Investigations on Reconfigurable Planar Antennas and Devices

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Investigations on Reconfigurable Planar Antennas and Devices

Biswajit Dwivedy

Department of Electronics and Communication Engineering

National Institute of Technology Rourkela

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Antennas and Devices

Dissertation submitted in partial fulfillment of the requirements of the degree of

Doctor of Philosophy

in

Electronics and Communication Engineering

by

Biswajit Dwivedy

(Roll Number: 513EC1042)

based on research carried out under the supervision of Prof. Santanu Kumar Behera

and

Prof. Debasis Mishra (VSSUT Burla)

Sep, 2019

Department of Electronics and Communication Engineering

National Institute of Technology Rourkela

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National Institute of Technology Rourkela

Sep 13, 2019

Certificate of Examination

Roll Number: 513EC1042 Name: Biswajit Dwivedy

Title of Dissertation: Investigations on Reconfigurable Planar Antennas and Devices We the below signed, after checking the dissertation mentioned above and the official record book (s) of the student, hereby state our approval of the dissertation submitted in partial fulfillment of the requirements of the degree of Doctor of Philosophy in Electronics and Communication EngineeringatNational Institute of Technology Rourkela. We are satisfied with the volume, quality, correctness, and originality of the work.

Debasis Mishra (VSSUT Burla) Santanu Kumar Behera

Co-Supervisor Principal Supervisor

Subrata Maiti Prasanna Kumar Sahu

Member, DSC Member, DSC

Susmita Das

Member, DSC External Examiner

Kamalakanta Mahapatra Tarun Kumar Dan

Chairperson, DSC Head of the Department

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Prof. Santanu Kumar Behera Associate Professor

Sep 13, 2019

Supervisor’s Certificate

This is to certify that the work presented in the dissertation entitled Investigations on Reconfigurable Planar Antennas and Devices submitted by Biswajit Dwivedy, Roll Number 513EC1042, is a record of original research carried out by him under my supervision and guidance in partial fulfillment of the requirements of the degree ofDoctor of Philosophy inElectronics and Communication Engineering. Neither this dissertation nor any part of it has been submitted earlier for any degree or diploma to any institute or university in India or abroad.

Santanu Kumar Behera

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Dedication

This thesis is dedicated to to

my beloved parents & sister.

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I,Biswajit Dwivedy, Roll Number513EC1042hereby declare that this dissertation entitled Investigations on Reconfigurable Planar Antennas and Devicespresents my original work carried out as a doctoral student of NIT Rourkela and, to the best of my knowledge, contains no material previously published or written by another person, nor any material presented by me for the award of any degree or diploma of NIT Rourkela or any other institution. Any contribution made to this research by others, with whom I have worked at NIT Rourkela or elsewhere, is explicitly acknowledged in the dissertation. Works of other authors cited in this dissertation have been duly acknowledged under the sections “Reference” or “Bibliography”.

I have also submitted my original research records to the scrutiny committee for evaluation of my dissertation.

I am fully aware that in case of any non-compliance detected in future, the Senate of NIT Rourkela may withdraw the degree awarded to me on the basis of the present dissertation.

Sep 13, 2019

NIT Rourkela Biswajit Dwivedy

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Acknowledgment

There are no suitable words to express my immense gratefulness and respect for my thesis supervisor, Dr. Santanu Kumar Behera. He has always given me the freedom to pursue various research works independently and shared knowledge through insightful discussions during the five year Ph.D. time. He has always motivated and helped me to face different challenges patiently during this long journey. He has been very supportive of me in both academic learning and other non-academic activities which helped me in pursuing my research smoothly. It was a great pleasure to work with him for this long time and certainly, it will be in future too.

I would like to thank my co-supervisor Dr. Debasis Mishra of VSSUT Burla for his kindness, support and constant encouragement throughout the course of research work.

I am highly obliged to my Doctoral Scrutiny Committee (DSC) members Prof. K. K.

Mahapatra, Prof. S. Maiti, Prof. S. Das and Prof. P. K. Sahu for their valuable suggestions regarding the progress of my research and improvement of the Thesis. I would like to thank Prof. T. K. Dan, Head of Department, ECE for allowing me to purchase different devices from the departmental fund for several times. My sincere thanks to the faculties of Microwave group Prof. Sudipta Maity and Prof. Prasun Chongder for their insightful discussions about the research work during the last year. My heartiest thanks to Prof. S.

Meher, Prof. S. K. Das, Prof. A. Swain and Prof. U. C. Pati for sharing their experience and giving me moral support for all-round development along with research.

I thank all the non-teaching staffs and technical staffs of Dept. of ECE especially Pramod babu, Iswar babu, Mr. Kujur and Mr. B. Das for their invaluable support.

I am profoundly grateful to Prof. A. Biswas, Director, NIT Rourkela for his support in the development of Microwave & Antenna Design lab by providing some essential equipment in the last two years. I would like to acknowledge TEQIP-II, NIT Rourkela for providing financial assistance for my visit to San Diego State University (SDSU), USA, for research work.

I am highly thankful to Dr. Runa Kumari for the kind consent to use her polarization reconfigurable dielectric resonator (DRA) antenna design as a part of this thesis work (Contents are used in Chapter-5 of this Thesis).

I would like to thank Suvendu Sir of VSSUT Burla for inspiring me to be an independent and qualitative researcher during my course of M.Tech. His constant encouragement has always motivated and given me inner strength to work hard with full dedication in many

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Department of Electronic Sciences, Berhampur University for his scientific advice and insightful suggestions. His deep knowledge of Microstrip Antennas and devices has been extremely beneficial and helped me as a vital resource for getting my science questions answered at all-time of research work.

I am very much grateful to Prof. S. K. Sharma, Dept. of ECE, SDSU, USA, for providing me an opportunity to visit his lab and work with his research team even though for a short period of time. His in-depth knowledge of antennas and direct guidance during my stay really helped me in getting a sound understanding of the subject as well as gave a proper direction to my research work. I heartily thank Mr. Amiya Kumar Samantaray of Phoenix Robotix, NIT-Rourkela for his invaluable support and helping hand during the fabrication of various antennas. I am deeply grateful to Mr. Vivek Kumar Singh (Scientist-D) for providing CATR measurement facility along with wonderful stays in SAMEER Kolkata campus. I am also very much thankful to the professionals of CATR facility SAMEER Kolkata campus for their effort during antenna measurement.

