Transparent and Flexible MIMO Antenna Technologies for 5G Applications : Transforming 5G with Transparent & Flexible MIMO Antennas [1 ed.] 9783031424854, 9783031424861

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EAI/Springer Innovations in Communication and Computing

Jayshri Kulkarni · Arpan Desai · Heng Tung Hsu · Brian Garner · Yang Li · Chow-Yen-Desmond Sim · Vigneswaran Dhasarathan

Transparent and Flexible MIMO Antenna Technologies for 5G Applications Transforming 5G with Transparent & Flexible MIMO Antennas

EAI/Springer Innovations in Communication and Computing Series Editor Imrich Chlamtac, European Alliance for Innovation, Ghent, Belgium

The impact of information technologies is creating a new world yet not fully understood. The extent and speed of economic, life style and social changes already perceived in everyday life is hard to estimate without understanding the technological driving forces behind it. This series presents contributed volumes featuring the latest research and development in the various information engineering technologies that play a key role in this process. The range of topics, focusing primarily on communications and computing engineering include, but are not limited to, wireless networks; mobile communication; design and learning; gaming; interaction; e-health and pervasive healthcare; energy management; smart grids; internet of things; cognitive radio networks; computation; cloud computing; ubiquitous connectivity, and in mode general smart living, smart cities, Internet of Things and more. The series publishes a combination of expanded papers selected from hosted and sponsored European Alliance for Innovation (EAI) conferences that present cutting edge, global research as well as provide new perspectives on traditional related engineering fields. This content, complemented with open calls for contribution of book titles and individual chapters, together maintain Springer’s and EAI’s high standards of academic excellence. The audience for the books consists of researchers, industry professionals, advanced level students as well as practitioners in related fields of activity include information and communication specialists, security experts, economists, urban planners, doctors, and in general representatives in all those walks of life affected ad contributing to the information revolution. Indexing: This series is indexed in Scopus, Ei Compendex, and zbMATH. About EAI - EAI is a grassroots member organization initiated through cooperation between businesses, public, private and government organizations to address the global challenges of Europe’s future competitiveness and link the European Research community with its counterparts around the globe. EAI reaches out to hundreds of thousands of individual subscribers on all continents and collaborates with an institutional member base including Fortune 500 companies, government organizations, and educational institutions, provide a free research and innovation platform. Through its open free membership model EAI promotes a new research and innovation culture based on collaboration, connectivity and recognition of excellence by community.

Jayshri Kulkarni • Arpan Desai Heng Tung Hsu • Brian Garner Yang Li • Chow-Yen-Desmond Sim Vigneswaran Dhasarathan

Transparent and Flexible MIMO Antenna Technologies for 5G Applications Transforming 5G with Transparent & Flexible MIMO Antennas

Jayshri Kulkarni School of Engineering and Computer Science Baylor University Waco, TX, USA

Arpan Desai International College of Semiconductor Technology National Yang Ming Chiao Tung University Hsinchu, Taiwan

Heng Tung Hsu International College of Semiconductor Technology National Yang Ming Chiao Tung University Hsinchu, Taiwan

Brian Garner School of Engineering and Computer Science Baylor University Waco, TX, USA

Yang Li School of Engineering and Computer Science Baylor University Waco, TX, USA

Chow-Yen-Desmond Sim Department of Electrical Engineering Feng Chia University Taichung City, Taiwan

Vigneswaran Dhasarathan Department of Electronics and Communication Engineering Centre for IoT and AI (CITI) KPR Institute of Engineering and Technology Coimbatore, India

ISSN 2522-8595 ISSN 2522-8609 (electronic) EAI/Springer Innovations in Communication and Computing ISBN 978-3-031-42485-4 ISBN 978-3-031-42486-1 (eBook) https://doi.org/10.1007/978-3-031-42486-1 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland Paper in this product is recyclable.

Preface

The emergence of 5G technology, with its low latency and high data transfer capabilities, has become a critical component of wireless communications. As this advanced technology continues to evolve, the antennas used in 5G networks will require upgrades to meet the increasing demands. With the trend toward more compact devices, there is a growing need for antennas with compact structures, wider bandwidth, and higher gain. The introduction of 5G/B5G, Wi-Fi 6E, and low-earth orbit (LEO) satellite wireless technology has further intensified this demand. Traditional antenna designs, such as loop, PIFA, monopole, and dipole, may become outdated in this rapidly advancing landscape. Consequently, there is an urgent need to develop novel antenna design techniques and intelligent antenna systems to keep pace with these advancements. This book fills a crucial gap in the literature by providing a comprehensive treatment of flexible antennas. It covers various types of novel, high-performance flexible MIMO antenna designs that cater to the entire Sub-6 GHz frequency band. This frequency range is vital for modern and upcoming wireless devices with futuristic capabilities. By addressing the challenges and opportunities in flexible antenna design, this book serves as a valuable resource for researchers, professionals, and engineers seeking to understand and implement cutting-edge antenna technologies for wireless communications. It paves the way for the development of innovative and efficient antenna systems that will shape the future of wireless communication devices. Flexible antennas have a fascinating history and offer a vast array of research opportunities in the field of antenna technology. The demand for flexible antennas is experiencing exponential growth due to various factors such as the rise of wearable devices, the Internet of Things (IoT), utmost care devices, personal medicine platforms, 5G technology, wireless sensor networks, and the need for smaller communication devices. Chapter 1 focuses on the importance of flexible antennas, the materials used in their fabrication, the manufacturing processes involved, and the

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influence of different material properties on antenna performance. It also addresses the challenges faced in the development and implementation of flexible antennas, including issues related to mechanical reliability, manufacturing processes, signal propagation, and integration with other components. Lastly, it delves into the potential future applications of flexible antennas in wireless solutions, highlighting their role in advancing wireless technologies and paving the way for further research and development in the field. Chapter 2 focuses on the design and analysis of a unique flexible, transparent, and wideband 4-port MIMO antenna with a connected ground plane, specifically tailored for Sub-6 GHz 5G and WLAN frequency bands. The chapter starts by discussing the single antenna geometry and its evolution mechanism, along with the incorporation of the MIMO configuration and isolating structure. Through comprehensive analysis, it demonstrates how the antenna’s geometry, material selection, and fabrication techniques meet the stringent requirements for impedance bandwidth, gain, radiation efficiency, and radiation patterns. To validate the design and analysis, numerical computation and experimental measurement studies are employed, providing reliable results and ensuring the antenna’s performance. Additionally, the chapter presents a detailed bending analysis, evaluating the antenna’s behavior under different bending conditions, and examines the MIMO diversity performance parameters for the desired operational frequency bands. By combining simulation and measurement studies, the chapter showcases the effectiveness and performance of the flexible MIMO antenna design, making it a valuable resource for researchers and engineers involved in Sub-6 GHz 5G and WLAN applications. Chapter 3 focuses on the design of a novel flexible antenna that operates in the ultra-wideband (UWB) frequency range, specifically from 3.1 to 10.6 GHz. The UWB band has gained considerable attention due to its authorization by the Federal Communication Commission (FCC) and its suitability for various modern applications. The chapter presents a 4-port MIMO antenna design with an isolating structure, enabling it to operate simultaneously in the UWB, Ku, and X bands. This multiband capability makes the antenna versatile and adaptable to a wide range of applications. Furthermore, it discusses the detailed characterization of the antenna’s parameters using real measurements. The performance of the antenna is thoroughly analyzed, providing valuable insights into its efficiency, gain, and effectiveness in real-world scenarios. The optimization process of essential parameters for the antenna, including the dimensions of antenna elements and MIMO configuration, is explained in detail. This optimization ensures that the antenna design meets the desired performance criteria. An important aspect addressed in this chapter is the evolution of the decoupling structure among the antenna elements. The analysis focuses on achieving effective isolation between the different antenna ports to enable efficient MIMO operation. The obtained results are properly validated, enhancing the reliability of the presented findings. Finally, it presents the measurement results

Preface

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of the 4-port flexible MIMO antenna. The measurements encompass various aspects, such as impedance bandwidth (both with and without bending), gain, efficiency, and MIMO diversity parameters. These measurements provide comprehensive insights into the antenna’s performance under different conditions. In summary, it offers a comprehensive exploration of a novel flexible antenna design operating in the UWB, Ku, and X bands. It covers the characterization of antenna parameters through real measurements, optimization of essential parameters, analysis of the decoupling structure, and extensive measurements to assess the antenna’s performance in terms of impedance bandwidth, gain, efficiency, and MIMO diversity parameters. Chapter 4 analyzes a wideband flexible self-isolating MIMO antenna designed for Sub-6 GHz 5G and WLAN smartphone terminal applications. The chapter explores a specific design technique that involves the insertion of four octagonal rings between opposite sides of the main radiator. This technique aims to achieve a wider bandwidth for the antenna. The utilization of a flexible polyamide substrate for printing the radiators is a key focus in this chapter. This allows the antenna to operate across the entire Sub-6 GHz 5G and WLAN bands, making it suitable for smartphone terminal applications. Measurement data are presented in this chapter to facilitate comparison and validation of the proposed antenna prototype. By examining the measurement results, the performance and effectiveness of the antenna design can be assessed. Additionally, this chapter includes an analysis of the Specific Absorption Rate (SAR) for the conformability of the antenna design to smartphone terminal applications. SAR analysis is essential to ensure that the antenna operates within the safety limits for human exposure to electromagnetic radiation. Overall, this chapter provides an in-depth analysis of a wideband flexible selfisolating MIMO antenna designed for Sub-6 GHz 5G and WLAN smartphone terminal applications. It discusses the design technique for achieving a wider bandwidth, emphasizes the use of a flexible polyamide substrate, presents measurement data for validation, and examines SAR analysis to ensure compliance with safety regulations. Chapter 5 focuses on the design and development of a flexible 4-port circularly polarized (CP) MIMO antenna specifically designed for Sub-6 GHz applications. The chapter explores the evolution of the isolating structure deployed among the four antenna elements, providing insights into its design and functionality. In this chapter, experiments are conducted to analyze the bending characteristics of the antenna. The results obtained from these experiments are presented for comparison, enabling a better understanding of the antenna’s performance under different bending conditions. Furthermore, the chapter investigates the CP technique used in the antenna design, specifically examining its contribution to achieving a wide 3-dB axial ratio bandwidth. The analysis includes a detailed investigation of the surface current distribution, shedding light on the behavior and performance of the antenna.

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Preface

Overall, this book serves as an essential platform for novice researchers, professionals, and antenna designers from various industries and universities. It provides valuable insights and knowledge for those working on the design of miniaturized, flexible antennas without compromising their performance. The book is particularly relevant for individuals involved in the integration of antennas into compact and futuristic wireless devices for wireless applications. Waco, TX, USA Hsinchu, Taiwan Hsinchu, Taiwan Waco, TX, USA Waco, TX, USA Taichung City, Taiwan Coimbatore, India

Jayshri Kulkarni Arpan Desai Heng Tung Hsu Brian Garner Yang Li Chow-Yen-Desmond Sim Vigneswaran Dhasarathan

Contents

1

Introduction to the Flexible and Transparent Antennas . . . . . . . . . . 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Categories of Flexible Antennas . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Advantages and Drawbacks of Flexible Antenna . . . . . . . . . . . . . 1.3.1 Challenges of Flexible Antenna . . . . . . . . . . . . . . . . . . . 1.3.2 Difference Between Conformable and Non-conformable Antenna . . . . . . . . . . . . . . . . . . . 1.4 Substrate Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.1 Dielectric Constant . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.2 Loss Tangent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.3 Thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.4 Dielectric Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 Choice of Substrate for Flexible Antennas . . . . . . . . . . . . . . . . . 1.5.1 Substrate Losses and Issues . . . . . . . . . . . . . . . . . . . . . 1.5.2 Temperature Expansion . . . . . . . . . . . . . . . . . . . . . . . . 1.5.3 Conductive Material Properties . . . . . . . . . . . . . . . . . . . 1.6 Flexible Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6.1 Kapton Polyimide . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6.2 Polydimethylsiloxane (PDMS) Substrates . . . . . . . . . . . 1.6.3 Textile Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6.4 Plastic Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6.5 Paper Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6.6 Other Substrate Materials . . . . . . . . . . . . . . . . . . . . . . . 1.7 Fabrication Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.7.1 Screen Printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.7.2 Inkjet Printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.7.3 Sewing and Embordering . . . . . . . . . . . . . . . . . . . . . . . 1.7.4 Chemical Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 1 2 2 3 3 3 4 4 4 5 5 5 6 6 7 7 8 9 12 12 14 14 14 15 16 18

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Contents

1.8 1.9

Applications of Flexible Antenna . . . . . . . . . . . . . . . . . . . . . . . . Case Studies: Flexible Antenna Design, Geometry, and Simulation Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.9.1 Dual-Band Flexible Concentric Rings Single Input Single Output Antenna . . . . . . . . . . . . . . . . . . . . 1.9.2 Decagon Antenna for GSM, LTE, 5G, and WLAN Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.9.3 Two-Dimensional (2-D) MIMO Antenna . . . . . . . . . . . . 1.10 Transparent Antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.10.1 Oxide based Transparent Antennas . . . . . . . . . . . . . . . . 1.11 Case Study of CPW-Fed Transparent Flexible Antenna . . . . . . . . 1.11.1 Antenna Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.11.2 Experimental Results and Discussion . . . . . . . . . . . . . . . 1.11.3 Bending Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.11.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

3

18 19 19 26 34 41 42 47 47 54 58 62 63

Flexible, Transparent, and Wideband 4-Port MIMO Antenna . . . . . 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 5G MIMO Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1 Conventional MIMO Configurations . . . . . . . . . . . . . . . 2.2.2 Applications of MIMO . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Printed Transparent 5G MIMO Antenna Design . . . . . . . . . . . . . 2.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2 A Review of Transparent MIMO-Printed Antennas for 5G Applications . . . . . . . . . . . . . . . . . . . . 2.4 Transparent 4-Element 5G MIMO Antenna Case Study . . . . . . . . 2.4.1 Antenna Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2 Numerical Computation and Parametric Studies . . . . . . . 2.4.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

71 71 72 72 73 74 74 75 77 77 79 82 93 97

UWB Flexible Antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Mutual Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Methods for Mutual Coupling Reduction . . . . . . . . . . . . 3.3 Fundamental MIMO Antenna System Parameters . . . . . . . . . . . . 3.3.1 Isolation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2 TARC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3 Envelope Correlation Coefficient (ECC) . . . . . . . . . . . . Diversity Gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.4 3.4 Flexible 4-Element UWB MIMO Antenna Case Study . . . . . . . . 3.4.1 Antenna Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.2 Results and Discussions . . . . . . . . . . . . . . . . . . . . . . . . 3.4.3 MIMO Diversity Analysis . . . . . . . . . . . . . . . . . . . . . .

101 101 103 103 103 104 105 105 106 106 107 111 114

Contents

3.4.4 3.4.5 3.4.6 References . .

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Bending Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Time Domain Analysis . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...........................................

120 122 127 127

4

Advancements in Flexible MIMO Antennas for 5G Smartphones . . . 4.1 Introduction to Flexible MIMO Antenna Advancement . . . . . . . . 4.2 Existing Research Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Geometry and Design of Mobile Antenna . . . . . . . . . . . . . . . . . . 4.4 The Findings and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1 Coefficient of Reflection (dB) . . . . . . . . . . . . . . . . . . . . 4.4.2 Coefficient of Transmission (dB) . . . . . . . . . . . . . . . . . . 4.4.3 Distribution of Surface Current (A/m) . . . . . . . . . . . . . . 4.4.4 (2D) Radiation Pattern . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.5 3D Radiation Pattern . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.6 Gain and Efficiency of Suggested MIMO Antenna . . . . . 4.5 MIMO Diversity Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.1 ECC and DG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.2 TARC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.3 MEG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.4 Channel Capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6 Analysis of Flexibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6.1 Coefficient of Reflection with Antenna Bending . . . . . . . 4.6.2 Coefficient of Transmission with Antenna Bending . . . . 4.6.3 Gain and Efficiency with Antenna Bending . . . . . . . . . . 4.6.4 ECC and DG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7 Impact of Smartphone Antenna on User (SAR Analysis) . . . . . . . 4.7.1 Effect of SAR Analysis on 3D Radiation Pattern . . . . . . 4.7.2 SAR’s Impact on Human Beings Tissue . . . . . . . . . . . . 4.8 Performance Assessment of the Suggested MIMO Antenna . . . . . 4.9 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

131 132 132 134 137 137 138 139 139 143 144 145 146 147 148 149 150 150 150 151 152 153 153 154 156 158 158

5

Flexible 4-Port CP MIMO Antenna . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Geometry and Configuration of a Single, Flexible Monopole Antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1 Working Principle of the Antenna . . . . . . . . . . . . . . . . . 5.2.2 Analysis of Surface Current Distribution in the Decagon-Shaped Flexible Antenna . . . . . . . . . . . . 5.3 CP Mechanism of the Decagon-Shaped Flexible Antenna . . . . . . 5.4 Bending Analysis of the Decagon Shaped Flexible Antenna . . . . 5.5 Designing a Dual-Polarized Flexible Four-Port MIMO Antenna with ILS . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

161 161 164 167 169 170 172 174

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5.6

Design Steps of the Four-Port Decagon-Shaped Dual-Polarized Flexible MIMO Antenna . . . . . . . . . . . . . . . . . . 5.7 Polarization Diversity of the Proposed Flexible MIMO Antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7.1 Results and Analysis of the Suggested, Flexible, Dual-Polarized MIMO Antenna with a Decagon Form . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7.2 Measured and Simulated Reflection Coefficient . . . . . . . 5.7.3 Simulation and Measured Transmission Coefficient . . . . 5.7.4 Axial Ratio: Measured and Simulated . . . . . . . . . . . . . . 5.7.5 Gain and Efficiency: Simulations and Measurements . . . 5.7.6 Measured and Simulated Radiation Patterns . . . . . . . . . . 5.7.7 Diversity Gain (DG) and ECC . . . . . . . . . . . . . . . . . . . 5.7.8 TARC or Total Active Reflection Coefficient . . . . . . . . . 5.7.9 Mean Effective Gain (MEG) of Suggested Antenna with MIMO Structure . . . . . . . . . . . . . . . . . . . 5.7.10 Channel Capacity of Antenna with MIMO Structure . . . . 5.8 Bending Analysis of Proposed Flexible MIMO Antenna . . . . . . . 5.8.1 Bending Analysis of Proposed Flexible MIMO Antenna Along X-Axis . . . . . . . . . . . . . . . . . . . . . . . . . 5.8.2 Bending Analysis of the Proposed Flexible MIMO Antenna Along the Y-Axis . . . . . . . . . . . . . . . . . . . . . . 5.9 Performance Comparison of Proposed Flexible MIMO Antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.10 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

176 180

182 182 182 184 185 186 192 193 194 195 195 196 198 200 202 202

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205

Chapter 1

Introduction to the Flexible and Transparent Antennas

1.1

Introduction

Internetworking across the globe has made technology more intelligent to provide constant communication that should be affordable, noise-free, resilient, and adaptable to send or receive EM waves. Modern technology has given great importance to small size and reliability. Antennas are helpful in communication systems. Due to their characteristics such as flexibility, bending, and cost-effectiveness, flexible antennas are extensively preferred in modern communication systems. Such antennas with increased flexibility ensure that they can be applied to surfaces with non-planar shapes. Flexible antennas are applicable in the sectors like defense, communication, cars, bodycentric applications, and many more. The resonators can primarily be utilized as wearable ones for defense [1]. They are also used in WSNs and WBANs and have increased with the development of wearable low-power devices [2, 3]. Additionally, it is used in applications like gesture monitoring, blood pressure, heart rate, and additional healthcare services, as well as in firefighter, law enforcement, and military operations, industrial monitoring, patient monitoring via computer or mobile phone technology, and GPS (Global Positioning System) applications. As they can be incorporated into clothing, flexible antennas are also used in wearable applications like R.F. sensors [4, 5]. The sole difference between flexible antennas and microstrip patch antennas is that flexible antennas are built from different flexible substrates and conductive materials. It can be used for applications where antennas need to be integrated yet surfaces are uneven or there is a space restriction.

