Antenna Designs for Nfc Devices

Near-field communication (NFC) enables the exchange of information between close devices.

The antenna is the indispensable element to transform an electronic device into an NFC system. For both theory and practice, this book presents in detail the design technologies of different antennas. They must meet the NFC ISO 18 092 and 21 481 standards as well as specifications by the NFC Forum for industrial applications, by EMVCo for banking applications and payments, and by CEN for public transport.

In a particularly pedagogic way, Antenna Designs for NFC Devices enables designers of communicating object systems and the Internet of Things (IoT) to have access to the mysteries of the design of NFC antennas.

Dominique Paret is a consultant in radiofrequency identification, contactless technologies, NFC and indoor geolocalization. He teaches electronics and RFID in various engineering schools. He is also the author of several books on RFID technologies.

Acknowledgments xi

Preface xiii

Introduction xvii

Part 1. Context 1

Introduction to Part 1 3

Chapter 1. Recap of the Constraints Governing the Design of Antennas for an NFC Device 5

1.1. Normative constraints 6

1.1.1. Uplink from initiator to targets 7

1.1.2. Downlink from targets to initiator 8

1.1.3. “Contactless” standards versus NFC device antennas 10

1.1.4. Technologies 12

1.1.5. “NFC Forum Devices” and “NFC Forum Tags” 12

1.1.6. Modes of communication of an NFC Forum Device 14

1.1.7. Role of an NFC Forum Device 16

1.1.8. Beware of false advertising 17

1.2. Regulatory constraints 17

1.2.1. RF regulations 17

1.3. Constraints on the NFC market 18

1.4. Typological constraints of NFC 19

1.4.1. Application consequences and their direct constraints 20

1.5. Applicational constraints on antenna design 21

Chapter 2. Introduction to and Recap of the Principles Employed in NFC 23

2.1. The physical fundaments of “contactless” and NFC 23

2.1.1. Phenomenon of propagation and radiation 23

2.1.2. Classification of fields and spatial regions 24

2.1.3. Spatial regions 24

2.1.4. Far field: r >> λ/2π (Fraunhofer zone) 24

2.1.5. Intermediary field: r approximately equal to λ (Fresnel zone) 25

2.1.6. Near field: r << λ/2π (Rayleigh zone) … and by essence, the origin of the “NF – Near Field”, and hence NFC 25

2.1.7. Remarks on contactless, RFID and NFC application 25

2.2. The concept of NFC  27

2.2.1. Biot–Savart law  28

2.2.2. Field H at a point on the axis of a circular antenna 28

2.2.3. Decrease in the field H as a function of “d” 31

2.2.4. Field H at a point on the axis of a rectangular antenna 32

Part 2. Methods and Designs for NFC Device Antennas 35

Introduction to Part 2  37

Chapter 3. “Initiator” Antennas: Detailed Calculations 41

3.1. Introduction 41

3.1.1. There are initiators … and there are initiators 41

3.2. Design of an initiator antenna (without influence from the outside environment) 42

3.2.1. Operating mode  43

3.2.2. Instructive recap 45

3.2.3. Choice of integrated circuit 54

3.2.4. Legislational constraining aspects and EMC pollution 59

3.2.5. EMC filtering 60

3.2.6. Choice of target used and incidence of its H_threshold 73

3.2.7. Determining the inductance value of the initiator antenna 75

3.2.8. Simple antenna 82

3.2.9. Matching circuit for the impedance of the antenna 88

3.2.10. Calculating the current in the antenna coil of the initiator 93

3.2.11. Summary and examples 96

3.2.12. Simulations  98

3.2.13. Value of the field H radiated by the antenna 100

3.2.14. Calculation and value of the working distance 101

3.3. Maximum quality coefficient Q of the initiator antenna 101

3.3.1. Q and cutoff of the field 103

3.3.2. Decrease in the ISO field  106

3.3.3. Measuring Q in the application 108

3.3.4. Measurement of the bandwidth in the application 109

3.4. Brief handbook on the process of designing an antenna initiator 110

Chapter 4. Examples of Applications of Initiator Antennas 113

4.1. Large antennas 113

4.1.1. Communication with a mono-NFC device in “card emulation – battery-assisted” mode 114

4.1.2. Communication multi-NFC devices in “tag batteryless” mode 114

4.2. Large antenna in mono-device 115

4.2.1. Mechanical formats of the NFC device targets 115

4.2.2. “Form factors” and sizes of antennas of the targets 115

4.2.3. Application distances required for operation 116