I want to acknowledge my lab-mate Mr. Tanmaya Kumar Das for his true friendship and support which really mean a lot to me. I cannot forget my friends who shared many good moments of my life, cheered me on during my hard times and celebrated each accomplishment: Dheeren Ku Mahapatra, Bibhu Sir, Sangram Ku. Samal, Kumar Samal and Satyajit Das. I feel happy to acknowledge Mr. Soumya Swarup Nandi, Shakti Somadutt Rout, Priyabrata Sethy, K. D. Sa, Monalisha Nayak and Debashish Rout for their well-wishes, motivations and sharing good thoughts over the phone. I would like to thank the entire Ph.D. badminton team for supporting me in maintaining good health and mental status.

I deeply thank my parents, Mr. Bijay Kumar Dwivedy and Mrs. Sandhyarani Dwivedy for their unconditional love, trust, all-time encouragement, endless patience and blessings.

Thanks to my beloved sister Sigma Dwivedy (Kuka) for her sweet words of inspiration. She is a great friend of mine and I can not imagine my life without her.

Last but not the least, I must acknowledge and thank The Almighty‘Jhankada Basini Maa Sharala’ for blessing, protecting and navigating me towards the success throughout this period. —

Sep 13, 2019 NIT Rourkela

Biswajit Dwivedy Roll Number: 513EC1042

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Abstract

In this growing technological trend, there always has been a requirement of multifunctional wireless devices which can provide all-around connectivity with assorted entertaining features. To incorporate different features in these devices, multiple radios are integrated into a single wireless platform which increases the density and complexity of hardware following Moore’slaw. Along with multiband wireless support, the requirement of enhanced signal quality at diverse climatic conditions for stability of connectivity and high data-rate adjoin more complications to the radio frequency (RF) end terminals. Therefore, the antenna reconfigurability approach has become the principal attraction for all researchers which is considered as one of the best ways to overcome the above-discussed problems. In other words, it can be said that reconfigurability has become salient and most desired attribute for state-of-the-art RF systems used in wireless and satellite communications, imaging and sensing.

This dissertation is aimed at building new design methodologies about multifunctional antennas/microwave devices to overcome the above-discussed challenges as well as enhance overall microwave system performance by proposing new structures and setups.

In this research work, the design of a frequency reconfigurable square shaped microstrip antenna with an ability to provide both right and left hand circularly polarized radiation (RHCP and LHCP) is investigated. The frequency tuning and radiation characteristics of the antenna are evaluated by a mathematical modelling, full-wave analysis as well as its physical measurement. The antenna generates very consistent circularly polarized radiation patterns with a wide 3 dB beamwidth of150^◦ ( for both LHCP & RHCP) at all the tunable bands within 1.98 GHz-2.44 GHz. The antenna shows a maximum realized gain of 2.2 dBc at 2.43 GHz and has a very wide CP bandwidth (21%) which is the same as its total tunable range.

A frequency, circular polarization as well as pattern reconfigurable microstrip triangular patch array is introduced in this research work which can withstand both spectrum and climatic or orientation based signal quality concerns. From physical measurement, it is found that the operating frequency of the antenna can be altered between 1.97 GHz to 2.54 GHz (25.3% tunable bandwidth). Investigation confirms that the antenna produces both types of CP radiation with minimum axial ratio of 0.98 dB and beamwidth >24^◦ at all tunable frequency bands using the proposed feed network. In pattern reconfiguring mode, antenna pattern can also be rotated at120^◦ angle in the complete azimuth plane at different

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antenna (DRA) array with four types of polarization variability. Full-wave analysis of the antenna was carried out using the simulated and measured responses of the phase shifting cum switching network. From the analysis, it is revealed that the antenna shows a wide axial ratio bandwidth of 39% and 19.4% when operated in LHCP and RHCP mode respectively.

It is also verified that the antenna can be fairly operated in horizontal and vertical linear polarization mode within the whole bandwidth of 39%. During the CP modes, the antenna shows the maximum realized gain value of 9.95 dBc whereas, in LP modes, it has a maximum gain value of 9.75 dB.

At the end of this research work, an active elements based wideband frequency reconfigurable rat-race hybrid is presented which has many attractive characteristics like wide operational bandwidth of 68.4%, smaller size (30% of conventional design), insertion loss of 4.8 dB, maximum amplitude imbalance of 0.4 dB, maximum phase imbalance of5^◦ and ease of design. The device performances are evaluated from mathematical modeling, full-wave analysis, as well as physical measurement and it is found that the device is a good candidate to challenge today’s multifunctional requirements.

Keywords: reconfigurable; circular polarization; axial ratio; wideband; rat-race;

linear polarization.

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1.1 Block diagram of a typical wireless communication system. . . . 3 1.2 Pictorial representation of different types of reconfigurable antennas using Venn diagram. . 5 1.3 Schematic representations of a tunable matching network and a tunable antenna [2]. . . . 6 2.1 Classification tree of reconfiguration techniques [5]. . . . 17 2.2 (a) Layout of the reconfigurable mini-nested patch antenna using MEMS (b) Fabricated

prototype [24]. . . . 18 2.3 Measured reflection coefficients of the antenna for different ON/OFF states of MEMS

switch [24]. . . . 19 2.4 Frequency reconfigurable multiband antenna [25], (a) Layout (b) Front side of the fabricated

model (c) Back side of the fabricated prototype. . . . 20 2.5 PIN diode and its lumped element equivalents in reverse and forward biased modes. . . . 20 2.6 Surface current distribution of the antenna [25], (a) D1-OFF & D2-OFF (3.2 GHz) (b)

D1-OFF & D2-ON (2.4 GHz) (c) D1-ON & D2-OFF (5.2 GHz) (d) D1-ON & D2-ON (4.4 GHz). . . . 20 2.7 Simulated and measured reflection coefficient responses [25], (a) ( D1-OFF & D2-OFF)

measured: solid line & simulated: dotted/dashed line; (D1-OFF & D2-ON) measured:

dotted line & simulated dashed line (b) (D1-ON & D2-OFF) measured solid line &

simulated: dotted/dashed line; (D1-ON & D2-ON) measured: dotted line & simulated:

dashed line. . . . 21 2.8 Design and performance of the ‘Filtenna’ designed using varactor diodes [26], (a)

Fabricated prototype (b) Tuning of the reflection coefficient of the ‘filtenna’ for different voltage levels. . . . 22 2.9 Comparison of the properties of different switching techniques [27]. . . . 23 3.1 Layout of the proposed antenna for simultaneous frequency and CP reconfigurability. . . 36 3.2 Analytical model of the proposed antenna using lumped elements. . . . 37 3.3 Frequency reconfigurability evaluated from the analytical model using MATLAB (a)

Reflection coefficients (b) Imaginary part of antenna input impedance (c) Real part of antenna input impedance. . . . 38 3.4 Reflection coefficients evaluated from the full-wave simulation. . . . 38