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 J. Kulkarni et al., Transparent and Flexible MIMO Antenna Technologies for 5G Applications, EAI/Springer Innovations in Communication and Computing, https://doi.org/10.1007/978-3-031-42486-1_1

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1.2

1

Introduction to the Flexible and Transparent Antennas

Categories of Flexible Antennas

The types of flexible antenna are displayed in Fig. 1.1. The categories in which the flexible antennas are divided is non-transparent and transparent antennas. The non-transparent antennas are subdivided into polymer-based, textile-based, and microfluidic antennas. Constructed on the substrate material, the antennas can be used for specific applications. Another category is transparent antennas which need transparent substrate and conductive material for their realization. The low conductivity of such antennas leads to low gain and efficiency.

1.3

Advantages and Drawbacks of Flexible Antenna

The flexible antenna advantages and drawbacks are shown in Table 1.1.

Fig. 1.1 Types of flexible antennas

Non Transparent

• Textile • Polymer • Microfluidic

• • • Transparent •

Plastic Oxide based Sputtering based Inkjet Printed based

Table 1.1 Flexible antenna advantages and drawbacks Advantages Good modulus of elasticity Compact size Bendable Lightweight It supports linear and circular polarization

Drawbacks Lower gain and bandwidth due to thin substrate Measurement needs to be done carefully as the extra bending may cause variation in the results Flexible substrate needs to be selected carefully else it may break easily while bending Fabrication may be difficult depending on the method of fabrication

1.4

Substrate Properties

1.3.1

3

Challenges of Flexible Antenna

• The degree of bending or curvature may cause gain to degrade and the radiation pattern to be distorted. The mismatched impedance and altered effective capacitance will also alter the resonant frequency and S11 behavior. Resonance shift and bandwidth shifts are impacted by the use of a cylinder with a constant radius (R) to bend the antenna. • The more the crumpling amplitude increases, the worse the antenna performance is under a surface cosine-type crumbling condition. In this case, S11 never drops below 10 dB. Thus, substantial deformations are not anticipated in a realistic usage situation due to the tiny size and compact antenna shape. • Since several crucial fabrication processes require particular procedures, like thermal annealing and ultraviolet treatment, antenna is impacted by high temperatures, pressures, and humidity. • Perfect Electric Conductors (PECs) for patch and ground are not used in flexible antennas. The height of quarter wavelength is necessary to boost the radiation efficiency. As the antenna gets thicker, it gets difficult to use it for flexible applications. • The human body has an asymmetrical form, conductivity, and frequency dependence. High body dielectric constant will alter the antenna’s frequency, gain, radiating pattern, and size. • Fabric’s performance qualities will be impacted by stretching and compression, making it challenging to produce antennas in large quantities. • The antenna’s proximity to human tissues suggests two problems: 1. The impedance is challenging to match. 2. The greater implantation of electromagnetic energy into tissues poses health risks (mainly due to hyperthermia).

1.3.2

Difference Between Conformable and Non-conformable Antenna

The comparison between conformable and non-conformable antennas is carried out in Table 1.2.

1.4

Substrate Properties [6]

In designing of antenna, the substrate plays crucial role and so the various properties of the substrate are discussed below.

4

1

Introduction to the Flexible and Transparent Antennas

Table 1.2 Difference between conformable and non-conformable antenna Conformable antenna Flexible substrate (good modulus of elasticity) Fabrication is complex Less correlation among simulated and measurement results when such antennas are not handled carefully Low gain due to thin substrate Good power handling capacity due to low power device Bendable but bending loss occurs due to flexible substrate Can be integrated into a very less space

1.4.1

Non-conformable antenna Rigid substrate (poor modulus of elasticity) Ease of fabrication Great correlation between software and experimental results if fabrication is done with precision Decent gain Poor power handling capacity Not bendable so no bending losses The antenna can occupy the space as per the actual size of antenna

Dielectric Constant

The relative static permittivity, or εr, is a measure of the fraction of electrical energy that a substance can store whereas a vacuum cannot. The low dielectric constant value functions as a good insulator and has a lower loss. It varies based on the substrate, temperature, and frequency. Different methods, such as the non-resonance and resonance methods using VNA, are used to measure a material’s dielectric constant.

1.4.2

Loss Tangent

The amount of EM waves moving in the dielectric that is lost as heat is calculated using the loss tangent (tan δ/dissipation factor). The loss tangent determines how much power is lost; thus, a low value for the loss tangent is needed whenever possible to produce a low loss. Antenna efficiency suffers due to the loss tangent high, which causes increased loss at high frequencies. The antenna’s output power is 1000 W, and its loss tangent is 0.01. The power loss is therefore estimated at 10 W.

1.4.3

Thickness

Smaller substrate thickness results in less loss, but it also radiates less power. If the substrate’s thickness is high, it radiates more power but also weighs more due to its high Q value. As a result, a thin substrate with a high Q value requires a low dielectric constant.

1.5

Choice of Substrate for Flexible Antennas

5

When the εr is fixed, the following happens: • Widen the feed line Z0 is reduced. • The substrate height Z0 is rising. • Z0 will decrease as conducting material thickness increases.

1.4.4

Dielectric Strength

The amount of voltage a dielectric material can withstand before failing is known as its dielectric strength (also known as electric strength). When a dielectric substance is damaged or ceases to function as an insulator, a dielectric breakdown occurs. Only applications requiring high power/high voltage will benefit from this characteristic. The thickness of the substrate will improve when frequency, temperature, and humidity change.

1.5

Choice of Substrate for Flexible Antennas

• High resistance to effects such as bending, rolling, flexing repeatability, high temperature, and humidity • Extremely flexible and mechanically durable • Low profile and lightweight • The desired range of dielectric constants The flexible substrates are enlisted in Table 1.3.

1.5.1

Substrate Losses and Issues

A portion of the energy from an EM wave is stored in the dielectric material that causes heat to be lost as a result of friction between the atoms in a dielectric material. Table 1.3 Flexible substrate materials [7, 8] Material Rogers Duroid 3210 Jeans Felt Paper Glass Epoxy (FR-4) PET (polyethylene terephthalate) Kapton Neoprene Foam

Dielectric constant 2.9 1.7 1.22 3.45 4.4 3.2 3.5 5.2 1.06

Loss tangent 0.003 0.05 0.016 0.065 0.02 0.05 0.05 0.03 0.001

6

1

Introduction to the Flexible and Transparent Antennas

Inductance loss due to reflection losses that produce impedance changes. The feed width, substrate dielectric constant, and thickness all affect these changes. The antenna will absorb water, moisture, or perspiration from the weather that will lead to change in the frequency bandwidth, εr, surface thickness, and the impedance matching.

1.5.2

Temperature Expansion

As the temperature rises, some materials’ characteristics will alter. Temperature T =

P k
Δf

ð1:1Þ

where Δf = bandwidth, k = Boltzman constant (1.38 * 10-23JK-1), and P = input power Input is applied to the antenna through SMA or end-launch connectors. For connections without soldering, silver-loaded epoxy conductive glue is employed. Patch and ground are attached to the substrate using self-adhesive copper tape using nonconductive glue, although this alters the frequency of the antenna because of the substrate’s changing dielectric constant (square root of dielectric constant is inversely proportional to the frequency). Surface roughness, fabrication errors, poor impedance matching, and occasionally fiber thread coming out during the fabrication of some textile antennas are common. This changes the antenna’s settings eventually. The material used to design the patch and ground should have the following properties:

1.5.3

Conductive Material Properties

• It should have high conductivity/low resistance. • It should have the flexibility to bend, crumple, and stretch. • It should not be subject to material deterioration brought on by environmental elements like oxidation and corrosion. • The substance must be resilient to repetitive pressure, deformation, etc. all have ε = 1 and μ = 0.9. • The electrical conductivity is measured using Eq. 1.2, δDC =

1 R×t

where R = sheet resistivity, t = sheet thickness.

ð1:2Þ

1.6

Flexible Substrates

1.6

7

Flexible Substrates

Selecting the proper substrate from the variety of available possibilities is crucial. Each substrate, including polyethylene terephthalate (PET), polyimide (PI), paper, foil, polydimethylsiloxane (PDMS), and others, has disadvantages and advantages affecting the circuits and applications. The main necessities for conformable substrates are a dielectric constant that is not near 1 and a lower loss tangent between 0.017 and 0.025 for FR4. Other requirements include the ability to recycle, good flexibility, thermal stability, dimensional stability, a two-dimensional surface, low costs, and excellent solvent resistance. The selection also depends on applications like RFID, displays, antennas, and sensors. The common substrates like Taconic, FR4, Teflon, or Rogers, can be utilized to print flexible antennas. On a flexible 0.2-mm-thick Rogers RO4003C, a bow-tie-shaped CPW-fed wideband resonator with a 60 × 80 mm2 size is proposed in [9] for WLAN or WiMAX bands. Another CPW-fed quasi-Yagi-type antenna was created by researchers in [10, 11] that resonates at 1.55 GHz. The antenna printed on Rogers RT5880 with a thickness of 0.127 mm realized a 5.76 dBi gain. A 3D antenna made from this kind of substrate is the four-brace spiral conical resonator that was created by etching, cutting, and wrapping Rogers RT5880 with a thickness of 0.127 mm [12]. For designing and producing stable antennas, Roger’s materials offer exceptional physical and electrical characteristics. The antenna’s deformation flexibility enables dependability, durability, and deformation without breaking, fracturing, loosening, or a tendency to unravel.

1.6.1

Kapton Polyimide

It is well known for its high quality and hardly any loss characteristics. Many researchers suggested building antennas utilizing this substrate, such as the dualband [13] monopole CPW-fed resonator resonating at 2.45 and 5.5 GHz and another CPW-fed monopole antenna resonating at 2.2 and 5.3 GHz, respectively [14]. Kapton is an excellent choice for applications involving wearable, adaptable telemedicine and wireless body networks (WBANs). Y. H. Jung et al. proposed a microfabrication technology in [15] that can be used to create a dual-band antenna using Kapton substrate (0.127 mm thick) and covered in a biocompatible Parylene C (10-m-thick) film for biomedical implantable solutions. Additionally, IFA antennas proposed in [16] also prefer biological flexible substrate, and laptop applications need such flexible antennas. Standard Kapton-based or conformable substrates provide consistent characteristics and excellent performance, but printing is not an option. Some Kapton-based resonators are publicized in Fig. 1.2.

8

1

Introduction to the Flexible and Transparent Antennas

Fig 1.2 Antennas using flexible Kapton polyimide (a) [17] (b) [18]

1.6.2

Polydimethylsiloxane (PDMS) Substrates

Polydimethylsiloxane (PDMS), used mostly in medical and bodily communication networks, is also recognized in several studies as elastic, flexible, and optically transparent. The polymer used should be organically compatible. With antenna and microwave systems operating at 2.68–3.0 GHz, loss tan ranges of 0.02–0.04, and FR4 matching, PDMS substrates are allowed. The authors of [19] came up with strategies for stabilizing and picking PDMS surface-based conductive components. The bond of the PDMS surface metal layers is significantly affected by the oxygen plasma ion surface treatment. The flexible PDMS is an implantable microelectrode for subretinal prosthesis having gold-plated electrodes. For the transparent UWB flexible micro-tripped resonator, a research team from Rennes, France, utilized fabric tissues with optical clarity in PDMS sub-substrates

1.6

Flexible Substrates

9

[20]. Such resonators find applications in wireless technologies like 5G in the future. The resonator with a microstrip connected to the conductive fibers over the PDMS substrate is another illustration of a flexible structure [21] resonating from 3.43 to 11.1 GHz. A conductive NCS95R-CR antenna using two varactors and a Marktek Inc. PDMS material is designed for the body-centered transmission operating at 2.4 and 5.8 GHz [22]. A circular polarized PDMS antenna using AgNW ink was introduced by Z. H. Jiange et al. for usage in WBAN applications [23]. For the creation of the resonator and a microwave circuit, PDM’s electromagnetic properties are suitable. Biocompatible substrates, flexible optics, and clear optics are beneficial in a variety of applications. However, the use of direct metal deposition writing is prohibited by such substrates, which makes low-cost mass production challenging. Figure 1.3 shows the PDMS-based antennas.

1.6.3

Textile Substrates

For wearable and biological purposes, electronic circuitry or textile-based substrates can be combined with clothes. According to [27, 28], these substrates’ properties can support flexible electronics and antennas. Conductive spray, liquid textile glue, sewing, copper tape, and ironed sheets are a few of their well-known techniques. The technique of metal deposition is also fairly popular. Additionally, you might use inkjet and screen-printing techniques. Numerous textile-based antennas are proposed in [29–40]. Frequency-selective surfaces (FSS) having periodic structures can be used for on-body communication applications that use copper over an electro-textile filter [29]. It acts like a band reject filter spanning from 10 to 12 GHz. FSS is frequently used for many objectives in antenna design, for improving the bandwidth and gain. In [30], a periodic structure working at millimeter wave frequencies is proposed. A spiral antenna using Kevlar fabric and textile threads made of conductive material was recommended by Jingni Zhong et al. with an excellent IBW of 0.3–3 GHz with 6.5 dBi gain. The patch resonator spanning from 2.4 to 2.48 GHz is proposed as a textile-based antenna [31–33]. Three textile substrates are used to realize an antenna for Bluetooth application [34] using different textile materials, including goch, denim, and leather, and they demonstrate that leather is the best option for production and performance. Inkjet printing can also be utilized for realizing flexible resonators [35]. The electrical capabilities of intelligent fabrics, such as transistors and capacitors used to print antennae, are tested by the authors. The screen-printing technique used to create an ultra-high frequency (UHF) RFID antenna (868 MHz) [36] is adapted for incorporation into a firefighter outfit. The meander antennas and an illustration of their use on a suit are shown in Fig. 3.3. Additionally, the substrate-integrated waveguiding (SIW) approach for designing antennas was examined in [37, 38]. A rectangular cavity-backed SIW folded

10

1

Introduction to the Flexible and Transparent Antennas

Fig. 1.3 Antennas using PDMS material (a) [24] (b) [25] (c) [26]

resonator connected to a shielded stripline feed that resonates at a 2.45 GHz band is proposed in [39]. In [40], a two-element SIW MIMO textile antenna is proposed for Wi-Fi bands (2.4/5.2/5.8 GHz). Some textile-based antennas are displayed in Fig. 1.4.

1.6 Flexible Substrates

Fig. 1.4 Textile-based antennas (a) [41] (b) [42] (c) [43]

11

12

1

Introduction to the Flexible and Transparent Antennas

Textile substrates are very practical for applications in biological and wearable products. However, because of its compressibility, porosity, and thickness, at small pressure is challenging to control. Additionally, since the majority of the antennas intended to be integrated with these substrates may become wet from human perspiration, this can alter the tissue’s dielectric characteristics, subsequently decreasing the antennas’ radiating abilities.

