à proximité de machines de soudage RF dans un environnement industriel est étudiée. Ces machines, fonctionnant à 27.12 MHz, émettent de forts rayonnements et l'exposition a eu lieu dans la région de champ proche. La distribution spatiale des champs électromagnétiques dans cette région est étudiée à la fois dans des simulations numériques et des mesures réelles.
Mots clés: Scan champ proche, sonde de champ magnétique, dosimétrie, exposition, RFID, machine de soudage RF, algorithme de prédiction.
164