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frequencies evaluated using MATLAB (b) Surface current distribution (vector plot) at 2.16 GHz showing LHCP (using HFSS) (c) Surface current distribution (vector plot) at 2.35 GHz

showing RHCP (using HFSS). . . . 40

3.6 Schematic of the wideband phase shift cum switching network and the fabricated model. . 41

3.7 General coupled line phase shifter and its even-odd circuit equivalent. . . . 42

3.8 Measured and simulated responses of the wideband phase shifter cum switching network (a) S-Parameters for state-1 of DPDT switch (LHCP) (b) S-Parameters for state-2 of DPDT switch (RHCP), Phase response of the feed network for (c) LHCP (d) RHCP. . . . 44

3.9 Measurement of the antenna (a) Fabricated model of the antenna and radiation patterns measurement in the anechoic chamber (b) Measured reflection coefficients (S11) and (S22) of the antenna. . . . 45

3.10 Simulated and measured CP radiation patterns (a) LHCP at 2.43 GHz (b) RHCP at 2.43 GHz (c) LHCP at 2.16 GHz (d) RHCP at 2.16 GHz. . . . 46

3.11 Variation of Axial ratio versusθ(AR beamwidth) (a) 2.43 GHz (b) 2.16 GHz. . . . 47

3.12 Realized Gain versus frequency responses of the antenna. . . . 47

3.13 Variation of axial ratio (AR) versus frequency responses of the antenna.. . . 48

4.1 Layout of the proposed triangular patch array and the fabricated prototype. . . . 52

4.2 Loci of 50Ωimpedance point for an equilateral triangular patch [153]. . . . 52

4.3 (a) Surface current distribution of the triangular patch at 2.05 GHz (b) Reflection coefficient of the triangular patch antenna (c) 3D radiation pattern at 2.05 GHz. . . . 54

4.4 (a) H-field distribution of the triangular patch (b) Perfect magnetic boundary symmetry of the patch (c) Analytical model of the equivalent patch using lumped elements. . . . 55

4.5 Surface current distribution of the triangular patch (a) 1.94 GHz (4.27 pF) (b) 2.52 GHz (1.15 pF) (c) 3.14 GHz (Very low capacitance=0.05 pF.) . . . 55

4.6 Simulated reflection co-efficients of the antenna for different values of capacitances obtained from full-wave analysis using HFSS. . . . 56

4.7 Linearly polarized wave vectors constituting a CP wave. . . . 57

4.8 Surface current distribution of the antenna at 2.2 GHz (2.24 pF) for different time instants showing LHCP (a)t= 0(b)t=T/3(c)t= 2T/3. . . . 58

4.9 Normalized simulated radiation patterns at 2.2 GHz (2.24 pF) when different ports of the antenna are excited (a) Port-1 (b) Port-2 (c) Port-3. . . . 59

4.10 Designed feed network to provide necessary amplitude and phase difference when the antenna is operated in receiving mode (a) Schematic (b) Fabricated prototype. . . . 61

4.11 Simulated and measured performances of the feed network for operating the antenna in LHCP mode (a) S-parameters (b) Phase response. . . . 62

4.12 Complete antenna with feed circuit and its measurement in CATR anechoic chamber. . . . 63

4.13 Measured reflection coefficients of the antenna for different values of capacitances. . . . 63

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4.15 Measured and simulated axial ratio (AR) beamwidth at (a) 2.23 GHz (b) 2.54 GHz. . . . 65 4.16 Measured and simulated axial ratio (AR) versus frequency for various values of varactor

capacitance (LHCP radiation). . . . 65 4.17 Measured and simulated realized gain versus frequency for different set of varactor

capacitances (LHCP radiation). . . . 66 4.18 Simulated and measured radiation patterns (polar plot) at 2.54 GHz when individual port

of the antenna is excited by the feed network (a) Port-1 (ϕ= 0^◦) (b) Port-2 (ϕ= 120^◦) (c) Port-3 (ϕ= 240^◦). . . . 67 4.19 Measured and simulated realized gain versus frequency for various values of varactor

capacitances when the antenna is operated in pattern reconfiguring mode. . . . 68 5.1 The proposed polarization reconfigurable DR antenna array (a) Single element DRA (b)

Layout of the DRA array (c) Top view of the fabricated model (d) Bottom side of the prototype. 72 5.2 Reflection coefficients of the proposed antenna array (a) Simulated (b) Measured. . . . . 73 5.3 Arrangement of a sequentially fed 2×2 array and their phase of excitation. . . . 74 5.4 E-field vector distribution of the DRA array at 3.3 GHz (*RHCP) for (a)t= 0(b)t=T/4

(c)t=T/2(d)t= 3T/4. . . . 75 5.5 Circuit configuration of the proposed multiple phase shifting feed network (Type-1). . . . 76 5.6 Simulated S-parameters and phase response of the wideband balun. . . . 77 5.7 Photograph of the fabricated multiple phase sifting feed network (Type-1). . . . 78 5.8 Simulated and measured S-parameters of the wideband multiple phase shifting feed network

(Type-1) (a) Reflection coefficients (b) Transmission coefficients. . . . 78 5.9 Simulated and measured phase responses of the quad phase shifting feed network (Type-1). 79 5.10 Simulated and measured isolations of the quad phase shifting feed network (Type-1). . . . 79 5.11 The proposed phase shifter and switching network (Type-2) for polarization

reconfigurability (a) Schematic (b) Fabricated model. . . . 80 5.12 S-parameter responses of the proposed feed network (Type-2) for operating the antenna in

LHCP mode (a) Simulated reflection coefficients (b) Measured reflection coefficients (c) Phase differences. . . . 82 5.13 Radiation patterns of the antenna while operated in left hand circular polarized (LHCP)

mode, at (a) 2.7 GHz (b) 3.0 GHz (c) 3.3 GHz (d) 3.7 GHz. . . . 84 5.14 Radiation patterns of the antenna while operated in right hand circular polarized (RHCP)

mode, at (a) 2.7 GHz (b) 3.0 GHz (c) 3.3 GHz (d) 3.7 GHz. . . . 85 5.15 (a) Gain versus frequency response of the antenna (b) Axial ratio (AR) versus frequency

response of the antenna. . . . 86 5.16 Radiation patterns of the antenna while operated in vertical linear polarized (VLP) mode,

at (a) 2.7 GHz (b) 3.0 GHz (c) 3.3 GHz (d) 3.7 GHz. . . . 88

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at (a) 2.7 GHz (b) 3.0 GHz (c) 3.3 GHz (d) 3.7 GHz. . . . 89 5.18 Gain versus frequency response of the antenna when operated in both linear polarized modes. 90 6.1 Circuit schematic of miniaturized frequency reconfigurable rat-race using low-pass

equivalent of theλ_g/4transmission line. . . . 94 6.2 Variation of operating frequency versus capacitance for different values of transformed

impedance. . . . 95 6.3 Lumped element model of the frequency reconfigurable rat-race using variable capacitors

instead of static capacitors. . . . 96 6.4 Circuit model of the frequency reconfigurable rat-race using varactor and its line of symmetry. 97 6.5 Even and odd mode circuit equivalents. . . . 97 6.6 Variation of insertion loss versus transformed impedance for different values of series

resistance of the varactor. . . . 99 6.7 Simulated S-parameters and phase responses of the circuit model of the proposed rat-race. 100 6.8 Designed wide-range frequency tunable rat-race (a) Layout of the proposed device (b)