1.6.4

Plastic Materials

Plastic polymers including polyethylene terephthalate (MEP), polyethylene phthalate (PEN), and plastic isolating sheets are potential options for a range of applications. Due to its great degree of transparency and flexibility, PET is frequently used for these purposes. A variety of research teams looked into antennas created and printed on PET. An antenna having a Z shape constructed on PET and constructed using a printer at 2.45 GHz is proposed in [44]. The antenna has a good resonance frequency, omnidirectional patterns, and radiation efficiency greater than 60%. Researchers in [45] developed and produced a circularly polarized microstrip patch antenna (7.7–8.3 GHz) using PET, and it produced gains of over 15 dBi and efficiency levels of over 75% throughout the defined band. The spacing between silver nano component droplets had a significant impact on performance when the authors of [46, 47] studied inkjet-printed monopole CPW-fed 20-GHz antennas having 2 soft substrates, namely thick paper and PET. PET’s 96% radiation performance was significantly better than 77% using Epson’s paper. UWB tagless chip ink screens, copper, aluminum, and silver were used to construct and compare UWB-chipless paper tags [48]. It has been demonstrated that these metallic inks can produce the necessary RF characteristics and that doing so could lower production costs. Of course, compared to some other flexible materials, flexible plastic substrates are less expensive and have low loss tangent (less than 0.005). However, after printing, a heat treatment process that may be carefully controlled is applied when polyester starts to sink. Therefore, to produce an RFID antenna on PET, the researchers in [49] used a printer-friendly, low-cost self-sintering, and Inkjet printer method. Various plastic-based antennas are illustrated in Fig. 1.5.

1.6.5

Paper Materials

A practical option that is cheap and can be used as a flexible substrate is paper which permits better direct writing than polyester while supporting mass manufacture. For communications systems using paper as a medium, the following benefits are listed:

1.6

Flexible Substrates

13

Fig. 1.5 Plastic-based antennas (a) [50] (b) [51] (c) [52]

• Paper is a common organic substrate that is readily available. Paper is the best option for a flexible printed electronics substrate since it can easily handle the increased need for mass production. • It is the perfect alternative to conventional metal grafting technologies for direct writing procedures because of its modest outside profile. • Electronics can be printed on or inside paper substrates using a quick production method, such as inkjet printing. • Paper is safe for the environment and works well with green devices and elements.

14

1 Introduction to the Flexible and Transparent Antennas

Paper substrates are equally challenging due to below reasons: • Paper-based substrates suffer larger losses than those made of other flexible materials. When integrating them into the substrate, high-frequency paperbased antennas require knowledge of their dielectric properties. Because of the paper’s thinness and non-homogeneity, the RF characterization process is difficult and should be performed with great precision. • Due to the manufacturing process for nanoparticulate components, printing ink costs are still expensive. Thin papers are frequently used to increase their flexibility; however, this makes it difficult to build a CPW feed line with a narrow slot. • The interconnectivity is still an issue due to the brittleness of paper substrates; thus, printing quality must be well managed. • Additionally, certain technical issues need to be fixed. In addition to the ink droplets’ inertial spread and the drying out of droplets on each other, which causes metallic coatings to be uneven and causes ink fluids to evaporate, there are other interactions between fluids and the substrate [53]. Another challenge is registering succeeding layers, particularly when substrates like paper have the propensity to change size during printing and drying. The issue of paper recycling de-inks is the last one. Paper-based antennas are shown in Fig. 1.6.

1.6.6

Other Substrate Materials

Other substrates, such as semiconductors, LCP, [56], silicone elastomer [57], natural rubber, printed material, and Flex 3-D [58], may also be used for flexible electronics in addition to the ones mentioned above. The production of flexible electroplating can also make advantage of the following.

1.7

Fabrication Methods

A proper fabrication approach must be used to ensure high stability and performance of conformable antennas. Depending on their needs, the researchers used various fabrication procedures. Among the most frequent fabrication techniques are inkjet printing, screen printing, embroidery, and stitching.

1.7.1

Screen Printing

It is a highly efficient, affordable printing technique [59]. Besides, it is one of the fastest and simplest antenna fabrication procedures. It is an adaptable technique since printing images is possible on almost any material. Figure 1.7 depicts several of the antennas created using such technique.

1.7

Fabrication Methods

15

Fig. 1.6 Paper-based antennas (a) [54] (b) [55]

1.7.2

Inkjet Printing

Additional production process is inkjet printing, which is considered as technology with low-cost printing over the flexible materials [63]. Through the consumption of ink droplets as small as a few picolitres, the inkjet printing technology may produce incredibly precise patterns [64]. As the nozzle projects a single ink droplet to the required spot without waste, it is also a cost-effective technique for manufacturing

16

1

Introduction to the Flexible and Transparent Antennas

Fig. 1.7 Screen-printed antennas (a) [60] (b) [61] (c) [62]

antennas. As a result, inkjet printing outperforms traditional etching methods [65]. It is not suitable for use with many types of conductive inks as it leads to nozzle congestion and the increased size of particle. Figure 1.8 depicts some inkjet-printed antennas.

1.7.3

Sewing and Embordering

Embroidering and sewing are presently used in numerous flexible antenna applications. No adhesive substance is applied to the fabric using this method that may alter the electrical qualities of the material. Furthermore, creases on the fabric are

1.7

Fabrication Methods

17

Fig. 1.8 Inkjet-printed antenna (a) [66] (b) [67]

generated during the sewing process, resulting in altered antenna properties. This approach, however, is not appropriate for a special substrate called as spacer textile [68]. The method has been used for the embroidery process to let a layout or digital picture that can be embroidered on the substrate with a machine supported through a computer. A superior solution is offered by such antennas in the field of flexible electronics than standard antennas, which is far more elastic as compared to metallic antennas [69]. Figure 1.9 depicts the manufactured antennas using such technique.

18

1

Introduction to the Flexible and Transparent Antennas

Fig. 1.9 Sewing and embordering-based antennas [70]

1.7.4

Chemical Etching

Chemical etching is developed as a technique for creating metallic patterns utilizing etchants and photoresists to corrosively mill out a chosen area. Chemical etching is frequently used with photolithography. It is the finest option out of all the fabrication techniques for producing accurate, high-resolution fabrications of complicated designs [71]. Organic polymers are appropriate for photoresists because when they are exposed to UV light, their chemical properties change. Positive resists are currently used more frequently than negative resists in the antennas fabricated using photolithography and RF circuits sector because of their superior resolution. In a previous work [72], a flexible multilayer monopole antenna for wearable glasses was manufactured on a polyimide substrate having an advantage of being transparent. The antenna conducting parts of the wearable glasses were made using a transparent oxide having transparency of 81.1% that is 100 nm thick and made of (IZTO)/Ag/IZTO (IAI). This multilayer form of flexible antenna is made using the physical vapor deposition (PVD) method.

1.8

Applications of Flexible Antenna

• In the defense, military, law enforcement, and industrial monitoring industries. • In vehicles. • Patient monitoring, body motion analysis, and other healthcare services. Applications include missiles, microwaves, spacecraft, and aircraft.

1.9

Case Studies: Flexible Antenna Design, Geometry, and Simulation Analysis

19

• Wireless communications include Wi-Fi, Bluetooth (2.4–2.485 GHz), Zigbee Band Application (2.4 GHz Worldwide), ISM band, Wi-Max and UWB, and WBANs (wireless body area networks) applications. GPS functions at 1.57 GHz and 1.22 GHz frequencies. • RFID tags, mobile devices, Google Glass, and routers. • Flexible display, organic LED, E-textile, solar cell, and flexible sensor.

1.9

Case Studies: Flexible Antenna Design, Geometry, and Simulation Analysis

This section provides a comprehensive overview of novel antenna geometries and their meticulous analysis using the CST software. The researchers have documented diverse antenna configurations utilizing readily accessible commercial substrates such as FR-4 and Rogers, which incorporate copper as a highly conductive material.

1.9.1

Dual-Band Flexible Concentric Rings Single Input Single Output Antenna

A flexible interlocking concentric circle coplanar waveguide (CPW)-fed antenna working in the 2.4 GHz, Sub 6 GHz, and Wi-Fi 6E frequency bands was the subject of a groundbreaking study by the authors of reference [73]. This flexible antenna’s structure was made up of nine interconnected concentric circles and used the CPW feeding technology. The rings were built using copper as the conductive material and a 0.2mm thick Rogers RT 5880 flexible substrate. Maximum impedance matching was made possible by interlocking, thanks to the way the rings were arranged on the substrate. The fractional impedance bandwidths produced by the simulations were 63.35% (2.20–4.24 GHz) and 25.67% (5.84–7.56 GHz), respectively. The simulation results showed twin resonances at 3.01 GHz and 6.63 GHz. The simulated performance also demonstrated a gain above 2.56 dBi and an efficiency above 91.50% across the intended operational bands. By applying compressive bending in the X-direction to an interlocked concentric circular antenna, the authors also looked at the impact of bending on the range of impedance bandwidth.

1.9.1.1

Overview of Existing Research Work

Due to its intrinsic qualities, which enable seamless communication with biomedical applications, vehicle navigation systems, wearable devices, and more, flexible and compact wireless electronic devices have attracted a lot of research attention.

20

1

Introduction to the Flexible and Transparent Antennas

Within this comprehensive system, the antenna assumes a pivotal role, necessitating flexibility, compactness, and stretchability to ensure device suitability. Previous studies (Refs. [74–76]) have documented non-flexible antennas that cater to specific applications such as smartphones, laptops, and wireless devices, employing substrate-dependent performance adaptations in accordance with the surrounding environment. Numerous flexible antenna designs have been reported in recent literature [77– 80]. In a work on the development as well as performance evaluation of a flexible multiband antenna for flexible wireless devices, [77] used an inkjet printing approach on a Kapton polyimide substrate. The authors in [78] designed and made an ultra-thin flexible antenna that operates in the 2.70 GHz and 5.80 GHz frequency bands specifically for Internet of Things (IoT) applications. Targeting WLAN bands of 2.4/5.2/5.8 GHz, WiMAX bands of 3.5/5.5 GHz, and 5G bands, the authors of [79] investigated the use of Liquid Crystal Polymer (LCP) as a preferred flexible substrate in their small triple-band antenna design. For wearable telemedicine applications, the authors of [77] presented a paper-based flexible antenna that operates in the Industrial, Scientific, and Medical (ISM) frequency spectrum between 2.30 and 2.35 GHz. A coplanar waveguide (CPW)-fed approach with interconnected circular rings was examined in reference [73] for an antenna design. This flexible antenna had significant characteristics like a thickness of 0.2mm, a relative permittivity of 2.2, and a loss tangent of 0.009; it was built on a 32 × 32 mm2 Rogers RT 5880 substrate. With a gain above 2.50 dBi and an efficiency above 91.50%, the suggested flexible circular ring antenna successfully met the bandwidth requirements for 2.4 GHz (2.401–2.484 GHz) WLAN, n77 (3.80–4.20 GHz)/3.5 GHz 5G, and sixth generation of Wi-Fi (2.4GHz and 5GHz).

1.9.1.2

Design and Geometry

Figure 1.10 illustrates the antenna design, which involves etching conductive copper material onto a flexible Rogers RT 5880 substrate with dimensions of 32 × 32 × 0.2 mm3. As shown in Fig. 1.10, a U-shaped structure is formed on the flexible substrate by tri interconnected circular rings that are placed strategically in horizontal as well as vertical directions. The antenna is made more compact by this setup. Each ring is the same size, with an 8 mm outside diameter and a 6 mm inner diameter. The CST software has been used to optimize the rings’ position and radius. A symmetrical coplanar waveguide (CPW)-fed approach is used to excite these rings, and a 0.6 mm gap is maintained between the ground plane and the feed line. The size of the two CPW grounds is 15 × 10 mm2. As depicted in Fig. 1.10, the feed line is attached at the base of the center of the concentric ring.

1.9

Case Studies: Flexible Antenna Design, Geometry, and Simulation Analysis

21

Fig. 1.10 Geometry of antenna [73]

Fig. 1.11 Simulated surface current concentration (A/m) (a) at 3.0 GHz (b) at 6.60 GHz

1.9.1.3

Evolution of Antenna Design

Figure 1.11 shows a simulated surface distribution of current at 3.0 GHz and 6.60 GHz to shed light on how the two resonances were created. According to Fig. 1.11a, the horizontal rings experience the greatest magnitude of current flow at 3.0 GHz. On the other hand, Fig. 1.11b shows that at 6.60 GHz the main current

22

1

Introduction to the Flexible and Transparent Antennas

passes via the two vertical concentric rings. This discovery supports the creation of twin resonances at 3.0 GHz and 6.60 GHz by the concentric flexible antenna that has been suggested. The wide bandwidth produced by these resonances spans from 2.20 to 4.25 GHz and from 5.85 to 7.56 GHz, respectively.

1.9.1.4

Simulated Analysis

The performance analysis of the proposed antenna design is the main topic of this section. The evaluation takes into account a number of important factors, such as the characteristics of the S11 reflection coefficient, the distribution of radiation in the E and H planes, gain (dBi), and radiation efficiency (%). These parameters are essential for evaluating and comprehending the overall performance of the antenna.

1.9.1.4.1

Reflection Coefficient Characteristics (S11)

The S11 (dB) curve in Fig. 1.12 shows how well the reported flexible circular ring antenna performed. A notable feature of the S11 curve is the presence of two separate resonances at 3.0 GHz and 6.60 GHz, which correspond to frequency bands with respective ranges of 2.20–4.26 GHz and 5.84–7.56 GHz. The bandwidth requirements for the 2.4 GHz WLAN, n77/3.5 GHz 5G, and sixth generation Wi-Fi frequency bands are thus efficiently covered by the reported concentric ring flexible antenna. Additionally, it is noted that the amplitude of S11 measures -18 dB and -23 dB at 3.0 GHz and 6.60 GHz, respectively. These numbers confirm that maximal impedance matching was achieved with the antenna shape that was used, demonstrating an ideal impedance match at these frequencies.

Fig. 1.12 Simulated S11 curve vs. Frequency

1.9

Case Studies: Flexible Antenna Design, Geometry, and Simulation Analysis

23

Fig. 1.13 The distribution of the circular ring antenna’s radiation pattern (a) E-plane operating at 3.0 GHz (b), H-plane operating at 3.0 GHz (c), E-plane operating at 6.60 GHz (d)

1.9.1.4.2

Radiation Patterns

The analysis of polar power distribution patterns in the E and H planes at resonating frequencies of 3.0 GHz and 6.60 GHz is presented in Fig. 1.13. According to the results, the disclosed circular ring antenna efficiently transforms electric current into electromagnetic radiation as shown by the development of power patterns with few nulls and a balloon-like shape. This observation highlights the antenna’s capability to facilitate smooth communication within flexible wireless devices. Therefore, the reported antenna demonstrates promising potential as a reliable candidate for efficient signal transmission and reception in such applications.

24

1.9.1.4.3

1

Introduction to the Flexible and Transparent Antennas

Gain and Efficiency Simulations

The peak gain and efficiency of radiation characteristics for a flexible concentric ring antenna are shown in Fig. 1.14. According to the data, the antenna consistently maintains a gain more than 2.5 dBi in the 2.40 GHz, n77, as well as 3.5 GHz 5G frequency bands. Additionally, the antenna has a gain greater than 4.0 dBi in the sixth-generation Wi-Fi frequency range. Additionally, the antenna achieves a radiation efficiency of over 91.50% across the 2.40 GHz, n77, 3.5 GHz 5G, and sixthgeneration Wi-Fi working bands. These findings affirm the antenna’s stable and efficient performance, making it well-suited for applications requiring reliable signal reception and transmission in the aforementioned frequency bands.

1.9.1.5

Bending Analysis

CST software was used to perform a compressive bending study along the X-axis in order to assess the concentric ring flexible antenna’s flexibility. This investigation sought to determine the effect of bending on the antenna’s performance, particularly as it related to S11 as shown in Fig. 1.15. Rolling the antenna on a cylinder with a 20 mm radius caused it to bend. Figure 1.16 illustrates the results of the bending analysis, revealing a slight deviation in the S11 values. However, it is noteworthy that the suggested flexible concentric ring antenna’s impedance bandwidth is unaffected by bending. This finding supports the inherent flexibility of the antenna design, enabling it to be easily bent according to spatial requirements without compromising its impedance 10

100

9

90

8

80

Gain (%)

7

70

Simulated Gain Simulated Efficiency

6

60

5

50

4

40

3

30

2

20

1

10

0

0 2

3

4

5

6

Frequency (GHz)

7

8

Radiation Efficiency (%)

Fig. 1.14 Simulated gain and radiation efficiency

1.9

Case Studies: Flexible Antenna Design, Geometry, and Simulation Analysis

25

Fig. 1.15 Antenna geometry after bending

–5

S-Parameters (dB)

Fig. 1.16 Antenna geometry after bending along the X-axis direction

–10

–15

–20

S11 Simulated –25 2

3

4

5

6

7

8

Frequency(GHz)

bandwidth. Therefore, the described circular ring flexible antenna holds promise for seamless integration within flexible wireless devices, as it can be bent and interfaced without adverse effects on its performance.

1.9.1.6

Concluding Remarks

The authors have demonstrated a dual-band flexible concentric rings antenna design for future compact flexible wireless devices, operating in the 2.40 GHz, n77, 3.5 GHz 5G, as well as sixth-generation Wi-Fi frequency bands. The antenna’s performance was evaluated through simulation using CST software. The design exhibits simplicity, ease of manufacturing, and a low profile, making it highly suitable for effective integration within versatile wireless devices for communication. It demonstrates excellent impedance bandwidth and distribution patterns of radiation, making it useful in various wireless purposes.

26

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Introduction to the Flexible and Transparent Antennas

Moreover, the antenna achieves a gain exceeding 2.50 dBi and a radiation efficiency surpassing 91.50% in the operational frequency range. These performance metrics further enhance its suitability for 2.40 GHz WLAN, n77, 3.5 GHz 5G, and sixth-generation Wi-Fi applications. Furthermore, the bending analysis confirms that the antenna experiences minimal impact on its bandwidth when subjected to bending, further establishing its robustness and flexibility. Overall, the reported flexible concentric ring antenna design presents an attractive solution for next-generation wireless devices, offering excellent performance characteristics and adaptability for various wireless communication needs.