4.2.4. Estimation of the “loading effects” of the distance or working range 117

4.2.5. Environment (copper, ferrite, battery, etc.) 117

4.2.6. Several measures for illustrating our proposal 117

4.2.7. H_d field necessary for the NFC device target 119

4.2.8. H_0 necessary to create at the antenna level of the initiator 120

4.2.9. Power P (in watts) 120

4.2.10. Field H which must be produced by the initiator for a specific 120

4.2.11. Definition of the initiator antenna: format of the “landing area” of the reader (where one puts the target) 121

4.2.12. “System” considerations of the application 121

4.2.13. Market integrated circuits for direct attack of the antenna 122

4.2.14. Booster amplifiers 124

4.2.15. Problem of the retro-modulation value 128

4.3. Large antennas in multi-antennas 130

4.3.1. In simultaneous mode (temporarily non-multiplexed) 130

4.3.2. In multiplexed mode temporarily 133

4.4. Large antennas in multi-devices 135

4.4.1. Conclusions  137

4.5. Other examples of initiator antennas  138

Chapter 5. Antennas for Targets and Tags: Detailed Calculations 141

5.1. Introduction: … there is a target and target  141

5.2. NFC Forum Tags 141

5.2.1. “Technology Subset”  142

5.3. Introduction to problems of antenna targets/tags 146

5.3.1. Tuning of the targets/tags 146

5.3.2. The inductance L 146

5.4. State-of-the-art of the antenna sizes  154

5.4.1. Sizes of the target antennas 155

5.4.2. Examples of applications of targets with antennas in ISO classes 157

5.5. Technological aspect of the NFC targets and tags 165

5.5.1. Data specific to integrated circuits for usage by NFC targets 165

5.5.2. Data specific to the additional capacities 165

5.5.3. Industrial data specific to antenna technology 165

5.5.4. Technology at stake 166

5.5.5. Estimation of the minimum number of antenna coils of the target to guarantee its remote power supply 171

Chapter 6. Detailed Examples of Designs of Target Antennas 173

6.1. Case of small antennas  173

6.1.1. Examples in classes 4, 5, 6… or close by 174

6.1.2. Example of design in class 5 175

6.1.3. Example  180

6.1.4. Example of design in class 6  182

6.2. Case of very small antennas 189

6.2.1. Example of design in classes 11, 12, 13  190

6.3. Case of the large NFC target/tag antennas: format A4 203

6.3.1. NFC bib number antennas for marathon and triathlon runners 203

6.3.2. Technical properties required by the NFC target/tag  204

6.4. Case of very large antennas targets: format A3  205

6.4.1. Context and technical frame of the large antennas 205

6.4.2. Retained concept 206

6.4.3. Example of network with four antennas 213

6.4.4. Simplification of the equation 216

Chapter 7. The Initiator–Target Couple and Its Couplings  233

7.1. Circuits and their couplings 234

7.1.1. Mutual induction and mutual inductance 235

7.1.2. Perfect mutual 237

7.1.3. Non-perfect mutual 238

7.1.4. Coupling coefficient “k” 242

7.2. Tuned circuits coupled by mutual induction 244

7.2.1. Why “almost”? 245

7.2.2. Coupling index “n” 246

7.2.3. In conclusion, an important point 247

7.3. Identical coupled circuits, tuned to the same frequency 248

7.3.1. Transfer function, A(􀈦) = V2/V1, in terms of the voltage of the secondary 250

7.3.2. Transmission coefficient “Kt” 251

7.3.3. In summary  252

7.3.4. Operation in the vicinity of the resonance frequency f0 255

Chapter 8. The Initiator–Target Couple and the Loading Effect 271

8.1. Loading effect by coupling  271

8.2. Coupled tuned antennas in terms of the primary current 272

8.2.1. Primary (initiator) non-loaded (no target within the field) 273

8.2.2. Primary (initiator) with a load (presence of target(s) in the field)  274

8.2.3. Value of R2 in view of the environment 277

8.3. Some food for thought 278

8.4. Loading effect 281

8.4.1. Definition and comments  281

8.4.2. Parameters involved in the loading effect 282

8.4.3. Variation of the working distance and thus of the coupling 285

8.4.4. Magnetic coupling and its consequences 285

8.4.5. Performances required by the initiator: loading effect on the value of the remote power supply to the target  286

8.4.6. Quality of the emitted magnetic field 287

8.4.7. Examples of coupling coefficients and loading effects  295

8.4.8. “Shunt” circuit in NFC 302

8.5. Appendix: how do we approach an NFC project? 307

Conclusion 309

Bibliography 317

Index 319

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