Fabricated prototype. . . . 101 6.9 Simulated and measured S-parameters at different reverse biased voltages (a) 0.5 V (1.77pF)

(b) 7.0 V (0.48pF) (c) 19.0 V (0.3pF).. . . 102 6.10 Simulated and measured phase difference (a)0^◦between port-1 and 3 when port-2 is taken

as input. (b)180^◦between port-2 and 4 when port-1 is taken as input. . . . 102

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3.1 Dimensional details of the antenna . . . 36

3.2 Comparison of the proposed antenna with some previously reported multiple parameters reconfigurable antennas . . . 49

4.1 Parameters and values of the proposed triangular patch . . . 52

4.2 Different switching conditions and corresponding antenna reconfiguring modes of operation 60 4.3 Measured performance of the feed network at different frequencies for CP and pattern reconfiguring modes . . . 62

4.4 Simulated and measured gain data at different frequencies for LHCP radiation . . . 66

4.5 Performance comparison with previously reported works . . . 68

5.1 Parameters and values of the proposed polarization reconfigurable DRA array . . . 73

5.2 Summary of switching conditions for different reconfiguring modes of the antenna . . . . 83

5.3 Performance comparison of the proposed antenna with previously reported works . . . . 90

6.1 List of all design parameters with their dimensional details . . . 100

6.2 Performance comparison of the proposed device with previously reported models . . . . 103

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List of Abbreviations

ADC Analog to Digital Converter

AR Axial Ratio

AR-BW Axial Ratio Bandwidth

BFN Beam Forming Network

BST Barium-Strontium-Titanate

BW Bandwidth

CATR Compact Antenna Test Range

CMOS Complementary Metal-Oxide-Semiconductor

CP Circular Polarization

CPW Coplanar Waveguide

CST Computer Simulation Technology DAC Digital to Analog Converter

dB Decibel

DC Direct Current

DCS Digital Cellular System

DIP Dual in-line Package

DPDT Double Pole Double Throw DRA Dielectric Resonator Antenna

DTV Direct Television

DVB-H Digital Video Broadcasting-Handheld

EM Electromagnetic

FBR Front-Back Ratio

FBW Fractional Bandwidth

FEM Finite Element Method

GPRS General Packet Radio Services GPS Global Position System

GSM Global System for Mobile Communication HFSS High Frequency Structure Simulator

IF Intermediate Frequency

IL Insertion Loss

LAN Local Area Network

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LP Linear Polarization

LTCC Low-Temperature Co-fired Ceramics

LTE Long Term Evolution

MEMS Micro-Electro-Mechanical Systems MIMO Multiple Input Multiple Output

MMIC Monolithic Microwave Integrated Circuit

MT 50ΩMatch Termination

PCB Printed Circuit Board

PCS Personal Communications Service PIFA Planar Inverted-F Antenna

PIN Positive Intrinsic Negative RADAR Radio Detection and Ranging

RF Radio Frequency

RFID Radio-Frequency Identification RHCP Right Hand Circular Polarization

RL Return Loss

RLC Resistor (R), Inductor (L), Capacitor (C) SAR Synthetic Aperture Radar

SDR Software Defined Radio

SIW Substrate Integrated Waveguide

SLL Side-Lobe Level

SMA SubMiniature version-A

SMD Surface-mount Devices

SPDT Single Pole Double Throw

TE Transverse Electric

TM Transverse Magnetic

TR Tunable Range

UMTS Universal Mobile Telecommunications System

VNA Vector Network Analyzer

VSWR Voltage Standing Wave Ratio

WiMax Worldwide Interoperability for Microwave Access WLAN Wireless Local Area Network

XPD Cross Polarization Discrimination

µW Microwave

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List of Symbols

ε₀ Permittivity of free space ε_r Relative permittivity

λ Wavelength

λ₀ Free-space wavelength λ_g Guided wavelength tanδ Loss tangent

c Speed of light in free space γ Propagation constant Γ Reflection coefficient α Attenuation constant

β Phase constant

f_c Centre frequency ε_{ef f} Effective permittivity µ₀ Permeability of free space µ_r Relative permeability

η₀ Free space intrinsic impedance

ϕ Phase difference

ρ Impedance ratio

ω Angular frequency

k₀ Free space wave number Ω Ohm (Unit of resistance)

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CHAPTER 1 Overture

1.1 Background and Motivation

Radio transmission technique was developed before ten decades and the use of the term wirelesswas technically initiated by Marconi. In the last few decades, we have experienced a phenomenal growth in wireless communication sectors which seems, wireless technology as a contemporary invention used in modern communication systems. A block diagram of a common wireless communication system consisting of several functional blocks is shown in Fig. 1.1. As seen from the block diagram, the main functions of the baseband and IF sections are analog-to-digital (A/D) conversion or digital-to-analog (D/A) conversion and modulation/demodulation of the signals. During modulation, the baseband spectrum is transformed to a higher frequency band called intermediate frequency (IF). The purpose of this conversion is to optimize the bandwidth usage and boost the signal quality for better transmission. Before transmission through the channel, the frequency of the signal is again up converted to a higher range (RF/µW) for further processing. The RF/µW transmitter and receiver mainly consist of some vital microwave components like RF oscillator, mixer, power amplifier, low noise amplifier (LNA), filter and antenna etc. Combinedly, the RF transmitter and receiver are known as RF front end which operates in RF/µW frequency ranges and the antennas are responsible for converting the signals to electromagnetic (EM) waves at the desired frequencies.

Microwave components are most crucial and non-detachable parts of all types of modern wireless communication systems. Implementation of microwave systems can be found in innumerable applications like RADAR, satellite, mobile phones, point to point communication, wireless local area network (WLAN), remote sensing, energy generation, power systems, RFIDs, surveillance systems, medical imaging etc. Due to the massive impact of microwave engineering and technology in our daily life, it is worth justified saying that microwave systems are the backbone of modern wireless communication.