1.9.2

Decagon Antenna for GSM, LTE, 5G, and WLAN Applications

A novel design for a flexible, multiband, linearly polarized decagon ring monopole antenna is provided by the authors in their research work [81]. The suggested antenna has a decagonal ring structure with a 5 mm thickness that includes a chamfered patch to produce linearly polarized waves in a range of frequency bands. Two resonances at 2.21 GHz and 4.11 GHz are produced when the antenna’s radiators are activated using a coplanar waveguide (CPW) fed-based approach. These resonances correspond to the bandwidth requirements of various communication standards, including WLAN, Fifth Generation (5G), Long-Term Evolution (LTE) 2300/2600, and Global System for Mobile (GSM) 1800 bands. A -10 dB impedance bandwidth of 48.62% (1.65–2.71 GHz) and 42.58% (3.66–5.64 GHz) at the resonant frequencies of 2.21 GHz and 4.11 GHz, respectively, have been proven by simulations to be achieved by the antenna. Furthermore, a bending analysis was conducted to assess the impact of bending on the matching of impedance bands and radiated performance in the operational bands of the decagon antenna. The outcomes proved that the antenna maintained its performance characteristics and exhibited minimal deviation in the operating bands even when subjected to bending. Overall, the reported design of the flexible, multiband decagon ring monopole antenna presents a promising solution for wireless communication systems operating in multiple frequency bands. It offers wide impedance bandwidth, linear polarization, and robust performance even in flexible applications.

1.9.2.1

Overview of Existing State of Arts

In numerous wireless applications, including wireless sensor networks (WSN), the Internet of Things (IoT), wearable sensors, 5G technologies, as well as compact wireless handheld devices, flexible antennas have grown in significance [82]. There are now available antennas that may be integrated into gadgets like computers and

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Case Studies: Flexible Antenna Design, Geometry, and Simulation Analysis

27

mobile phones [83–86], but they might not be appropriate for fragile or wearable gadgets. Recent work has described a number of flexible antennas based on various substrates and manufacturing techniques [87–94] to solve this constraint. For instance, a flexible antenna designed using a polyimide substrate is reported in [82]. This antenna, with a volume of 99 mm3, operates within the WiMAX channels and exhibits two resonant frequencies of 2.6 GHz and 3.4 GHz. For IoT applications running at 2.70 GHz and 5.80 GHz, [88] develops a CPW-fed antenna employing three flexible substrates (ceramic + Arlon 25N + polypropylene). The bandwidth requirements of WiMAX, WLAN, as well as 5G applications are met by another antenna in [89] that has a 20 × 32 mm2 footprint and a liquid crystal polymer substrate of 0.1 mm thick. An UWB (ultra-wideband) antenna is presented in [90] to achieve wideband performance with a single resonance. This type of antenna operates in the 3.90–17.4 GHz frequency range and is printed on a flexible substrate with a thickness of 0.07 mm. [91] describes a semi-flexible antenna for wearable telemetry purposes using an RT/duroid 5880 substrate with a 0.787 mm thickness. This antenna, with a modified rectangular patch, operates in the 2.40 GHz Bluetooth/ WLAN frequency for telemetry purposes. In [92], a reconfigurable antenna is developed to achieve radiation properties while maintaining mechanical flexibility. [93] proposes a wearable antenna for the 2.4 GHz Wi-Fi band that uses inkjet printing technology. The antenna is 40 × 35 mm2 and is 0.3 mm thick, which is suitable for wearable applications. Additionally, [94] presents a monopole antenna in a circular shape excited using a CPW-fed method for use in GSM, Wi-Fi, and X-band frequencies. This antenna’s overall dimensions are 35 mm × 35 mm, and it is printed on FR-4 material that is 0.1 mm thick. The multi-band decagon monopole antenna for applications in wireless communication is described in this release. The antenna can transmit and receive in the GSM 1800 (1.71–1.88 GHz), (2.3–2.31 GHz/2.35–2.36 GHz) LTE 2300 uplink/ downlink, and (2.50–2.57 GHz/2.62–2.69 GHz) LTE 2600 uplink/downlink frequencies. It can also operate in the (3.65–5 GHz) as well as (5.15–5.5 GHz) namely 5G/Wi-Fi frequencies, respectively. It has broadside radiation patterns and a maximum gain of 5.5 dBi. In order to evaluate the antenna’s flexibility and its implications on the impedance matching bandwidth, a bending analysis is also carried out.

1.9.2.2

Antenna Geometry

The decagon antenna, which features chamfered patches and a decagon ring on a flexible substrate, is schematically depicted in Fig. 1.17. The substrate used is the Rogers RO 3206 lossy substrate, known for its flexibility, featuring a thickness of 0.4 mm and low tangent losses of 0.0027. The antenna is ideal for seamless integration into wireless devices, thanks to its small profile, which measures 50 mm × 60 mm.

1

Introduction to the Flexible and Transparent Antennas

R

2

28

Chamfered Patch Decagon Ring

Lp Wp

Lc

Ls

R1

O Wc

Ground Plane Wf

Flexible Substrate

Lf Wg Lg Ws

(a)

(b)

Fig. 1.17 Antenna construction (a), (b) Layered view of the front

A transmission line with the dimensions (Lf × Wf) makes up the antenna design, and it is encircled by two partial grounding planes. These ground planes, which are each (Lg × Wg) in size, are positioned with an air gap of 0.5 mm to allow for the proper connection of a SMA connector. A decagon ring is created with optimum outer and inner radii (R1 and R2) imprinted from the center “O” in order to achieve a wide resonant frequency near 2.20 GHz, as shown in Fig. 1.17. The bottom of the decagon ring can meet the transmission line, thanks to its carefully chosen position. Through optimization, the CST software yields the values of R1 and R2. The decagon ring also includes a rectangular patch with a specified size of (Lp × Wp). The rectangular patch has 45-degree chamfers on the opposing corners to improve performance. Another resonance near 4.11 GHz is brought up by this integration at the transmission line’s top. As a result, the complete structure made up of the chamfered patch and decagon ring successfully generates the appropriate frequency bands. The antenna demonstrates 2.21 GHz and 4.11 GHz resonant frequencies, resulting in frequency bands of 1.65–2.71 GHz and 3.66–5.64 GHz, respectively. As a point of reference, Table 1.4 lists all the intended parameters of the decagon antenna together with their optimal values.

1.9

Case Studies: Flexible Antenna Design, Geometry, and Simulation Analysis

Table 1.4 Dimensions of antenna

Parameters Ls Ws R1 R2 Lf W f Lg Wg Lc Wc

Size (mm) 50 60 25 20 30 2 10 23.5 10 20

A/m

A/m 3.99

3.99 v

v 3 w

29

u

w

u

3

2.4

2.4

1.8

1.8

1.2

1.2

0.6

0.6 0

0

(a)

(b)

Fig. 1.18 Distribution of current (a) 2.21 GHz (b) 4.11 GHz

1.9.2.3 1.9.2.3.1

Simulated Analysis Distribution of Current (A/m)

The decagon ring monopole antenna’s current distribution study at 2.21 GHz together with 4.10 GHz is shown in Fig. 1.18, which also reveals how the antenna functions. The decagon ring exhibits a sizable magnitude of current at 2.21 GHz, while the chamfered patch exhibits a nearly zero magnitude current, as seen in Fig. 1.18a. This finding demonstrates that the decagon ring is essential for producing frequency at 2.21 GHz, which permits the current to run for a long time. The antenna successfully meets the uplink and downlink bandwidth requirements of GSM 1800, along with LTE 2300/2600, by achieving a widened bandwidth in the frequency range of 1.65–2.71 GHz.

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Introduction to the Flexible and Transparent Antennas

1

The loading effect of the chamfered rectangle patch also causes another resonance to be formed at 4.10 GHz, as seen in Fig. 1.18b. The decagon ring also displays concurrent current flow, and a sizable quantity of current is shown flowing through the patch. This finding confirms that the resonance at 4.11 GHz may be attained without interfering with the impedance matching at 2.21 GHz.

1.9.2.3.2

Reflection Coefficient Curve (S11)

Figure 1.19 shows the decagon antenna’s reflection coefficient (S11) in dB as a function of frequency (GHz). The graph clearly demonstrates two separate resonances at 2.21 GHz and 4.11 GHz, with corresponding -10 dB impedance bandwidths of 48.62% (1.65–2.71 GHz) and 42.58% (3.66–5.64 GHz). According to these data, the decagon antenna appears to have excellent matching of impedance within the necessary operating frequencies.

1.9.2.3.3

Radiation Performance

Figure 1.20 displays radiation distributions in the E-plane and H-plane at 2.21 GHz and 4.11 GHz. The radiation patterns at both resonances exhibit a broadside radiation direction, indicating maximum radiation in the desired bands of interest. There are negligible nulls, ensuring a consistent and uniform radiation pattern. Additionally, it was verified that the antenna demonstrates gain greater than 2.5 dBi in both operating bands, further emphasizing its effectiveness and efficiency in wireless communication applications. Figure 1.21 illustrates the radiation pattern distribution at 2.21 GHz and 4.11 GHz. The radiation patterns clearly exhibit cross-polar patterns with significantly lower magnitudes compared to the co-polar patterns at both resonant Fig. 1.19 Characteristics of antenna in S11 (dB)

0

S11 (dB)

-10 -20

2.70 GHz

1.64 GHz

5.65 GHz

3.65 GHz

-30 -40 S 11 (dB)

-50 1

2

3

4

Frequency (GHz)

5

6

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Case Studies: Flexible Antenna Design, Geometry, and Simulation Analysis

31

y

d Bi

y

d Bi

3.61

2.62

–2.45

–3.45 Phi z

x

Theta

–8.51

–9.51

Phi

–15.6 z

–21.6

x

Theta

–14.6 –20.6 –26.7

–27.7

–36.4

–37.4

(a)

(b)

Fig. 1.20 Pattern distribution antenna (a) At 2.21 GHz (b) At 4.11 GHz 0o

5

-5

-15

-15

-25

-25

-35 -45

0o

5

-5

-35

270o

-45

90o

-45

270o

-35

-35

-25

-25

-15

-15

-5

-5

5

5

180o

180o

(a)

(b)

0o

5

0o

5

-5

-5

-15

-15

-25

-25

-35 -45

90o

-45

-35

270o

90o

-45

-45

270o

90o

-45

-35

-35

-25

-25

-15

-15

-5

-5

5

180o

(c)

5

180o

(d)

Fig. 1.21 2D radiation pattern: E-plane operating at (a) 2.21 GHz (b) 4.10 GHz, H-plane operating at (c) 2.20 GHz (d) 4.10 GHz

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Introduction to the Flexible and Transparent Antennas

frequencies. This observation confirms that the decagon antenna maintains linear polarization characteristics, making it well-suited for various wireless communication applications where linearly polarized signals are desired.

1.9.2.3.4

Signal Strength and Efficiency

The decagon antenna’s gain and efficiency parameters are shown in Fig. 1.22. The antenna has a maximum gain of 3.5 dBi at frequencies between 1.65 and 2.71 GHz and a gain of 5.5 dBi at frequencies between 3.66 and 5.64 GHz. Furthermore, the antenna displays excellent efficiency values of more than 90% in both frequency ranges of 1.65 GHz to 2.71 GHz and 3.65 GHz to 5.65 GHz. The decagon ring monopole antenna is a remarkable option for a wide range of cutting-edge wireless applications because of its exceptional radiation qualities.

1.9.2.4

Bending Analysis

The decagon antenna’s performance along the X and Y axes is examined in the bending analysis shown in Fig. 1.23. A 30 mm-radius cylinder is used for the bending analysis in order to assess the antenna’s flexibility. Figure 1.23a shows the antenna bending along the X-axis, whereas Fig. 1.23b shows the antenna bending along the Y-axis. The impact on the antenna’s impedance bandwidth is seen in Fig. 1.23c by comparing the S11 curve before and after bending. Remarkably, minimal deviation is observed when the X and Y axes of the antenna are both bent, indicating that it maintains operation within the desired frequency bands. This observation confirms the inherent flexibility of the decagon antenna, enabling seamless integration into wireless devices with space constraints. 100

9

90

8

80

7

70

Simulated Gain Simulated Efficiency

6

60

5

50

4

40

3

30

2

20

1

10 0

0 1

2

3

4

Frequency (GHz)

5

6

Radiation Efficiency (%)

10

Gain (dBi)

Fig. 1.22 Gain and efficiency

1.9

Case Studies: Flexible Antenna Design, Geometry, and Simulation Analysis

(a)

33

(b)

0

S11 (dB)

-10

-20

-30

-40

S11 (Bending Along X-Axis) S11 (Bending Along Y-Axis) S11 (Without Bending)

-50 1

2

3

4

5

6

Frequency (GHz)

(c) Fig. 1.23 Bending study of suggested decagon antenna (a) X-axis bending (b) Y-axis bending (c) Comparison of S11

1.9.2.5

Concluding Remarks

It has been successful to evaluate and simulate the decagon antenna for a variety of wireless communication applications. It has a broad frequency band coverage that includes GSM1800, LTE 2300/2600, 5G new radio, and Wi-Fi. The antenna stands out because of its straightforward design, compact size, and straightforward manufacturing process.

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Introduction to the Flexible and Transparent Antennas

The decagon ring monopole antenna has outstanding frequency coverage with a -10 dB impedance bandwidth of 48.84% (1.64–2.70 GHz) and 43.01% (3.65–5.65 GHz). In both frequency ranges, it maintains a minimum efficiency of 90% and a maximum gain of 5.5 dBi. Its versatility further enhances its appeal, making it a great contender for 5G and wireless communication applications.

1.9.3

Two-Dimensional (2-D) MIMO Antenna

In this section, we delve into a comprehensive case study on 2D MIMO antennas, as presented by the authors [95]. The main emphasis of the work is the development and analysis of a 2D, two-input, flexible multiple MIMO antenna that operates in the sub-6 GHz frequency band for wireless applications. By using two identical antenna arrays positioned over the X-axis, the MIMO antenna is ingeniously designed. An intelligently placed inverted-U isolating structure is used to efficiently isolate the antenna elements. On a FR-4 substrate that is 0.2 mm thick and has been chosen for its excellent electrical characteristics, the complete design is meticulously implemented. With dimensions of 52 × 30 mm2, the resulting MIMO antenna structure has a small form factor. With an excellent impedance bandwidth of 32.18% extending from 3.05 to 4.22 GHz, it exhibits a broad resonance around 3.35 GHz. In addition to the design and resonance analysis, the authors have meticulously investigated several vital MIMO diversity parameters. These parameters include gain exceeding 2.8 dBi, efficiency surpassing 78%, an ECC less than 0.05, and DG above 9.75 dB. These findings underscore the exceptional performance and effectiveness of the reported MIMO antenna design. Moreover, the authors have undertaken a rigorous bending analysis to assess the flexibility of FR-4 material employed in the described antenna structure. Antenna’s bending analysis validates the material’s flexibility by demonstrating its ability to withstand bending along the X direction up to a certain radius. To confirm these findings, a simulated bending analysis experiment has been conducted using CST software, yielding insightful results.

1.9.3.1

State of Arts in MIMO Designs

With the development of 4G wireless communication technology, the next wave of mobile and portable wireless devices has been made possible. Because of their small forms, these gadgets require higher data rates and the most recent standards. Antennas are essential for enabling continuous and unbroken connection between these devices. MIMO (numerous Input Multiple Output) technology, which makes use of numerous antenna elements, has consequently become a crucial technology in the field of 5G wireless communications. It is essential to design antennas that take

1.9

Case Studies: Flexible Antenna Design, Geometry, and Simulation Analysis

35

up the least amount of space possible and can modify to fit in confined spaces for optimal MIMO operation. Flexible antennas have gained significant attention as they offer the ability to bend and conform to various form factors, enabling easy integration into spaceconstrained devices. Consequently, several studies have reported both unbending as well as flexible MIMO antennas [96–102]. In [96, 97], two-port MIMO antennas with excellent isolation among antenna elements are presented. Due to the use of strong FR-4 substrates, these antennas are inflexible and have a small range of adaptation. In [98], a 2.4 GHz and 5.2 GHz reconfigurable MIMO antenna with dual-band and flexibility is presented. However, because it is made on a semiflexible Rogers RO3003 material having a thickness of 1.52 mm, this antenna is expensive. Flexible UWB (Ultra-Wideband) MIMO antennas, offering advantages such as extreme security, low usage of power, and high data rates, are cited in [99, 100]. These antennas were created utilizing inkjet printing techniques on a Kapton polyamide substrate. [101] presents a 3D paper-based 3 × 3 MIMO antenna that can operate in the 2.4 GHz and 5 GHz WLAN bands, but it also needs an ABS plastic support structure for increased stability. Another study [102] describes a MIMO antenna for wearable applications that operates in the 2.4/5 GHz WLAN and 3.5 GHz WiMAX bands and is built on a liquid crystal polymer (LCP) substrate. However, the aforementioned flexible MIMO antennas have limitations such as expensive substrates, the need for additional materials, fabrication complexity, and more. Therefore, the objective of this case study is to evaluate a two-port flexible MIMO antenna built on a cheap FR-4 substrate with a 0.2 mm thickness, 4.3 dielectric constant, and 0.0025 loss tangent. The suggested flexible MIMO antenna offers isolation greater than 15 dB over the specified frequency band and a large impedance bandwidth of 32.18% (3.05–4.22 GHz).

1.9.3.2

MIMO Antenna Design

Figure 1.24 shows the optimal MIMO antenna shape and its isolating structure, demonstrating the careful design considerations. The FR-4 substrate, which is flexible and has a thin 0.2 mm thickness, serves as the foundation for the MIMO antenna. Twin antenna elements, Ant-1 and Ant-2, are used in the antenna array and are positioned along the X-axis. As shown in Fig. 1.24a, each antenna element has a radiating structure in the form of a butterfly. To enable maximal signal power transfer, a microstrip feed line with a dimension of 10 mm × 0.6 mm is linked to the butterfly structure’s belly. This line is used to excite the structure. The structure of butterfly’s wings is elliptical in shape, with an 8 mm length from the center. All three wings on each side are angled at 45 degrees with respect to each other. The central structure of the butterfly, which is oval-shaped, possesses dimensions of 20 × 1.5 mm2. As shown in Fig. 1.24b, each antenna element integrates a partial ground plane of 25 mm × 12 mm to create the proper impedance matching between the feed line and the structure.