In recent days, the advancement of modern communication systems is oriented towards development of different versatile electronic gadgets supporting multiple radios, high data rates, uninterrupted connectivity and many advanced features. Along with this, requirements like low power consumption, circuit miniaturization, simplicity of design and low cost are adding additional complexity to the framework of RF terminals. Therefore, optimum efforts are expended by all researchers to overcome all these challenges by enhancing all traditional microwave components not only at the system level but also at the level of hardware design in various RF modules. To cope with various demands of end-users in this proliferating electronic market, our vision and research must be emphasized for innovation of new techniques to replace traditional mechanisms which will be no more capable of withstanding different communication challenges in the present and future situations.

Every microwave communication system consists of several passive devices (antennas, phase shifters, power dividers, filters, couplers and baluns) and active devices (switches,

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Figure 1.1: Block diagram of a typical wireless communication system.

amplifiers and mixers), each performing certain operations. It is not possible to completely develop new concepts of microwave components, rather inspired by the colossal potential of microwave technology, researchers are now focused towards development of flexible, multifaceted and smarter devices to ameliorate current situation. This leads to the development of various reconfigurable microwave devices/antennas to empower the communication system and increase the technical operability along with efficiency. As a whole, it can be stated that the concept of reconfigurable microwave components are meant as an alternative to a group of microwave devices/circuits performing hybrid functions and has become a focal point of exploration both in academia and industry. Over the last decade, several innovative approaches for the accomplishment of reconfigurability in microwave antennas (planar type) and devices have been demonstrated. Each proposed invention has its own uniqueness and limitation showing a new path for subsequent research work. This dissertation will focus on the design of some new tunable microwave components (planar antennas, and microwave devices) addressing the methods to overcome major challenges and shortcomings of the current reconfiguring systems.

1.2 Requirements of Tunable Antennas/Microwave Devices in Wireless Systems

In this growing technological trend, there always has been a requirement of multifunctional wireless devices with various advanced features. Therefore, the invention of different wireless devices to provide all-around connectivity with assorted entertaining features has

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become the key point for the researchers and scientists. These devices mainly include various types of mobile phones, laptops, tablets, Bluetooth devices, global positioning system (GPS) receivers, wireless routers, security sensors and different satellite communication devices. To incorporate different features in these devices, multiple radios are integrated into a single wireless platform which increases the density and complexity of hardware following Moore’s law. Along with multiband wireless support, the requirement of enhanced signal quality at diverse climatic conditions for stability of connectivity and high data-rate adjoin more complications to the RF end terminals. In this scenario, the development of compact, multifaceted microwave devices and electrically small antennas having increasing bandwidth with efficiency have become the most crucial challenges for the RF system engineers.

Nowadays, the reconfigurability approach has become the principal attraction for all researchers which is considered as one of the best ways to overcome the above-discussed problems. In other words, it can be said that reconfigurability has become salient and most desired attribute for state-of-the-art radio frequency (RF) systems used in wireless and satellite communications, imaging and sensing. The actual movement of this technique is to make the system extremely dynamic for adapting frequency tunability, software-defined and cognitive radios to handle expandable multi-standard services with optimized spectrum and power utilization. Practically, the application of this reconfigurability concept to RF systems (both antennas and microwave devices) can extremely reduce the hardware complexity and overall cost.

Antennas are recognized as reconfigurable if they are engineered to reversibly and intentionally alter their performance metrics [1]. The variation of performance metrics of these reconfigurable antennas is mainly based on their design methodologies, which are decided from application and system point of view. A similar type of interpretations is also applicable for reconfigurable microwave devices. To achieve multifunctionality, several trade-offs are taken into consideration while designing these dynamic electromagnetic systems. The main trade-offs include minimization of operational and design complexity, efficiency, the feasibility of practical implementation, net performance in the system-level achieved through the addition of reconfigurable capabilities. From the antenna point of view, reconfigurability can strengthen the system performance by technically congregating the potential of several antenna systems into a single platform to enable variable functionality.

These design requires integration of several active devices as well as non-antenna systems to the antenna system and compromise between various performance metrics to make it optimum dynamic. Therefore, tunable antennas can be considered as a subset of another class of antenna called active integrated antennas (AIAs), shown in Fig. 1.2 [2]. In AIAs, one or more active devices like transistors, oscillators, switches, mixers and multipliers are embedded within or connected to the antenna. Antennas can be designated as reconfigurable, if the active devices are integrated within the radiator itself or directly connected nearby but

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Figure 1.2: Pictorial representation of different types of reconfigurable antennas using Venn diagram.

not connected to external matching networks or tunable networks packaged in the transceiver module. This concept of antenna reconfigurability can be well explained by taking a simple example of frequency selectivity shown in Fig. 1.3 [2]. In the first section of Fig. 1.3, an external tunable matching network is used to filter the required signal band from the received broadband signal by the antenna. Alternatively, in the second part, a tunable antenna is directly used to effectively enhance the bandwidth performance and eliminate the requirement of high-level filtering circuit used in radio frequency (RF) front-end. Although, both these techniques result in frequency agility, yet reconfigurable antenna described later has certain advantages over the other one. Fixing of active components within the matching network are less complex, however, integrating them within the antenna lowers the insertion losses and makes the design more compact which are the most important requirements in many hand-held/mobile devices. Again integration of tunable antennas along with other tunable microwave devices will make the device more powerful, efficient and dynamic to handle various types of modern communication challenges.

1.3 Aim of the Thesis

Now-a-days, reconfigurable antennas/microwave devices are in high demand. Though numerous techniques already have been proposed to achieve various types of reconfigurability to date, yet some significant challenges still exist which must be addressed to fulfil modern day system requirements. So many major as well as recent contributions in this research area are discussed in the literature review section (Chapter 2) of this thesis which focuses on various features and lacunae in these works. This dissertation is aimed at building new design methodology about multifunctional antennas/microwave devices to overcome those discussed drawbacks as well as enhancing overall microwave

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Figure 1.3: Schematic representations of a tunable matching network and a tunable antenna [2].

system performance by proposing new structures and setups. Therefore, the key problems which have been deeply investigated in this research work and the proposed reconfigurable solutions are briefly explained as follows.

1. Now, there are adequate numbers of reconfigurable antennas (frequency or polarization or pattern) available which are currently serving various types of communication systems. But the potentiality of these antennas is not proven suffice to tackle current requirements of modern communication systems like multi-radio support, high data-rate, enhanced connectivity and signal quality at all climatic conditions etc. Therefore, in this thesis, design and analysis of two types of hybrid antennas are proposed which are capable of showing multiple types of reconfiguring states. One antenna is wideband frequency reconfigurable square microstrip antenna with the ability to produce both right-hand and left-hand circularly polarized (RHCP and LHCP) radiations. The antenna can be continuously tuned within a wide range of frequencies of 1.98 GHz to 2.44 GHz and simultaneously can provide any type of CP radiation at all these frequency bands. From the design point of view, the antenna is very simple, cost-effective due to use of very less number of active components and has a very stable wide-beam axial ratio (AR) response. The second proposed antenna is a frequency tunable microstrip triangular patch array which can be operated to generate both types of CP (RHCP & LHCP) radiation and also be used in pattern reconfiguring mode. The antenna shows consistent CP radiation patterns with wideband axial ratio responses at all the tunable frequencies. Additionally, the antenna radiation pattern can be rotated electronically when each element is excited in a linear polarized mode. From the meticulous analysis and practical verification, it is believed that the proposed hybrid antennas can withstand the above-discussed problems as well as can act as perfect alternatives for many types of radiating systems by performing diverse

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operations.

2. In the recent trend, dielectric resonator antennas (DRAs) are considered to be a more suitable candidate as compared to microstrip because of their inherent features like high radiation efficiency, less conductor loss and wider bandwidth etc. Therefore, DR antennas are chosen for the applications in which higher gain, wider bandwidth are the prime concern. In the current research work, a wideband polarization reconfigurable DRA array is proposed in which four types of polarizations (vertical linear, horizontal linear, RHCP and LHCP) can be altered depending upon different switching states of the wideband feed network. The main objective of designing this antenna is to achieve maximum realized gain with wide bandwidth circular/linear polarization responses as these two are quite critical and self-opposing requirements of any microwave systems.

3. One of the major advantages of the reconfigurable devices is its reduced footprint with variable functionalities. Much more effort has been already devoted by the researchers to make various conventional passive devices (branch line hybrid, Wilkinson’s power divider, filters etc.) to reconfigurable type. Yet, very limited approaches are available in the literature for the accomplishment of reconfigurable rat-race which has motivated for this research work of modelling and designing a wideband frequency tunable compact180^◦ coupler. The design of this wide frequency tuned rat-race is based on the transformation of the conventional quarter-wave transmission line to the active element (varactor diode) based equivalent circuit. The main objective of the proposed device is to achieve wideband frequency tunability, low insertion loss as well as compact size. Achieving all these characteristics in one device makes its design little complex and research work more challenging.

Looking at the various shortcomings discussed earlier, this thesis aims to provide multiple advanced solutions to overcome them to a large extent. Basically, in the current work, the main effort has been dedicated to the development of a new concept of hybrid planar antennas having multiple reconfiguring effectiveness. Also, we have attempted to design a reconfigurable microwave device (tunable rat-race) which is having diverse unique characteristics like wideband frequency tuning, size miniaturization, low-loss and low cost.

Moreover, it can be said that the objective of this thesis is to introduce the design of different types of reconfigurable planar antennas/microwave devices those can be easily realizable, directly useful for various devices as an alternative for conventional antennas/devices and combat various challenges of modern communication systems.

1.4 Problem Statement and Original Contribution

With the motivation and objectives mentioned in the above sections, some research problem statements selected for this doctoral research, are stated as follows:

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1. Investigation of a frequency and polarization reconfigurable microstrip antenna for the wireless devices operating in the S-band.

2. Design of a wideband frequency, polarization and pattern reconfigurable microstrip antenna for multi-featured wireless communication.

3. Investigation of a wideband polarization reconfigurable dielectric resonator antenna for RADAR applications.

4. Design and physical verification of a compact frequency tunable microwave device (rat-race hybrid).

1.4.1 Contributions in the Dissertation

All these above-listed research problems target to resolve various challenges discussed in the preceding sections and are focused on providing unique approaches for different types of characteristics nimble microwave component/antenna design. The techniques used for these problems, the associated challenges and the outcome presented as original contributions in this thesis are summarized as follows:

1. The main challenge in the design of wideband frequency tunable microstrip antenna with circular polarization (CP) reconfigurability is the implementation of two types of reconfiguring ability to a single antenna.

As the basic microstrip antennas are narrow band in nature, the first problem arises to increase its operating band range without changing its primary radiation characteristics. For this purpose, a square-shaped microstrip antenna embedded with varactor diodes is designed which can be easily operated within 1.98 GHz-2.44 GHz without altering its fundamental modes of radiation.

The next interest is to manipulate the antenna for both types of circularly polarized radiation (LHCP and RHCP) with wide beamwidth at all these altering frequency bands. In this research work, a wideband phase-shifting cum switching network was designed which was used to excite the two ports of the antenna to generate CP as well as alter its modes of radiation.

The performance of the proposed antenna was verified using analytical modelling, full-wave analysis followed by measurement of a prototype. Finally, the performance of the antenna was also compared with some recently reported designs which are presented at the end of theChapter 3of this thesis.

This work has been published in/reported to the following research articles.

X B. Dwivedyand S. K. Behera, “A Square Shaped Microstrip Antenna with Frequency and Circular Polarization Reconfigurability: An Approach,” (Provisionally Accepted inIEEE Antennas & Propagation Magazine.)

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X B. Dwivedyand S. K. Behera, “A Circular Polarization Alterable Square MPA with Wide Frequency Tuning,” In Proc. of IEEE Applied Electromagnetics Conference (AEMC-2017),Aurangabad, 2017, pp. 1-2.

2. The requirement of pattern reconfiguring ability along with frequency and polarization agility in a single antenna increases its design complexity. To accomplish three types of reconfigurability, a varactor based frequency tunable triangular patch array is proposed which is to be operated it in both linear and circularly polarized mode within a wideband of 1.97 GHz to 2.54 GHz.

Another concern is to excite each element of this array, which requires proper amplitude and phase difference along with suitable switching methodology for mode alteration. To overcome this problem, an external phase shiting network with necessary switching ability was designed and its effect on the antenna performance was evaluated. The measured performance of the antenna in different reconfiguring mode are also compared with some recently reported models which are presented at the end of theChapter 4of this thesis.

This work has been published in or reported to the following research articles.

X B. Dwivedy, S. K. Behera and V. K. Singh, “A Versatile Triangular Patch Array for Wideband Frequency Alteration with Concurrent Circular Polarization and Pattern Reconfigurability,”IEEE Transactions on Antennas & Propagation., vol. 67, no. 3, pp. 1640-1649, Mar 2019.