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Introduction to the Flexible and Transparent Antennas

Fig. 1.24 Design of reported MIMO antenna (dimensions are in mm unit), view: (a) Front (b) Back

The detached ground planes of the two antenna elements are separated by a distance of 2 mm, improving isolation between them. An inverted-U decoupling structure is installed between the two antenna elements to further improve isolation. With this design, coupling between antenna elements is substantially reduced. The claimed MIMO antenna has an astoundingly thin thickness of about 0.2 mm and an overall size of 52 × 30 mm2. The MIMO antenna may bend readily due to its astounding thinness without significantly impacting its performance. The effects of bending on the properties of the reflection coefficient curve are also thoroughly investigated and validated.

1.9.3.3

Performance Evaluation of MIMO Antennas

The reported MIMO antenna undergoes comprehensive analysis using state-of-theart Computer simulation technology (CST) software. Through careful consideration of CST simulations, a number of performance metrics, including electric field intensity in (A/m), reflection coefficient (S11) in dB, transmission coefficient (S12) in dB, gain (dBi), efficiency (%), ECC, and DG, are assessed. In order to do these tests, Ant-1 must be activated while Ant-2 is terminated with a 50-ohm load impedance and vice versa. Figure 1.25 visually presents the outcomes of the CST simulations. It is clear that the isolating structure considerably lessens the mutual coupling between Ant-1 and Ant-2, guaranteeing that the antennas operate within the targeted band of interest while being protected from one another’s electric fields. In particular, Fig. 1.25a shows that the field current primarily flows through Ant-1 when port 1 is excited, whereas Fig. 1.25b confirms that the opposite situation occurs when port 2 is stimulated. These findings highlight the isolating structure’s efficiency in keeping the antenna elements separate and preventing unintended interactions while it is in use.

1.9

Case Studies: Flexible Antenna Design, Geometry, and Simulation Analysis

37 A/m

A/m 11.8

11.8

10

10

8

8

6

6

4

4

2

2

0

1

0

2

(a)

(b)

Fig. 1.25 Field current density (a) Excited port 1 (b) Excited port 2

Fig. 1.26 Analysis of S-parameters

S-Parameters (dB)

0 -5 -10 -15 -20 -25 -30 -35 -40 -45 2.0

S11 Simulated S22 Simulated

2.5

3.0

3.5

4.0

4.5

5.0

Frequency (GHz)

To ascertain the matching bandwidth along with mutual coupling characteristics, a detailed analysis of the S-parameters is conducted. Figure 1.26 depicts the S11 (dB) and S12 (dB) curves obtained from the analysis. In particular, the S11 curve shows the resonance produced at 3.35 GHz by Ant-1 and Ant-2, with an outstanding bandwidth of impedance of 32.18% (3.05–4.22 GHz). Furthermore, the S12 curve reveals the minimal coupling between the two antennas, as evidenced by the consistently low coupling level of -17 dB across the entire band of operation. These observations support the MIMO antenna’s superior performance in terms of impedance matching as well as reduced mutual interference, proving its appropriateness for effective and dependable wireless communication applications. The distribution of radiation patterns of antenna elements namely Ant-1 and Ant-2 at the resonant frequency of 3.35 GHz is illustrated in Fig. 1.27, depicting both planes E and H. Notably, the distributed radiations of Ant-1 and Ant-2 exhibit a symmetrical and mirror-image arrangement, confirming their independent operation within the targeted band of operation. This characteristic ensures that both antennas function autonomously, allowing for efficient and reliable wireless communication without interference or crosstalk between them.

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Introduction to the Flexible and Transparent Antennas

0o

5

0o

5

-5

-5

-15

-15

-25

-25

-35

-35

-45

270o

90o

-45

-45

270o

90o

-45

-35

-35

-25

-25

-15

-15

-5

-5

5

180o

Crosspolar (Simulated) Copolar (Simulated)

5

180o

(a)

(b)

0

0o

o

5

5

-5

-5

-15

-15

-25

-25 -35

-35 -45

270o

90o

-45

-45

270o

90o

-45

-35

-35

-25

-25

-15

-15 -5

-5 5

Crosspolar (Simulated) Copolar (Simulated)

180o

(c)

5 Crosspolar (Simulated) Copolar (Simulated)

180o

Crosspolar (Simulated) Copolar (Simulated)

(d)

Fig. 1.27 Distributed polar radiation patterns at 3.5 GHz (a) Ant. 1 excited in E-plane (b) Ant.1 excited in H-plane (c) Ant.2 excited in E-plane (d) Ant.2 excited in H-plane

Figure 1.28 showcases the three-dimensional (3D) distribution of radiation patterns of Ant-1 and Ant-2, providing further evidence that these patterns mirror each other, just as observed in the two-dimensional radiation patterns. This close agreement between the 2D and 3D distribution of radiations underscores the consistent as well as independent operation of both antennas, ensuring optimal signal coverage and reception in the desired direction for effective wireless communication. Figure 1.29 shows the gain (dBi) and efficiency (%) characteristics of Ant-1 and Ant-2. The results show that over the full frequency band of interest, which ranges from 3.05 to 4.22 GHz, both antenna elements maintain a constant gain of over 2.8 dBi and an efficiency of over 78%. This analysis offers compelling proof that the described MIMO antenna can be integrated into flexible electronic devices that use the sub-6 GHz n77/n78 5G channels.

1.9

Case Studies: Flexible Antenna Design, Geometry, and Simulation Analysis y

39

y

dBi

dBi

3.86

3.86

–2.2

–2.2 Phi z

Theta

x

–8.26

Phi

–8.26 z

–14.3

x

Theta

–14.3 –20.4

–20.4

–26.4

–26.4

–36.1

–36.1 (a)

(b)

10

100

9

90

8

80

7

70

Gain (dBi)

Fig. 1.29 Gain and efficiency of MIMO antenna

60

6 5

Simulated Gain Simulated Efficiency

50

4

40

3

30

2

20

1

10

0 2.0

2.5

3.0

3.5

4.0

4.5

Radiation Efficiency (%)

Fig. 1.28 3D patterns of MIMO antenna (a) 3.35 GHz, Ant. 1 excited (b) 3.35 GHz, Ant. 2 excited

0 5.0

Frequency (GHz)

1.9.3.4

MIMO Antenna Diversity Performance Analysis

The MIMO diversity metrics, namely ECC and DG, are evaluated and presented in Figs. 1.30 and 1.31, respectively, based on the calculations outlined in [103]. Figure 1.30 clearly demonstrates that the ECC value remains consistently below 0.05, indicating a high level of diversity achieved by the MIMO antenna system. Additionally, Fig. 1.31 shows that the DG value exceeds 9.75 dB, highlighting the significant improvement in signal quality and robustness offered by the MIMO configuration. These results substantiate the suitability of the reported MIMO antenna for MIMO systems, as the antenna elements exhibit excellent isolation and minimal interference within the desired operating bands. The observed performance enhancements contribute to the overall capacity enhancement of wireless systems. As a result, the reported MIMO antenna holds promising potential for integration within compact wireless devices.

40

1

Fig. 1.30 ECC value calculated from far-field from reported MIMO antenna

Introduction to the Flexible and Transparent Antennas

0.10 0.09 0.08

ECC from Farfield

ECC

0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.00 3.0

3.5

4.0

4.5

5.0

4.5

5.0

Frequency (GHz)

10.0

Diversity Gain (dB)

Fig. 1.31 DG value calculated from far-field of reported MIMO antenna

9.9 9.8 9.7 9.6 9.5 3.0

DG from Farfield

3.5

4.0

Frequency (GHz)

1.9.3.5

Performance Evaluation of MIMO Antenna Bending

The FR-4 substrate of the MIMO antenna was tested for flexibility. A simulated bending experiment is conducted using CST software, focusing on the X-direction bending as depicted in Fig. 1.32. The impact of bending on the antenna’s impedance bandwidth as well as mutual coupling is examined and presented in Fig. 1.33. Notably, despite a slight deviation observed in both S11 and S12 results, it is clear that the bent MIMO antenna neither impairs the impedance bandwidth achieved nor interferes with the coupling between the antenna elements.

1.9.3.6

Concluding Remarks

The current study addresses developing and analyzing a novel, small, flexible two-port MIMO antenna for wireless applications operating and analysis of a novel, small, flexible two-port MIMO antenna for wireless applications that operates

1.10

Transparent Antennas

41

Fig. 1.33 S-parameters of MIMO antenna X-axis bent

S-Parameters (dB)

Fig. 1.32 Layout of MIMO antenna X-axis bent

0 -5 -10 -15 -20 -25 -30 -35 -40 -45 2.0

S11 Simulated S22 Simulated

2.5

3.0

3.5

4.0

4.5

5.0

Frequency (GHz)

in the sub-6 GHz n77/n78 5G bands. The proposed MIMO antenna’s adaptability makes it the perfect option for wireless devices with limited space because it is simple to integrate and modify to fit the space.

1.10

Transparent Antennas

Some of the applications also demand antennas, which can be fitted with no extra board space. Transparent antennas have come out as an efficient solution as there is no visual clutter, and it can be placed anywhere on the device with no extra space [104–108]. These antennas are optically transparent which makes them superior to the conventional antennas due to aesthetics. Transparent conductive sheets can be easily affixed on a glass of a building and motor vehicles. With the increasing demand for dual and multiband antennas which can incorporate various applications, a lot of work is going on in prototyping such antennas (ULVAC Corporate Center). The fields like automobile, satellite applications, biomedical applications, defense and energy harvesting applications demand antennas which are efficient, high gain, and with ease of installation [109–117].

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Introduction to the Flexible and Transparent Antennas

Broadly, transparent antennas are divided into two different categories:

Antennas which use a combination of conductive sheets along with nontransparent copper, silver, and gold for fabricating the antennas fall into the category of semi-transparent antennas [118–123]. The advantage of the semi-transparent antenna is its better performance due to the presence of non-transparent materials having higher conductivity value as compared to transparent materials while the disadvantage is that it is visible to the eyes in most cases.

1.10.1

Oxide based Transparent Antennas

Transparent antennas are constructed using transparent conductive oxides such as FTO, ZnO, ITO, AgHT, with aluminum oxide serving as the patch and ground, in conjunction with transparent substrates like glass, plexiglass, and PET. The advantage of being invisible needs to be compromised if reasonable antenna performance is required.

1.10.1.1

Transparent Materials Having Conductive Properties

Various transparent conducting oxides which can be used are indium tin oxide (ITO), silver tin oxide (AgHT), fluorine-doped tin oxide (FTO), aluminum-doped zinc oxide (ZnO: Al), and indium-doped zinc oxide (ZnO: In). Various ITO-based antennas are proposed by researchers due to its high conductivity and transparency. However, indium on the verge of depletion is a rare metal, and so in the near future, the availability of ITO is a concern. Researchers are focusing on tin oxide (SnO2) and zinc oxide (ZnO). Aluminum-doped zinc oxide and gallium-doped zinc oxide can replace ITO, but it is in the development phase and still not commercialized. Tin oxide has emerged as the best option over ITO. AgHT is conductive along with optically transparent. If the electron flow of such conductive materials can be increased, then the conductivity of such sheets can be improved a lot [122]. The frequency ω is used for calculating the conductivity of a TCO that can be determined by (1.3) where carrier concentration is given by Ne, the electron charge is given by qe, the relaxation time of electron is given by τ, and electron effective mass is given by m.

1.10

Transparent Antennas

43

σ=

N e q2e τ 1 , m
1 þ jωτ

ð1:3Þ

The transparent antennas can be used in antenna applications only if the oxide film used is adequately thin in the visible range to be called transparent. At the same time, it should also be thick so that it can efficiently work in the microwave range. Materials optical property depends on the wavelength. Material behaves as transparent dielectric above plasma frequency and as a metal below plasma frequency. Plasma frequency of the majority of transparent oxides is in the range of nearinfrared and is calculated using (1.4). ωp =

1.10.1.2

N e q2e 1 ε1 ε0 m
τ2

ð1:4Þ

Transparent Conductive Oxides Material Properties (Tcos)

The properties of AgHT-4, AgHT-8, and copper are listed in Table 1.5 in terms of transparency, surface resistance, and conductivity. Table 1.6 illustrates the order of conductivity and transparency for various transparent oxides. As the conductivity of oxides decreases, the transparency increases and vice versa. A comparison between various TCOs is listed in Table 1.7 in order to compare the conductivity values. Conductivity is a relation between mobility (μ), electric charge (e), and carrier concentration (n) given by Eq. 1.5. σ = neμ

1.10.1.3

ð1:5Þ

Applications of TCO Materials

• TCOs are used for making windows, which conserve energy due to its capability to reflect thermal heat. Solutia Inc, USA, has commercialized AgHT as a sun-shielding film. • TCOs are also used in ovens where oven windows are coated with TCOs to conserve energy. It also helps in maintaining the temperature on the outer side, which makes it safe to touch.

Table 1.5 Properties of AgHT-4, AgHT-8, and copper Parameters Visibility/transparency Surface resistance (Ohms per square)

AgHT-4 Min 75% 4 Ω/sq

AgHT-8 Min 82% 8 Ω/sq

Copper 0% 0.5 mΩ/sq

44

1 Introduction to the Flexible and Transparent Antennas

Table 1.6 Order of conductivity and transparency of TCOs

TCO Type Conductivity Transparency Ag-based

ZnO: Al

Increase

In2O3: Sn

Decrease

TiN

SnO2: F ZnO: F

Table 1.7 Comparison of TCOs S.No 1. 2. 3. 4. 5. 6. 7. 8.

TCO material Ag Si:Pn Si:P n++ ITO AZO GZO ZnO SnO2

Conductivity (S cm-1) 6.8 × 105 10-2 102 104 104 104 103 103

Carrier concentration (cm-3) 5.76 × 1022 1014 1019 1020–1021 1020 1020 1020 1020

Mobility (cm2V-1S-1) 72 1500 100 20–80 10–20 10–20 16 7.7

• It can be utilized in automobiles rear view mirrors, which can be dimmed automatically. It is also used in smart windows where electrochromic material between TCO films is used for defrosting windows by passing current through TCOs. • TCOs are used to fabricate transparent antennas, which can be attached on routers, smart devices, automobile windows, or windscreens.

1.10.1.4

Other Properties of Transparent Oxides

Properties that make the TCOs a superior choice for antenna fabrication are as follows:

1.10

Transparent Antennas

1.10.1.4.1

45

Transparency

AgHT, which is a three-layered film, comprises of tin oxide and a silver layer which is a conductive film with visible transparency of 75–80%. The AgHT film contains 40% of its thickness due to PET and coating. The overall thickness of AgHT is 0.177 mm. The transparent features of the AgHT are beneficial in antennas for use in various wireless applications where board space is limited with no visual clutter.

1.10.1.4.2

Electrical Conductivity

AgHT

AgHT-4 AgHT-8

Sheet resistance is indicated by the number. Sheet resistance is linked to the film thickness (d ) and specific resistivity, ρ, that is, Sheet resistance = ρ=d

ð1:6Þ

Electrical conductivity and resistivity are inversely proportional to each other.

1.10.1.4.3

Finding Conductivity of Thin Sheet from Sheet Resistance

The steps to calculate the conductivity from the sheet resistance are as shown as follows: L A L R=ρ W ×T ρ L R= × T W L R = RS × W R=ρ

R: resistance, ρ: resistivity, L: width and T: sheet thickness, Rs: sheet resistance. Example: AgHT-4 (Sheet Resistance = 4 Ω/Sq) Thickness = 0.177 mm = 0.000177 m

according

to

the

datasheet,

46

1

Introduction to the Flexible and Transparent Antennas

RS =

ρ T

1 σ×T 1 σ= RS × T RS =

where σ is the conductivity. σ=

1 4 × 0:000177

σ = 1412:43s=m

1.10.1.4.4

Hardness of TCOs

The TCO’s durability is measured using the hardness of the crystal, which is used to fabricate it. Hardness is expressed in Mohs where harder materials have higher values. The tin oxide, which is harder than glass, is used for the manufacturing of TCOs. Zinc oxide can be scratched easily. Thin silver films are brittle, and it needs to be covered using protective layers. Figure 1.34 shows the AgHT structure with a silver layer sandwiched between two protective layers in the form of tin oxide. Single patch antennas using AgHT-8 operating at 2.3 GHz and 9.5 GHz are demonstrated by [104]. The two antennas are structured using Plexiglas and AgHT, fed through microstrip feed and probe feed. Mias (2000) [119] did a performance analysis of antennas fabricated using various transparent conductors. The performance comparison was carried out using materials like gold, silver, copper, aluminum, and ITO. The study compared how ITO trace performed against other conductive traces, which are used for fabricating a dipole on a substrate made up of transparent glass. The conclusion of their work was that gold and silver owing to its high value of conductivity have potential in making antennas. In [120], authors validated a multilayer transparent antenna as shown in having wide-band and high directivity using AgHT as conductive oxide material. The antenna was developed mainly for its use in satellite and space exploration. The NASA engineers attempted to improve both the parameters to increase the usefulness of the patch antenna. The antennas were mainly developed for the applications where compact and slim profiles are not needed.

Fig. 1.34 AgHT conductive sheet cross-sectional view

1.11

Case Study of CPW-Fed Transparent Flexible Antenna

1.11

47

Case Study of CPW-Fed Transparent Flexible Antenna

The polyethylene terephthalate (PET) substrate is used in a transparent flexible co-planar waveguide-fed patch antenna that is presented. At the center frequency of 4.2 GHz, the wideband high gain antenna has an overall dimension of 0.81λ × 1.09λ × 0.12λ and is constructed from a transparent sheet of silver tin oxide (AgHT-8). Four additional non-transparent, non-flexible, and semi-transparent flexible antennas are compared to the performance of the proposed antenna. For the designed design, the substrate and patch materials are changed while the patch geometry and feeding mechanism remain constant. The full wave, finite element method (FEM)-based high-frequency structure simulator (HFSS) is used for simulations before the antennas are built and tested. The suggested flexible transparent antenna has a bandwidth of around 40%, covering the frequency range of 3.89–5.9 GHz. It also has a significant gain of more than 3 dBi and a frequency band efficiency of more than 80%. The flexible transparent antenna is also put to the test under bending conditions, and it performed well for sub-6 GHz 5G and WLAN applications.

1.11.1

Antenna Geometry

The flexible, transparent antenna geometry with optimum sizes is shown in Fig. 1.35a. The antenna is constructed using double circular rings connected by a star-shaped engineering framework. CPW feed is used to match impedance and create a smooth transition for electromagnetic waves. Silver and tin oxide-based conductive oxide sheet (AgHT-8) is printed and adhered to the substrate’s top side using thin glue sheet, creating the patch and ground plane, respectively. To provide total transparency and flexibility of the antenna, a substrate made of PET with a thickness of 0.9 mm, a dielectric constant of 3.2, and a loss tangent of 0.22 is utilized. The antenna dimensions are optimized to produce wideband characteristics, as illustrated in figure. Figure 1.35b, c illustrates resonator view from above and from below, respectively. The fabricated resonator is depicted in Fig. 1.35d. For the purpose of evaluating performance, four additional antennas with comparable patch antenna geometry have been simulated and constructed. Table 1.8 provides the antenna parameters. Antennas 1 and 2 are made of FR4 sheets having 1.6- and 0.5-mm thicknesses, respectively, with dielectric constants of 4.4 and 0.2, which classify them as flexible and non-transparent non-flexible antennas. Antennas 3 and 4 are built of flexible Cu and PET that forms patch and substrate, respectively, and have thicknesses of 0.7 mm and 0.9 mm. This antenna falls within the category of semi-transparent flexible antennas due to the patch and CPW feed made of -.05 mm thick flexible

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1 Introduction to the Flexible and Transparent Antennas

Fig. 1.35 Antenna structure (a) Front view (b) Perspective view (c) Cross-section dimensions (d) Fabricated antenna

1.11

Case Study of CPW-Fed Transparent Flexible Antenna

49

Fig. 1.35 (continued)

copper sheet. When PET sheet and AgHT-8 are combined, the antenna is completely transparent. PET and AgHT-8 are both transparent and flexible materials. Table 1.9 lists the comparison of antennas based on flexibility and transparency. Before constructing the antenna as illustrated in Figs. 1.36 and 1.37, the suggested flexible transparent antenna’s performance is analyzed by optimizing the antenna geometry. Figure 1.36a, b shows that as R1 and R2 fluctuate, the reflection coefficient varies at lower and higher resonances with little change in the antenna’s bandwidth, but an extra resonant mode is also being excited outside the suggested frequency range. R1 and R2’s ideal dimensions are chosen to be 15.9 mm and 15 mm, respectively. The impact of changing the radius of the circular conducting component (R3) fed by the CPW on reflection coefficient is shown in Fig. 1.36c. As can be observed from the figure, as the radius (non-conducting surface) increases, the return loss increases as well. However, as the radius increases, an undesired frequency is resonated; therefore, the value of R3 is carefully selected as 26.5 mm for maximum performance. According to Fig. 1.36d, the R4 parameter modification results in a slight increase in return loss at higher resonance frequencies and a decrease at lower resonance frequencies. The fluctuation in reflection coefficient caused by variations in CPW feed width (S) reveals that |S11| increases at lower resonance and decreases at higher resonance as the width changes. When the breadth shrinks, the bandwidth also slightly shrinks. For optimal performance, S is set to 1.6 mm. It is also examined how different shapes affect how the inner and outer rings of the patch are coupled, and it is shown that using a circle as the coupling element improves performance while also generating more resonance. An engineered star-shaped coupling element aided in reaching the desired results because a rectangular shape as a coupling element indicates a significant drop in bandwidth.

Non-conducting substrate material FR-4

FR-4 PET PET PET

S. No Ant 1

Ant 2 Ant 3 Ant 4 Ant 5

0.02 0.02 0.02 0.02

Non-conducting layer (tan δ) 0.02 0.30 0.65 0.85 0.72

Non-conducting layer thickness (in mm) 1.40

Conducting material Copper (Cu) Cu Cu Cu AgHT-8 0.20 0.05 0.05 0.177

Conducting layer thickness (in mm) 0.20

0.50 0.70 0.90 0.90

Overall thickness (T) (in mm) 1.60

1

4.40 3.20 3.20 3.20

Nonconducting layer (εr) 4.40

Table 1.8 Different proposed antennas for performance analysis

50 Introduction to the Flexible and Transparent Antennas

1.11

Case Study of CPW-Fed Transparent Flexible Antenna

Table 1.9 Comparison of proposed antennas transparency and flexibility

Antenna number Ant 1 Ant 2 Ant 3 Ant4 Ant 5 a

51 Transparenta N N S S Y

Flexible N Y Y Y Y

N no, Y yes, S semi

Fig. 1.36 S11 by varying (a) Outer ring outer radius (R1) (b) Outer ring inner radius (R2) (c) CPW-fed circle radius (R3) (d) Inner ring radius (R4)

52

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Introduction to the Flexible and Transparent Antennas

Fig. 1.36 (continued)

The comparison of all the suggested antennas in terms of reflection coefficient is shown in Fig. 1.38. Analysis is done on the impact of changing the substrate, the conductive material, and the thickness of the antenna while maintaining the same antenna geometry. When compared to semi-transparent flexible antennas, it has been

1.11

Case Study of CPW-Fed Transparent Flexible Antenna

53

Fig. 1.37 S11 by varying (a) CPW feed width (S) (b) Various shapes for interconnection of rings

found that FR4-based flexible and non-flexible antennas exhibit narrow bandwidth. This appears to be caused by PET having a lower dielectric constant than FR4 in comparison. Additionally, the PET-based antenna is thicker than the flexible FR4-based antenna. Transparent flexible antenna yields respectable results with respect to S11 and bandwidth.

54

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Introduction to the Flexible and Transparent Antennas

Fig. 1.38 Comparison of simulated reflection coefficient of flexible transparent antenna with other designed antennas

Fig. 1.39 Simulated (solid) and measured (dashed) reflection coefficient of flexible transparent antenna

1.11.2 Experimental Results and Discussion The exhibited antennas are simulated using HFSS software that is FEM-based. After building the suggested antenna, the return loss, gain, and radiation patterns are measured. Figure 1.39 illustrates the computed and measured reflection coefficient plot of a flexible transparent antenna, which demonstrates that the antenna exhibits 10 dB return loss over the 3.89–5.8 GHz (40%) bandwidth. The observed value of the reflection coefficient |S11| illustrates the 3.91–5.76 GHz bandwidth,

1.11

Case Study of CPW-Fed Transparent Flexible Antenna

55

Fig. 1.40 Co-/cross-pol pattern of flexible transparent antenna (a) H (X-Z plane), ɸ = 90° (b) E (Y-Z plane), ɸ = 0°

demonstrating the consistency between the results of the simulation and the measurement. The adhesion procedure used to adhere the AgHT-8 sheet to the PET substrate is what causes the discrepancy between the two findings. The manufacturing of the patch may have different dimensions from the simulation’s antenna, and its actual antenna may have different characteristics. The suggested flexible transparent antenna has the lowest return loss at two resonance frequencies, 4.28 GHz and 5.20 GHz, which are taken into consideration for further investigation. Figure 1.40 depicts co-polarization and cross-polarization radiation patterns of the antenna at 4.28 GHz and 5.20 GHz in H-plane (X-Z) and

56

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Introduction to the Flexible and Transparent Antennas

Fig. 1.41 Pattern measurement (a) E-plane setup (b) H-plane setup

E-plane (Y-Z) for ɸ = 90° and 0°, respectively. At both the frequencies, the antenna attains a cross-pol isolation of 15 dB between co-/cross-polarization patterns. An omnidirectional radiation pattern is observed in H plane at 4.28 GHz; however, in E plane at 5.20 GHz, the radiation pattern turns directive. Figure 1.41 depicts the configuration for pattern measuring in an acoustic chamber for the E and H plane at both resonance frequencies. At either of the resonant frequencies, as illustrated in Fig. 1.42’s 2D radiation pattern, there is no null, with the exception of 5.20 GHz in the H-plane, where a dipole-like pattern is seen. Figure 1.43 displays the current distributions at 4.28 GHz and 5.20 GHz. In spreading the current, the feed line and V-shaped feeding network are crucial components. At 4.28 GHz, it is noted that the current distribution is more even between the star and circular ring, as well as near the sides of the central circular ring and along the margins of the outer ring. The dispersion at 5.20 GHz is primarily on the bottom side of the antenna. Wider frequency response is caused by the surface current flow in the CPW feed and patch. The gain and efficiency curve for the whole target frequency band is shown in Fig. 1.44. For the planned 5G and WLAN application bands, the simulated gain is greater than 3 dBi, and the efficiency is greater than 80%. The measured findings are in good accord with small differences brought on by the SMA connection interfacing approach, which uses conductive graphene silver glue to attach the SMA connector

1.11

Case Study of CPW-Fed Transparent Flexible Antenna

57

Fig. 1.42 2D radiation pattern (a) E-plane (b) H-plane

to the antenna rather than hot soldering, which would otherwise cause the AgHT8 sheet to lose its conductivity. Additionally, the manufacturing process has a significant impact on lower gain and efficiency values since patch and CPW feed are patterned using a laser, which lowers conductivity values around the edges as a result of very little heat generation. After building the antennas as illustrated in Fig. 1.45, the performance of other antennas is evaluated in terms of the reflection coefficient. The patch and CPW feed of PET-based antennas are manually patterned, which slightly affects the precision, in contrast to the FR4-based antennas, which are patterned using an etching process.

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Introduction to the Flexible and Transparent Antennas

Fig. 1.43 Current distribution (a) 4.28 GHz (b) 5.20 GHz

Fig. 1.44 Plot for (a) Gain (b) Efficiency

As can be seen from the picture, simulated and measured results are similar with a small amount of manufacturing and soldering process variation. Because simulated and real losses differ, the findings of the substrate loss tangent modeling are likewise impacted. In comparison to other antennas, antenna 2’s decreased bandwidth is principally caused by a thinner construction and a greater dielectric constant.

1.11.3

Bending Analysis

As the antenna will be incorporated for use in flexible smart devices, it is vital to evaluate the performance under bending conditions, which entails measuring return loss since antennas are vulnerable to fluctuations in effective electrical length. The

1.11

Case Study of CPW-Fed Transparent Flexible Antenna

59

Fig. 1.45 S11 of (a) Ant 1 (b) Ant 2 (c) Ant3 (d) Ant 4

analysis of the reflection coefficient under bending conditions is shown in Figs. 1.46 and 1.47, respectively. An antenna is shown bending along the X and Y axes in Fig. 1.46. By bending a flexible antenna along the Y-axis around a semi-cylindrical piece of Styrofoam with a radius of 55 mm (r = 1.03), as shown in Fig. 1.10a, the S11 is measured using a VNA. The resonator is similarly bent along the X-axis to measure the S11. The fundamental justification for choosing Styrofoam as the semi-circular surface is that it has a permittivity value that is nearly equal to that of air. The values of the simulated and observed S11 are shown in Fig. 1.13a, b, and they amply demonstrate the impact of structural bending on resonance frequencies. The antenna still resonates in the band that has been proposed for sub-6 GHz 5G and WLAN applications, despite the measured S11 showing deflection from the results of simulations. Table 1.10. depicts the comparison of proposed flexible transparent antennas with existing state-of-art. These are examples of various transparent and flexible antenna designs proposed for different applications:

1. Laptop Application: A transparent and flexible antenna made of indium zinc tin oxide (IZTO) and silver is proposed to be used in laptops. It operates in the frequency range of 5.18– 5.32 GHz [124].

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Fig. 1.46 Bending positions and measurement setup in (a) Y-axis (b) X-axis

2. Monopole Antenna: A flexible and transparent monopole antenna is designed using a Kapton polyimide substrate and conductive indium tin oxide. This antenna resonates at a frequency of 2.48 GHz [125]. 3. Wearable Glasses Application: An antenna fabricated using the physical vapor deposition method is proposed for flexible wearable glasses. It consists of IZTO/Ag/IZTO layers and operates in the frequency range of 2.4–2.5 GHz [126]. 4. 5G Wireless Communications: A transparent and flexible antenna, based on AgHT and PET materials, is proposed for 5G wireless communications. It operates in the frequency range of 23–29.5 GHz [127].

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Case Study of CPW-Fed Transparent Flexible Antenna

61

Fig. 1.47 S11 after bending antenna along (a) Y-axis (b) X-axis

5. WLAN Application: An antenna designed for WLAN applications uses an ITO and flexible glassbased structure. It operates at a frequency of 5.8 GHz [128]. 6. Wireless Networks: A transparent antenna designed for wireless networks uses a polydimethylsiloxane substrate and conductive fabric tissue as the patch material. It operates in the frequency band of 2.2–2.5 GHz [129].

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Table 1.10 Evaluation of proposed flexible transparent antennas with other similar resonators from literature

References [124]

Substrate material used Polyamide

[125]

Polyamide KAPTON

[126]

Polyamide

[127]

PET

[128] [130]

Glass PDMS

Proposed antenna

PET

Patch material used Multilayer film (IZTO/Ag/ IZTO) ITO (patch plane) sprinkled Copper (ground plane)

Dimension (mm) 24 * 24 * 0.1

Transparency (%) 86.00

Impedance bandwidth 5.18–5.32 GHz (Measured)

26.5 * 25

80.90

IZTO/Ag/ IZTO Aght-8

27 * 10

81.10

139 MHz at 2.4 GHz (Simulated) 123 MHz at 2.485 GHz (Measured) (2.4–2.5)

12 * 12 * 3.22

70–80

NA 50 * 40 * 2.05

75–85 80.00

58 * 78 * 0.9

80.00

ITO Conductive fabric AgHT-8

106 MHz at 27.09 GHz 4–10.5 GHz 167% at 2.2–25 GHz 42.2% at 3.89–5.97 GHz

7. Wideband Applications: A flexible and transparent coplanar waveguide (CPW) fed antenna is proposed for wideband applications. It utilizes ITO and PET materials [130]. These antennas offer transparency and flexibility, making them suitable for various applications where aesthetic considerations and space-saving are important. Each design targets specific frequency ranges and applications, showcasing the versatility of transparent and flexible antenna technology.

1.11.4

Conclusion

We show a flexible, transparent, high gain, wideband antenna made with PET and AgHT-8. Laser technology is used to achieve the conducting layer patterning’s accuracy. The antenna achieves an average gain of about 3 dBi over the whole frequency band of operation while having an impedance bandwidth of 40%. The gain and efficiency figures are clearly impacted by the dielectric constant and thickness, as shown by the performance analysis with various manufactured antennas. AgHT-8’s poorer conductivity as compared to copper has been made up for by carefully determining the overall height and substrate dielectric constant while preserving the antenna’s flexibility and transparency. After comprehensive

References

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parametric study, the suggested flexible transparent antenna is built. Antenna’s operating bands average efficiency is greater than 80%. Additionally, bending study was done to test the performance of the antenna when bent, and the results are encouraging for use in sub-6 GHz 5G and WLAN applications where the antenna needs to be bent while still keeping the device’s aesthetics.

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Chapter 2

Flexible, Transparent, and Wideband 4-Port MIMO Antenna

2.1

Introduction

Communication practices employing MIMO technology have grown extra popular across the past decades, with well-known cases existing Wi-Fi systems and cellular 5G and 6G systems that make up a considerable part of our new information foundation. The idea of extensive or large-scale MIMO orders having more than 100 antenna components is needed for portable interfaces using both fifth (5G) and sixth (6G) generations, respectively [1, 2]. More lately, MIMO has found its method into fast-developing businesses such as licensed performance video and a more significant data rate handling various Tx and Rx sequences. Frequency variety and frequency carriers can be used for communication, decreased signal deformity due to the multipath appearance, and contributes time variety – a data package can be given at various time openings. With the greater requirements of bandwidth for audio, video, and data applicability in need, MIMO offers an excellent communication solution, principally in urban locations where it is challenging to obtain free line-ofsite communication. The wealth of RF/microwave systems can pose choking problems. Also, the system needs a more extraordinary level of signal processing at the antenna and receiver end. Currently being investigated are transparent antennas, which give engineers the ability to improve on the prevailing designs for applications in the field of IoT that discretely integrate into glazed artwork, overhead lighting, or windows. In order to boost signal strength and data throughput, more antennas can be deployed close to the points of utilization thanks to optical transparency [2].

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 J. Kulkarni et al., Transparent and Flexible MIMO Antenna Technologies for 5G Applications, EAI/Springer Innovations in Communication and Computing, https://doi.org/10.1007/978-3-031-42486-1_2

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2.2

2

Flexible, Transparent, and Wideband 4-Port MIMO Antenna

5G MIMO Technology

This section will discuss matters compared to the basics of getting MIMO information technology and combining it with printed transparent antennas for 5G applications that can be assumed by users of all levels of technological aspects.