X B. Dwivedy and S. K. Behera, “Design Approach of A Wideband Frequency Tunable Triangular Patch Array with Concurrent Polarization Alteration,” In Proc.

of International Conference on Wireless Communications, Signal Processing and Networking,Chennai, 2017, pp. 1007-1012.

3. Another major concern of achieving wideband response with the gain enhancement of an antenna is also addressed in this thesis. InChapter 5, a cylindrical shaped dielectric resonator antenna array was investigated in which bandwidth enhancement up to 39% (2.7GHz-4.0GHz) of the DRA array was accomplished while least perturbing its fundamental mode radiation pattern.

Other challenges such as the arrangement of the radiating elements for gain enhancement and development of a reconfiguring mechanism to excite the antenna for polarization alteration were also addressed in this work. For achieving wideband polarization reconfigurability, a four-way equal power divider providing phase differences of0^◦,90^◦,180^◦,270^◦and necessary switching capability was also realized.

The design of this reconfiguring feed network itself is a dilemma which involves many crucial steps of designing a wideband power divider with different phase-shifting

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ability and embedding different active devices to electronically control its responses.

Amplitude and phase matching of the various port outputs to generate required polarization and managing insertion loss of the device for increasing overall efficiency of the antenna increase further difficulty.

Finally, the LHCP, RHCP and linear polarization performance (horizontal & vertical) of the antenna were evaluated by using the measured responses of the feed network and compared with some previously reported works.

X B. Dwivedy, S. K. Behera and D. Mishra, “Wideband Multiple Phase Shift Quad Feed Network for Polarization Reconfigurable Antennas,”Electromagnetics (Taylor Francis), vol. 37, no. 8, pp. 483-492, 2017.

4. Apart from tunable antennas, this work is also dedicated for the development of a tunable microwave device which surmounts various challenges like wideband tuning, low loss, compactness and cost minimization.

Size reduction of the devices is becoming more compelling due to the increase in functionality and higher physical size of the components at lower frequencies.

Managing size of the device at diverse operating bands, making its loss frequency independent and the responses of active devices increase the burdens of the designer.

Therefore, in this work, a wideband frequency tuned rat-race was proposed whose design is based on the transformation of the conventional quarter-wave transmission line to the active element (varactor diode) based equivalent circuit which effectively reduces the dimension of the device without affecting the performance.

Along with these technical issues, the tunable rat-race coupler was also designed using FR-4 substrate for cost-effectiveness which is also essential to cope with this rapidly changing electronic market.

Overall, the device has a wide tunable frequency band, acceptable insertion, less amplitude and phase deviation due to which it can be considered as a unique implementation. A comparative assessment of different parameters of the proposed model with some recently reported tunable devices is included at the end of Chapter 6.

X B. Dwivedyand S. K. Behera, “Modelling, analysis and testing of an active element based wide-band frequency tunable compact rat-race hybrid,”AEU-International Journal of Electronics and Communications, ELSEVIER., vol. 103, pp. 24-31, 2019.

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X B. Dwivedy, S. K. Behera and D.Mishra, “Design of a Frequency Agile Rat Race Coupler,”In Proc. of 5^th IEEE Applied Electromagnetics Conference (AEMC)-2015, Guwahati, 2015, pp. 1-2.

1.5 Dissertation Organization

Chapter 1 provides a short prologue to the research background and motivation. It also emphasizes on requirements of modern communication systems, goal and the contributions achieved in this thesis.

Chapter 2 provides a brief literature review on different state-of-the-art reconfigurable planar antennas and microwave devices used in communication sub-systems. Basically, this chapter discusses some important arts on frequency, polarization, pattern reconfigurable planar antennas (Microstrip and Dielectric Resonator). It also gives insight into different wideband phase shifters used as feed network for tunable antennas and various tunable microwave couplers. The advantages, lacunae and challenges in the previously reported works are clearly outlined in this section.

InChapter 3, design of a wideband (1.98 GHz-2.44 GHz) frequency tunable square patch antenna with an ability to provide both senses of circular polarization (RHCP and LHCP) is presented. Detail mathematical modelling and performance analysis of the antenna by physical measurement of radiation patterns in an anechoic chamber are neatly discussed.

This chapter also enlightens different unique characteristics of the antenna like wideband frequency tuning, wide-beam LHCP/RHCP radiations at all the frequencies and wideband axial ratio performance followed by a comparison with some recently reported works.

Chapter 4 introduces a frequency reconfigurable triangular patch array which can be simultaneously operated to produce both types of CP radiation and also can be used for pattern alteration at all the frequencies. Various important parameters like S-parameters, radiation patterns, axial ratio, gain of the antenna operating at different modes within 1.97 GHz to 2.54 GHz are analyzed. Design of an exclusive feed network meant to provide desired wideband phase response with necessary switching conditions is also included and its effect on the antenna performance is summarized. It has been demonstrated that the antenna can be simultaneously used for three types of reconfigurability like frequency, polarization and pattern to withstand both spectrum related issue and degradation of signal quality in diverse climatic conditions.

A wideband polarization reconfigurable cylindrical dielectric resonator antenna (DRA) array using sequential rotation excitation technique is presented in Chapter 5. Two types of wideband feed networks, each providing four-way equal power division and four phase differences (0^◦,90^◦,180^◦,270^◦) intended for sequential rotation purpose are discussed in this section. The performances of one feed network after implementation of various RF switches (SPDT, DPDT) for automatic state alteration of the antenna are also included. Finally,

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response (LHCP & RHCP) and linear polarization responses (horizontal & vertical) of the antenna using the feed network within a wide bandwidth of 39% (2.7GHz-4.0GHz).

InChapter 6, an active element based wideband frequency reconfigurable180^◦ hybrid is investigated. The chapter contains mathematical modelling for evaluation of tunable range and insertion loss of the device. In this section, full-wave analysis, as well as measurement responses of the device, are given for validation purpose. Several features of the proposed model like dimensional compactness, wideband tuning, low insertion loss, cost effectiveness are highlighted and also compared with other existing models.

Finally, theChapter 7concludes the thesis by summarizing the overall work and obtained results. It also describes the shortcomings in this work and the scope for future work.

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CHAPTER 2 Literature Review

Techniques . . . . 15 2.2.1 Electrically Reconfigurable Antennas . . . 16 2.2.2 Reconfigurable Antennas Based on RF-MEMS . . . 17 2.2.3 Reconfigurable Antennas Based on PIN Diodes . . . 18 2.2.4 Reconfigurable Antennas Based on Varactor Diodes . . . 21 2.2.5 Summary of Switching Methods for Reconfigurability . . . 22 2.3 Review and Analysis of Reconfigurable MPAs . . . . 23 2.3.1 Frequency Reconfigurable MPAs . . . 23 2.3.2 Polarization Reconfigurable MPAs . . . 24 2.3.3 Pattern Reconfigurable MPAs . . . 25 2.3.4 Microstrip Antennas with Multiple Reconfigurability . . . 26 2.4 Reconfigurable Dielectric Resonator Antennas . . . . 27 2.4.1 Frequency Reconfigurable DRAs . . . 28 2.4.2 Pattern Reconfigurable DRAs . . . 28 2.4.3 Polarization Reconfigurable DRAs . . . 29 2.5 Passive and Tunable Microwave Devices . . . . 30 2.5.1 Review of Passive Wideband Power Dividers and Phase Shifters . 30 2.5.2 Review and Analysis of Rat-Race Coupler . . . 31 2.5.3 Tunable Microwave Devices . . . 32 2.6 Summary . . . . 32

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2.1 Introduction

Reconfigurability of an antenna is the technique of deliberately altering its electromagnetic behaviours like resonant frequency, polarization, or radiation characteristics. These types of alterations are basically accomplished by redistributing antenna current and thus varying the electromagnetic field components as well as its effective aperture. Reconfigurable antennas can address various complex requirements of modern communication systems, by replacing multiple antennas, embedding multiple functionalities, geometry alteration in electric means and adapting to environmental changes. Therefore, reconfigurability has become an important and non-detachable feature of futuristic radio-frequency (RF) systems meant for mobile communication, satellite communication, sensing, imaging and many more. This methodology of making system nimble can significantly reduce the number of components thus hardware complexity and cost compared to conventional as well as today’s technology.

Intensive research on reconfigurable antennas have been carried out for the last two decades for diverse applications and most of them have implemented various types of switching mechanism to achieve reconfigurability. During operation, these antennas can be reconfigured remotely without reconstructing the antenna or the platform upon which they are mounted, once they are designed and fixed on a particular platform. The first patent on reconfigurable antennas emerged in 1983, where Schaubert introduced a microstrip antenna having both frequency agility and polarization diversity [3]. In 1999, to investigate the advancements and potential applications of various types of reconfigurable antennas, the Defense Advanced Research Projects Agency (DARPA, USA) organized a multi-university program among 12 well-known universities, research institutes, and companies in the United States named as Reconfigurable Aperture Program (RECAP) [4].

While designing reconfigurable antennas, several types of compromises between various parameters are made to achieve a specific type of functionality. But, in the primary stage of design, RF engineers have to address three critical questions.

1. Which type of reconfigurability (Whether frequency, radiation pattern, or polarization)?

2. In which method radiating elements of the antenna should be reconfigured to achieve desired reconfigurability?

3. Identification of the reconfiguration technique which will have minimized negative effects on the antenna impedance characteristics and patterns.

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2.2 Classification of Reconfigurable Antennas and Reconfiguring Techniques

Reconfigurable antennas can be broadly classified into four major categories.

1. The radiating element which has the ability to alter or hop its operating/notch frequency band by means of electronically varying its effective length is called frequency tunableantenna.

2. A radiating structure that is able to alter the orientation, shape or gain value of its radiation beam is calledpattern reconfigurableantenna.

3. The antenna that can change electric field orientation of its radiated wave (polarization) is called polarization reconfigurable antenna. In this case, the antenna can switch between various types of linear (horizontal/vertical, slant ±45^◦), left-hand circular polarization (LHCP) or right-hand circular polarization (RHCP) etc.

4. These antennas are the hybrid combination of different types from the previously discussed three categories. For example, the antenna can be both frequency and polarization reconfigurable, the frequency with pattern reconfigurable or simultaneously frequency, polarization and pattern reconfigurable etc.

The reconfigurability corresponding to each of the four categories can be achieved by revamping the antenna surface current distribution, implementation of feeding network with switching ability, altering antenna physical structure, or altering radiating edges of the antenna. It is very important to note that the change in one parameter can affect the other characteristics of the antenna. Therefore, special care and caution must be taken during the design process to achieve the required reconfigurability without affecting the other parameters. Some advantages of using reconfigurable antennas in wireless systems are mentioned below [5–7].

1. Minimization of overall system cost;

2. Size reduction/ Miniaturization/ Reducing volume requirement;

3. No requirement of front end filtering;

4. Sound isolation between various wireless standards;

5. Ease of integration;

6. Adaptability to different conditions and learn;

7. Operate as an array or single element.

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While reconfigurable antennas are considered to be the best possible candidate for RF front ends of modern wireless systems, there are definitely some disadvantages associated with it which can be related to different parameters as summarized below.

1. Increase in complexity of antenna structure and associated systems due to insertion of the biasing network for activation/deactivation of the switching elements and active devices;

2. Incorporation of active components increases power consumption and also raises the system cost;

3. Higher order harmonics and inter-modulation products come into the picture;

4. The requirement of rapid tuning and switching in the antenna radiation characteristics for proper reconfiguring operation of the system.

Reconfiguration techniques used to implement reconfigurable antennas can be classified in to four categories, as shown in Fig. 2.1 [5]. Antennas in which radio-frequency micro-electromechanical systems (RF-MEMS) [8–11], PIN diodes [12–14] and varactors [15, 16] are implemented to vary their surface current distributions are called electrically reconfigurable. Antennas, on which photoconductive switches are used as controlling elements are called optically reconfigurable antennas [17, 18]. Physically reconfigurable antennas are achieved by directly modifying the structure, position and orientation of the antenna [19]. Finally, reconfigurability can also be achieved by using ferrites and liquid crystals as antennas components [20, 21]. Despite the large range of reconfiguring techniques available to make the RF devices more powerful and dynamic, this thesis work discusses only electrically controlled reconfigurable microwave devices/antennas due to their massive popularity and advantages over other techniques.

2.2.1 Electrically Reconfigurable Antennas

The electrically reconfigurable antennas are based on the use of electronic switching components (RF-MEMS, PIN diodes, or varactors) to alter the surface current distribution and/or vary the antenna radiation characteristics by the change of effective radiating edges.

Integration of active switches into the antenna gives the flexibility to the designer of operating it in desired reconfiguring states. Basically, planar (microstrip/DRA) antennas are found suitable for implementation of these reconfiguring techniques due to their planar geometry and the distinct ground plane on which various types of active tunable components and their required control circuitry can be easily accommodated. Therefore, many researchers have been attracted to design planar reconfigurable antennas using active switching elements despite the issues associated with such reconfiguration techniques. The