2.2.1

Conventional MIMO Configurations

MIMO wireless operations use many antennas to transmit and support various data streams at once. The radio operator determines the number of antennas for optimum communication. Standard forms are as follows [3]: • 2X2 MIMO • (2 Tx antennas, 2 Rx antennas)

• 8X8 MIMO • (8 Tx antennas, 8 Rx antennas).

MIMO Configurations

• 3X3 MIMO • (3 Tx antennas, 3 Rx antennas).

• 4X4 MIMO • (4 Tx antennas, 4 Rx antennas)

MIMO transmission, which goes by the acronym, uses a single communication channel to transmit an equal amount of data through a number of antennas and signals. To develop the signal strength and status of an RF link, this type of antenna variety makes use of numerous antennas. At the time of delivery, the data is split into several data streams, which are then recombined by various MIMO radios set up with identical antennas on the receiving surface. The receiver’s purpose is to reflect all signals’ brief time gaps between parties, any additional noise or interference, and even dropped communications. The MIMO communications include more data into data transmission that conventional elementary resonator structures (SISO: Single In, Single Out) cannot provide [4]. As a result, MIMO systems have numerous advantages over traditional SISO systems. Changing the overall throughput will result in better conditions and more video or other data being transmitted via the system. Figure 2.1 illustrates the SISO system, while Fig. 2.2 displays a different example of a 4 × 4 MIMO mode.

2.2

5G MIMO Technology

73

Fig. 2.1 Illustration of SISO mode, where elementary antenna is practiced on every surface of the RF connection [4]

Fig. 2.2 Illustration of a 4 × 4 MIMO mode, where the strength and bandwidth of the link connection are changed by communication between four antennas on the experienced radio and four antennas on the broadcast receiver [4]

2.2.2

Applications of MIMO

2.2.2.1

MIMO in LTE Superior

MIMO technology can be applied in LTE and LTE advanced wireless systems for enhancing the antenna system performance. With the initiation of MIMO technology, signal changes due to multipath have been significantly decreased. MIMO technology uses multipath to maximize the device by receiving dismissed signals from unreachable transmission lines [5].

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Flexible, Transparent, and Wideband 4-Port MIMO Antenna

MIMO in Wireless LAN

One of the traditional practices of MIMO technology now is broadcast LAN. Radio routers with multiple antennas have grown popular nowadays. The data rate can be increased or repeated several times with the efficient use of MIMO technology in radio routers and portable media. To have an effective system, both sending and holding things must be cooperative [5].

2.2.2.3

5G and IoT

5G and the IoT need a vast data rate. MIMO technology with beamforming is one of the critical communication technologies for supercharged 5G systems and IoT. The communication tower will be provided with multiple antennas. It will find a unique user at a particular place and transfer to working multiple antennas together [5].

2.3 2.3.1

Printed Transparent 5G MIMO Antenna Design Introduction

The 5G policy is viewed as an effective key across the well-known 5G operational band due to the augmented demand for mobile intelligence operations to combat some undesirable phenomena including considerable latency, multipath fading, and low data-rate communication [6, 7]. The available research on MIMO antenna systems frequently describes a well-known antenna diversity system to boost connection security, channel space, or spectrum power under 5G conditions. Despite the advantages of a MIMO system that have already been mentioned, researchers are still having trouble developing a small multi-antenna system for portable media that has improved performance in terms of low envelope correlation coefficient (ECC) and mutual coupling within the antenna components [8]. However, despite their ease of fabrication and surface-level construction features, planar antenna arrays have transmission line losses that can be avoided with good modeling [9, 10]. [9] describes a constructed array with nine components that has a high gain of about 15 dBi. With a 400 mm2 compact size, the antenna has a simple geometry; however, the provided bandwidth is inadequate. A simple 33 planar MIMO patch array antenna is demonstrated in related research [10] resonating at 28 GHz. It boasts a 1.7 GHz bandwidth and a significant peak gain of 15.6 dB. To combat the issues with multipath and fading effects, 5G and sub-6 GHz applications need a wide bandwidth and high gain. As nanotechnology advances swiftly, transparent conductive films (TCFs) are used to create antennas rather than traditionally controlled materials like copper. In order to make new antennas, TCF materials including metallic nanostructures, transparent conducting oxides, conductive polymers, and nanocarbon materials

2.3

Printed Transparent 5G MIMO Antenna Design

75

like graphene and carbon nanotubes are used. Transparent antennas were initially proposed as a way to put solar-powered antennas onto constrained spaceship surfaces [11]. The last few decades have seen research towards creating increasingly intricate and accurate nano-patterned transparent antennas as nanomaterial technology technologies have advanced. There are numerous non-transparent MIMO antennas presented in the literature that uses various structures and decoupling techniques for achieving the required isolation [12–19]. It is very challenging to design a transparent structure using isolation enhancement methods like inclusion of vias; uncoupling structures as same may affect the transparency while increasing the fabrication complexity. A few non-flexible transparent MIMO structures using 2-element [20, 21] or 4-element [22] are proposed by the researchers. Antennas in [23–25] are flexible and transparent where [24] achieves better performance due to NI metallic mesh which is highly conductive. The same is realized using the technique of electro-deposition. Such fabrication techniques need state-of-the-art facilities and so the flexible antenna proposed in [25] that uses commercially obtainable AgHT (oxide sheet with combination of silver and tin) sheet is preferred that has achieved satisfactory MIMO performance with 4-element flexible structure. Transparent antennas are suitable for 5G applications as more antennas can be mounted anywhere for increasing the signal strength and data speed owing to the property of blending into architectural fittings like glazed artwork, overhead fixtures, or windows.

2.3.2

A Review of Transparent MIMO-Printed Antennas for 5G Applications

2.3.2.1

Graphene

One of the replicable isotopes is graphene, where carbon atoms cluster to form a two-dimensional (2D) sphere. Hexagonal structure is formed by each carbon atom. The carbon atoms that are found in graphene thin films are responsible for chemical vapor deposition (CVD), epitaxial growth, and decreasing graphene oxide [26, 27]. Graphene is a semi metal or zero-gap semiconductor from an electrical perspective, and its combined conductivity nature enables the creation of plasmonic techniques at THz frequencies [28, 29 and 30]. Here a graphene-based antenna operating at THz frequency while handling the perfect conductivity features of graphene is proposed.

2.3.2.2

Carbon Nanotubes

Since their invention in 1991 [31], carbon nanotubes (CNTs) have been studied for a variety of electrical and biological properties and functions. Different types of CNTs are shown below:

76

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Flexible, Transparent, and Wideband 4-Port MIMO Antenna

Single-wall carbon nanotubes (SWCNT)

Carbon Nanotubes

Multi-wall carbon nanotubes (MWCNT)

Double-wall carbon nanotubes (DWCNT)

Reference [17] examines 2.4 GHz MWCNT and onion-like carbon (OLC) dipole antennas for IEEE 802.11 b/g applications. The specimen, which is rectangular and measures 3.5 mm, shows conductivity for MWCNT and OLC films of 9.0 S/cm and 1.5 S/cm, respectively. A borosilicate libation having 1mm thickness and 4.6 relative permittivity is used to link the dipole antenna (Fig 2.3).

2.3.2.3

Silver Tin Oxide (AgHT)

A compact 4-port wideband MIMO antenna having complete optically transparency is projected in [22] as shown in Fig. 2.4. The MIMO antenna has a low profile of 24 × 20 mm2. Antenna uses AgHT-8 for patch and ground, whereas Plexiglas material forms the substrate. The antenna material properties are listed in Table 2.1. Both the materials are transparent in nature which helps the MIMO antenna in achieving the optical transparency as illustrated in Fig. 2.6. Dual-band operation (24.10–27.18 GHz, 33–44.13 GHz) is achieved that covers applications in mm-wave 5G band. The compact transparent MIMO antenna accomplishes isolation > 16 dB, with acceptable diversity performance (ECC < 0.1, DG > 9, TARC < -15 dB, and MEG ratio = 1 dB).

2.3.2.4

Copper Nanowire

A meandering dual-band transparent monopole antenna is proposed in [23] having a conductive micro metal mesh coating. The tested antenna has overall measurements of 40 mm by 40 mm. According to Fig. 2.5a, the micro metal mesh conductive layer has optical transmittance value of about 75% and film stability of 0.05 W/sq. In Fig. 2.5b, S21 illustrates how conventional coupling between two antennas has a negligible effect on S21 below -15 dB at the operational frequency. Additionally, it enables maximum gains of 0.74 dBi at 2.44 GHz and 2.30 dBi at 5.5 GHz, which are suggestively more significant than those of other compounds like TCOs. It also permits high efficiencies of 43% and 46%.

2.4

Transparent 4-Element 5G MIMO Antenna Case Study

77

Fig. 2.3 (a) Proposed OLC antenna; (b) reflection coefficient of OLC, CNT, and copper antennas [31]. (With permission from AIP)

2.4

Transparent 4-Element 5G MIMO Antenna Case Study

This section describes a case study of a transparent 4-port flexible MIMO antenna [25].

2.4.1

Antenna Design

The single element geometrical model (part of 4-port MIMO structure) simulated using FEM-based ANSYS HFSS software of a transparent resonator (Fig. 2.5). The resonator geometry comprises of an upward-facing C-shaped conducting structure attached to a microstrip feedline housing a tilted stub connected to a solid centrally

78

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Flexible, Transparent, and Wideband 4-Port MIMO Antenna

Fig. 2.4 (a) Fabricated antenna geometry (b) Simulated S-parameters and (c) Measured S-parameters of the antenna [22]

2.4 Transparent 4-Element 5G MIMO Antenna Case Study

79

Table 2.1 Material properties AgHT-8 (patch/ground) Impedance of sheet Conductivity of sheet Thickness Transparency

8 Ω/Sq 2824 S/m 0.177 mm 80–82%

Plexiglas (substrate) εr tan δ Thickness Transparency

2.3 0.0003 1.48 mm 89%

located circle on the top with an inverted L-shaped stub connected to a partial ground at the back. The antenna uses two materials where the conductive patch/ground is realized using transparent flexible conductive oxide (AgHT-4) while the substrate uses transparent flexible Melinex. Table 2.2 illustrates the material properties. The 4-port structure realization needs careful consideration of maintaining the isolation with the common ground profile. The modified MIMO geometrical structure with grounds united together is shown in Fig. 2.6. The patch elements are arranged in a symmetrically mirrored fashion, while on the ground, a centrally located conductive oxide having a rectangular shape connecting all the 4 ground planes is added to realize a connected ground assembly. To understand the flexible operation, bending of antenna along both the axis (X and Y) is carried out, respectively. Selection of bending angle as 300° is selected after testing the maximum bending capability of the antenna structure. The bending investigation helps in understanding the RF performance of antennas along conformal surfaces. The 4-port bent antennas along X- and Y-axis are illustrated in Fig. 2.7.

2.4.2

Numerical Computation and Parametric Studies

The single antenna geometry is carefully chosen after analyzing the reflection coefficient curves by performing parametric variation as shown in Fig. 2.8. Figure 2.8a illustrates the variation of radius of the inner circle (R3). Significant increases in the |S11| levels are observed between 2.9 and 3.5 GHz as the radius rises; however, the bandwidth of the antenna is not much affected as the frequency increases. Value of 2.63 mm as inner-circle radius gives decent bandwidth as well as reflection coefficient results. While carrying out the outer circle radius variation, it is identified that broader impedance bandwidth (IBW) and the maximum |S11| are attained choosing 6 mm as the outer-circle radiu (Fig. 2.8b). The effect of substrate height (HS) variation is shown in Fig. 2.8c. The shift in the frequency is observed while negatively affecting the -10 dB impedance bandwidth at 0.508 mm and 0.762 mm, respectively. The value of Hs is selected as 0.635 mm because of the finest RF performance. The length of partial ground (GH) significantly affects the impedance bandwidth and |S11| throughout the complete operation band. To achieve the targeted band, a value of GH as 6 mm is selected (Fig. 2.8d).

80

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Flexible, Transparent, and Wideband 4-Port MIMO Antenna

Fig. 2.5 Antenna geometry views (a)Top (b) Back, and (c) Perspective (dimensions in mm)

2.4

Transparent 4-Element 5G MIMO Antenna Case Study

81

Table 2.2 Material properties AgHT-4 (patch/ground) Impedance of sheet Conductivity of sheet Thickness Transparency

4 Ω/Sq 1412 S/m 0.177 mm 70%

Melinex (substrate) εr tan δ Thickness Transparency

Fig. 2.6 Antenna geometry views (a) Top (b) Back, and (c) Layered

2.9 0.06 0.635 mm 81%

82

2

Flexible, Transparent, and Wideband 4-Port MIMO Antenna

Fig. 2.7 4-port antenna geometry by bending along (a) X-axis (b) Y-axis

The recommended stub position is proven to yield the finest outcomes in terms of bandwidth and reflection coefficient as depicted in Fig. 2.9a, which also evaluates the stub position. Figure 2.9b shows the ground plane geometry evolution steps. The necessary bandwidth and reflection coefficient are attained by adding vertical and horizontal lines over the partial ground plane. The optimized single and 4-port antennas are fabricated for the further experimental study where Fig. 2.10 illustrates the front and bending views of the antennas. Keysight VNA 9912A is used to measure the single and 4-port flexible antenna’s reflection coefficient.

2.4.3

Results and Discussion

Figure 2.11 displays the measured S11 parameter where the IBW of 73.2% is achieved spanning from 2.72 to 5.86 GHz which shows good harmony with the numerically computed results. To demonstrate the RF performance of the preferred common ground geometry of proposed transparent MIMO antenna, the current distribution over the surface is shown at 3 GHz by exciting ports 1–4 in Fig. 2.12. A tiny amount of current coupling is observed to the rest of the ground locations for a specific port excitation. Figure 2.13 shows the performance comparison for simulated and fabricated antenna in terms of S-parameters. The levels of isolation among elements greater than 20 dB are observed for frequency band greater than 3 GHz. The simulated S11 shows that -10 dB IBW ranges from 2.21 to 6 GHz, whereas in case of measured S11, a notch band is observed at frequency band amid 3.87–4.54 GHz, while rest of the operational band is below -10 dB. Figure 2.14 shows the simulated and measured two-dimensional patterns at the YZ (phi = 90) and XZ (phi = 0) planes, respectively. The co- and cross-pol pattern isolation level larger than 15 dB is seen. Figure 2.15a, b depicts the setup for measuring the far field patterns. The received power level at the receiving antenna

2.4

Transparent 4-Element 5G MIMO Antenna Case Study

83

Fig. 2.8 Parametric analysis of single antenna geometry (a) Parameter 1 (b) Parameter 2 (c) Parameter 2 (d) Parameter 4

84

2

Flexible, Transparent, and Wideband 4-Port MIMO Antenna

Fig. 2.8 (continued)

Fig. 2.9 Results by varying (a) stub position (b) ground plane

2.4

Transparent 4-Element 5G MIMO Antenna Case Study

85

Fig. 2.10 Fabricated prototype of (a) Single antenna (b) 4-port antenna (c) X-axis bending (d) Y-axis bending

side is used to conveniently determine a 3D radiation pattern for the preset RF input power when measuring the peak gain in an anechoic chamber. The total radiated power level is calculated by numerically integrating the radiated power level that the receiving antenna will measure. The calculated and observed S-parameters for the configuration with 300° bending are shown in Figs. 2.16a–c and 2.17a–c. Nearly equal impedance bandwidth is recorded from 2.16 to 4.01 GHz and from 4.35 to 5.2 GHz at both bands. The port

Fig. 2.11 |S11| of Single antenna

Fig. 2.12 The current distribution of 4-port antenna

2.4

Transparent 4-Element 5G MIMO Antenna Case Study

87

Fig. 2.13 S-parameters of 4-port antenna in unbend condition (a) Simulated (b) Measured

isolation is more than 15 dB that satisfies the isolation requirement for antennas in MIMO applications. The envelope correlation coefficient (ECC) of the transparent flexible antenna under normal and bent conditions are illustrated in Fig. 2.18. The value of ECC for all the conditions is well below 0.1 which meets the MIMO criteria however by bending along the Y-axis. The value of ECC > 0.1 between 3.81 and 4.24 GHz is observed which could be possible due to degraded isolation (S21) levels under bending conditions. Figure 2.19 shows that gain of the transparent flexible resonator is 0.53 dBi with 41% average radiation efficiency. Under X- and Y-axis bending, MIMO antenna achieves the maximum gain of 0.47 dBi, 1.07 dBi, and average efficiency of 34% and 33%, respectively (Fig. 2.20a, b).

88

2

330

5 -5 -15 -25 -25 -15

0

Flexible, Transparent, and Wideband 4-Port MIMO Antenna

300

270

90

240

120 210

180

150

E-plane Co-pol (Sim.) Co-pol (Mea.)

0

30

300

60

-5 5

330

30

60

270

90

240

120 210

180

150

H-plane Cross-pol (Sim.) Cross-pol (Mea.)

Fig. 2.14 2D radiation patterns at 3 GHz

Lower gain and efficiency are observed as the antenna is made up of conductive oxide AgHT-4 that has a sheet impedance of 4 Ω/Sq. Owing to low conductivity and flexible nature and higher tanδ value of melinex substrate leads to the low efficiency of the antenna. Additional factors include the connector losses. The values attained are still satisfactory considering the transparent, flexible, and lower conductivity material resonators in the literature. A thorough comparison of the performance of a few prominent MIMO antenna designs from the literature is shown in Table 2.3. Due to the various geometrical structures used in the design of these antennas, the type of radiation pattern is also included in the table. The suggested method achieves a respectable performance while guaranteeing a connected ground plane for practical application, as can be shown from Table 2.3 [34]. The proposed antenna has applications in the following: 1. Employing several antennas for enhanced channel capacity within a certain frequency range is the main benefit of employing MIMO antennas. The 4-element antenna structure can produce enough channel capacity compared to other single element patch antennas for smart indoor wireless devices like repeaters and routers working in the sub-6 GHz 5G range because the bands of interest include the 2.21–6 GHz band.

2.4

Transparent 4-Element 5G MIMO Antenna Case Study

89

Fig. 2.15 Setup for measuring far-field patterns (a) Block diagram (b) AUT inside anechoic chamber

2. A transparent thin flexible antenna is used in applications where available surface area is crucial. The solar cells are intended to occupy the majority of space on cube satellites, or satellites with a compact form factor. With such tiny antennas

90

2

Flexible, Transparent, and Wideband 4-Port MIMO Antenna

Fig. 2.16 4-port antenna characteristics by bending laterally (X-axis) (a) Reflection coefficient (b) Simulated (c) Measured

2.4 Transparent 4-Element 5G MIMO Antenna Case Study

91

Fig 2.17 4-port antenna characteristics by bending along sideways (Y-axis) (a) Reflection coefficient (b) Simulated (c) Measured

92

2

Flexible, Transparent, and Wideband 4-Port MIMO Antenna

Fig. 2.18 ECC of 4-port antenna (a) normal (b) bending along X-axis (c) bending along Y-axis

2.4

Transparent 4-Element 5G MIMO Antenna Case Study

93

Fig. 2.19 Gain of 4-port antenna in unbend condition

covering the solar panels’ outside, flexibility is increased without compromising the ability to capture energy. The deployment costs are further increased by the cube satellite’s weight, which can be greatly decreased by using thin film antennas [40, 41]. A spread of these adaptable antennas connected to cube-sats can aid in boosting the total gain. 3. By integrating thin film antennas, designers can reduce the amount of space needed for solar panels inside the chassis while minimizing transmission losses on the backside of solar panels. Leasing the available surface area to the telecom communication firms might reduce the cost of installing solar power systems for both commercial and residential buildings. AgHT-4, which is doped with titanium oxide and aluminum to boost the material’s overall conductivity and aid in reaching a larger gain, is suggested as a transparent antenna for solar panels [42]. The antenna is non-conforming, though, and intricate fabrication methods are required to dope other metals with AgHT-4. The suggested antenna can be a great alternative because it is versatile and simple to make.

2.4.4

Conclusion

A different ground plane connection option for achieving a planar monopole flexible transparent MIMO antenna is proposed. For the antenna to operate effectively overall under the severe manufacturing constraints of a transparent antenna construction, the design of these connecting lines is crucial. The final design exhibits an

94

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Flexible, Transparent, and Wideband 4-Port MIMO Antenna

Fig. 2.20 4-port antenna by bending along (a) X-axis (b) Y-axis

impedance bandwidth (IBW) of 3.79 GHz and isolation level >15 dB. Given the flexible structure and high sheet impedance value of 4 Ω/sq, the maximum gain is 0.53 dBi with the minimum efficiency being 41%, which is satisfactory. The suggested flexible transparent MIMO antenna has a flexible structure, no co-site placement concerns, and perfect transparency, which together with the reduction of visual clutter make it a practical option for IoT devices employing the sub-6 GHz 5G and WLAN frequency band.

4.27 mm

2 mm

126 × 70 (2 element)

38.1 × 38.1 (2 element) 180 × 180 (4 element)

[34]

[35]

1.52 mm

0.125 mm

22 × 31 (2 element)

[33]

[36]

Thickness 1.124 mm

AT 40 × 40 (2 element)

References [32]

20% (2.4–2.5 GHz) 0.7–1.01 2.6–3.18 5.3–6.06 6.7–6.94

9.13% (5.8 GHz center)

3.43–10.1

IBW 2.4–2.48, 5.15–5.8

Very low Very low

70

Very low

Very low

Sheet Impedance (Ω/Sq) 0.05

27

72

RE (%) 43% (2.44 GHz) 46% (5.5 GHz) 63

~10 (2.6 GHz band)

No No

12

15

I (dB) >15

Table 2.3 Comparison of proposed transparent structure with wideband 4-element MIMO antennas from literature

D and B

O

D

B

RP O

Yes

Yes

Yes

Yes

CG Yes

Yes

Yes

Yes

Yes

Flexible Yes

(continued)

Conductive silver nanoparticles ink Kapton (Substrate) Conductive copper foil flexible multilayered polydimethylsiloxane (PDMS) substrate Conductive copper Felt substrate Thick flexible Rogers RO3003

Material MMMC (micro-metal mesh conductive) film and glued on a glass substrate

2.4 Transparent 4-Element 5G MIMO Antenna Case Study 95

0.2 mm

0.625 mm

60 × 60 (4 element)

66 × 45 (4 element)

Prop ~41%

4

0.0025

0.00028

–

85%

Sheet Impedance (Ω/Sq) Very low

RE (%) –

>15

>20

>20

I (dB) >40

18 dB. Particularly, the differences can be the result of inaccurate fabrication. Figure 3.11a displays the measurement setup for the patterns. Line of sight is maintained between horn and AUT. The absorbers inside the chamber absorb the electromagnetic radiation that is reflected. The antenna is rotated along its azimuth, elevation, and polarization axes by a position controller that is connected to the AUT. The VNA feeds the horn antenna, which causes it to begin radiating. On the other end, an AUT that is likewise linked to a VNA operating in S21 mode (the marker must be on the desired frequency) receives electromagnetic radiations. The AUT then fetch the results and feeds them to PC software that has been installed to map the far field patterns. The co-/cross-pol patterns are depicted in Fig. 3.12. Figure 3.13 displays the antenna’s gain together with radiation efficiency. While efficiency more than 89% is seen across the proposed band, a slightly lower measured gain ranging from 3.4 to 6.8 dBi is observed as shown in Fig. 3.14b. The AUT placed within the chamber for gain and pattern monitoring is illustrated in Fig. 3.11. Figure 3.15 depicts the co- and cross-polarized radiation patterns at three frequencies (7, 10, and 15 GHz) across the H and E planes after stimulating port 1 of radiator and terminating rest of the ports with matching impedance. A nearly omnidirectional patterns are observed with 15 dB difference amid co-/cross-pol patterns.

114

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UWB Flexible Antenna

Fig. 3.9 Reflection coefficient of MIMO antenna

3.4.3

MIMO Diversity Analysis

The MIMO diversity is verified in terms of ECC, with values calculated using Eq. (3.4). ECC provides information about the quality of channel, with a lower ECC indicating improved inter-element isolation. The minimum threshold of ECC is 0.1, which is determined with the help of far- field characteristics [23, 24]. The ECC value of 0.02 is achieved for proposed configuration. The S-parameters can be used to compute the MIMO system’s TARC. The expression in Eq. (3.3) is used to compute the TARC. Figure 3.15 displays TARC curves with various phase pairings. The TARC is substantially < -10 dB, which satisfies the performance requirements for MIMO.

3.4

Flexible 4-Element UWB MIMO Antenna Case Study

115

Fig. 3.10 Simulated (a) and experimental (b) S-parameters of MIMO radiator

The formulae below were used to compute the CCL findings given in Fig. 3.16. It can be seen that the numbers are significantly lower than the 0.5 bits/s/Hz. C loss = - log 2 det ψ R

ψR =

ρ11

ρ12

ρ13

ρ14

ρ21

ρ22

ρ23

ρ24

ρ31 ρ41

ρ32 ρ42

ρ33 ρ43

ρ34 ρ44

4

ρii = 1 n=1

Sin Sni

ð3:6Þ

ð3:7Þ

ð3:8Þ

116

3

UWB Flexible Antenna

Shielded Anechoic Chamber 2D/3D Radiation Patterns Plotted Using Software

Horn Antenna (800 MHz to 18 GHz) Tx

MLMO Antenna Under Test Far Field Measurement

Rx

Absorbers

Control PC Ethernet Cable Vector Network Analyzer

Ultra low loss (ULL) phase stable RF Cable

Out Port

(a)

(b) Fig. 3.11 Setup for radiation pattern measurement

Position Controller

In ULL phase stable RF To PC Cable Port

3.4

Flexible 4-Element UWB MIMO Antenna Case Study

Fig. 3.12 Radiation patterns of MIMO antenna

117

118

3

UWB Flexible Antenna

Fig. 3.13 Gain and efficiency of MIMO antenna

0.14

ECC12

ECC13

ECC14

0.12

ECC23

ECC24

ECC34

ECC

0.1 0.08 0.06 0.04 0.02 0 2

4

6

8

10

12

14

16

18

20

Frequency (GHz)

Fig. 3.14 ECC of MIMO antenna

4

ρij = n=1

Sin Snj

ð3:9Þ

For i, j = 1, 2, 3 or 4. Using the Shannon formula in (number), the MIMO system’s channel capacity is determined.

Flexible 4-Element UWB MIMO Antenna Case Study

Total Active Reflection Coefficient (dB)

3.4

119

5

Port 1 and 2

Port 1 and 3

Port 1 and 4

0

Port 2 and 3

Port 2 and 4

Port 3 and 4

–5 –10 –15 –20 –25 –30 –35 –40 –45 –50 2

4

6

8

10

12

14

16

18

20

Frequency (GHz)

Fig. 3.15 TARC of MIMO antenna

Channel Capacity (bits/S/Hz)

24 23

22.70 Bits/S/Hz (Maximum limit for 4x4 MIMO)

22

Channel Capacity of 4x4 MIMO)

21 20 19 18 17 Calculated

16

Ideal

15 3

5

7

13 9 11 Frequency (GHz)

15

17

Fig. 3.16 CCL of MIMO antenna

C = k log 2 det ½I  þ η where: H: channel matrix. H*: Hermitian Transpose matrix. SNR: the mean SNR. K: rank of the matrix HH*. I: identity matrix.

SNR ½H ½H   k

ð3:10Þ

120

3

UWB Flexible Antenna

When determining the channel capacity, the channel model should be chosen first. Ray tracing models or corresponding statistical models are frequently utilized. The channel capacity is then determined using the correlation matrix approach. According to simulations of efficiencies (η) and an average of 80,000 Rayleigh fading realizations with SNR of 20 dB under independently and identically distributed propagation conditions [23, 24], the calculated capacity of the channel for proposed array ranges from 21 to 22 bps/S/Hz. When all channels are uncorrelated for maximum channel capacity, the fading matrix [H][H*] is transformed into an identity matrix. The ergodic channel capacity spanning the operating bandwidth of (3–17 GHz) is extremely close to the value of the ideal channel capacity.

3.4.4

Bending Analysis

The suggested MIMO performance is also evaluated by changing the electrical length while it is bent. The transmission and scattering characteristics with ECC are plotted by stimulating Port 1 to perform the antenna evaluation for flexible applications. Figure 3.17a, b shows the bending situations for simulated and realworld cases. The antenna bent along both the axes is depicted in Fig. 3.17c, d. After determining the radiators maximum bendable angle, the bent antenna is illustrated in

Fig. 3.17 Simulated bending prototype of MIMO antenna

3.4

Flexible 4-Element UWB MIMO Antenna Case Study

121

Fig. 3.18a, c. Along both the axis, antenna can be bent at 120°. In order to put the antenna in an anechoic chamber with the predetermined bending angle, a Styrofoam (εr = 1.03) is employed. The observed scattering and transmission curves are shown in Fig. 3.19a, b, and the measurements were made using a VNA. Due of the sparseness of the data, simulated outcomes are not shown. By bending the antenna along both the axes, respectively, an almost equal IBW of 126.81% and 123.36% is observed. For bending situations, a noticeable enhancement in the isolation is detected between parallel and vertically positioned elements; the overall isolation between elements is >15 dB. The effect of bending along both axes is seen to have very little impact on the vertical isolation. The ECC curve shown in Fig. 3.20a is substantially

Fig. 3.18 Fabricated bending prototype of MIMO antenna

122

3 0

S11

S33

S22

S44

S12

UWB Flexible Antenna

S13

S14

S Parameters (dB)

–10 S14 (Vertical Isolation)

–20 –30 –40

S12 (Horizontal Isolation)

S13 –50 (Diagonal Isolation) –60 2

4

6

8

10 12 14 Frequency (GHz)

16

18

S13

S14

20

(a)

S Parameters (dB)

0

S11

S33

S22

S44

S12

–10

S14 (Vertical Isolation)

–20 –30 –40 –50 –60 –70

S12 (Horizontal Isolation)

S13 (Diagonal Isolation) 2

4

6

8

14 10 12 Frequency (GHz)

16

18

20

(b) Fig. 3.19 S-parameters simulated (a) and measured (b) of bent MIMO antenna

0.1 from 2.6 and 4 GHz. As seen in Fig. 3.20b, the ECC < 0.1 is successfully attained for most of the band.

3.4.5

Time Domain Analysis

Time-domain properties of the antenna are investigated where S21 and group delay are plotted. Two identical single antennas are positioned 20 cm apart and facing each other in the time-domain analysis arrangement depicted in Fig. 3.21. The Tx antenna sends data, while the Rx antenna receives it.

3.4

Flexible 4-Element UWB MIMO Antenna Case Study

123

0.25 0.2

ECC12

ECC13

ECC14

ECC23

ECC24

ECC34

ECC

0.15

0.1

0.05

0 2

4

6

8

14 10 12 Frequency (GHz)

16

18

20

(a) 0.5 0.45

ECC12

ECC13

ECC14

ECC23

ECC24

ECC34

0.4 0.35 ECC

0.3 0.25 0.2 0.15 0.1 0.05 0 2

4

6

8

10 12 14 Frequency (GHz)

16

18

20

(b) Fig. 3.20 ECC simulated (a) and measured (b) of bent MIMO antenna

For creating a far-field atmosphere, 20 cm distance that is 2.5λ at 3.89 GHz is chosen. The antennas are excited using an Eq. (3.11) fifth order Gaussian pulse: 15t 10t 3 15t 5 þp -p xð t Þ = A - p 2π σ 7 2π σ 9 2π σ 11 where: t: time. A: amplitude. σ: indicates Gaussian pulse, respectively.

 x exp -

t2 2σ 2

ð3:11Þ

124

Fig. 3.21 Analysis in time domain (a) setup (b) group delay

3

UWB Flexible Antenna

3.4

Flexible 4-Element UWB MIMO Antenna Case Study

125

As shown in Fig. 3.21a, the forward transmission coefficient (S21) is below 25 dB throughout the whole proposed band, while the setup exhibits a steady group delay value in the region of 1 ns. The antenna is suitable for UWB applications where lower transmission coefficient, linear transmission characteristics, and constant group delay are achieved over the intended frequency band, as shown by time domain behavior analysis. The suggested antenna’s innovative contribution is as follows: 1. Flexible antennas are easy to integrate and save space on tiny circuit boards where space is at a premium. They can be positioned around the curves of a device using flexible materials, providing an integration flexibility unmatched by conventional antenna systems. 2. The use of flexible FR-4 substrate is made possible by the choice of the CPW feeding method, which yields fewer losses. Additionally, the CPW feed enables simple SMA connector interface for flexible radiators. The adaptable MIMO structure with four elements achieves an associated ground profile. 3. The mutual coupling is often lower when the ground planes are independent. However, common reference leads to enhanced coupling that lowers isolation and degrades the diversity performance. The 4-port structure offers good isolation. The common reference is guaranteed because none of the slots touch one another. The networked structure achieves isolation of greater than 15 dB. The proposed flexible MIMO antenna comparison is carried out with other flexible MIMO antennas in Table 3.2. The novel contribution of the proposed antenna is as mentioned as follows: 1. Where space is extremely limited on a compact circuit board, flexible antennas offer easy integration while saving space. Using a layer of flexible materials, they can be placed around the contours of a device, offering a degree of integration flexibility unrivalled by other antenna solutions. 2. Flexible structure, wide bandwidth, isolation levels above 15 dB, shared ground plane, satisfactory MIMO diversity characteristics with high gain, and radiation efficiency point out the proposed flexible MIMO antenna to be a commercially feasible solution for the smart devices useful in UWB, X, and Ku band applications. 3. The feeding method in the form of CPW fed is chosen as it delivers lower losses and thus enabling the use of flexible FR-4 substrate. Moreover, the CPW fed allows the easy interfacing of SMA connector in the flexible profile. 4. The flexible 4-element MIMO structure achieves a connected ground profile. Usually with separate ground planes in multi-port MIMO devices, the mutual coupling is less; however, when we go for common reference, the mutual coupling between the ports increases which leads to lower isolation and thus affecting the MIMO diversity performance. The conducting plane of the CPW fed is connected by implementing a plus-shaped slotted structure separated by a slotted diamond at the center realized in the 4-port structure. None of the slots touches each other thus ensuring the common reference. The interconnected structure achieves isolation >15 dB.

No. of elements 2 2

2 2

2

2 4

4

4

4

Size (mm3) 2.35λ × 1.30λ × 0.079λ 1.73λ × 2.43λ × 0.009λ

0.304λ × 0.304λ × 0.016λ 0.468λ × 0.230λ × 0.0008λ

0.24λ × 0.452λ × 0.0008λ

0.212λ × 0.299λ × 0.0012λ 0.55λ × 0.485λ × 0.007λ

0.42λ × 0.42λ × 0.0035λ

0.170λ × 0.762λ × 0.002λ

0.726λ × 0.881λ × 0.0025λ

References [29] [30]

[31] [32]

[33]

[34] [35]

[36]

[37]

Proposed

Yes

3.89–17.09

2.4–2.485 2.38–2.55 3.37–3.60 4.92–5.37 2.4–11.3 Notch Band: 3.1–4.3 2.9–12 1.5–3.8 4.1–6.1 0.7–1.01 2.6–3.18 5.3–6.06 6.7–6.94 3.2–14

Frequency (GHz) 5.8 24

5.87

0.05 4.4 6.9 5 5.6

3 4

5.35

1.6 3.79

Gain (dBi) 11.5 6

Duroid 5880 FR-4

RT Duroid

Kapton Jeans

LCP

Substrate PDMS Rogers 6002 Felt LCP

>15

>22

>10