Preface

Microwave bands range from 300 MHz to 300 GHz of the electromagnetic spectrum. Vast applications can be found in the signals that propagate in this band; they are employed in the fields of communication, networking, astronomy, and biomedical engineering, to name a few. This book provides a detailed elucidation on the physics of microwave signals, methods of modeling these signals, applications of these signals in various fields, and the underlying principles of some of the latest microwave devices.

Chapter 1 presents a comprehensive comparison between plane wave and orbital angular momentum (OAM) wave propagation using a patch antenna as a radiator at 2.45 GHz. This comparison allows the appreciation of the fundamental properties of the OAM wave when compared to its plane-wave counterpart. Two abbreviated terms are used for simplified comparison and discussion: PWPA for plane-wave patch antenna and OWPA for OAM wave patch antenna. PWPA refers to the planar patch antenna that produces plane waves in the far-field, whereas patch antennas that deliver OAM waves in the far-field are termed OWPA. All physical quantities for wave propagation such as electric field, magnetic field, wave impedance, wave vector, velocity, pitch, and propagation constant are theoretically studied for OAM waves and compared with plane waves. First, the OAM wave generation is studied through a widely used uniform circular antenna array (UCAA). Then, PWPAs and OWPAs are designed and verified through simulations and measurements. The OWPA is designed with characteristic mode analysis (CMA) based on a lossy substrate to excite a twisting wave at a determined patch location. With this in mind, a comparative investigation of PWPA and OWPA is conducted for different physical parameters. The cylindrical near-field scan clearly shows a helical wave motion for the OWPA, whereas a normal plane wave motion for the PWPA. Furthermore, the comparison of plane wave and OAM wave propagations is demonstrated using the combination of a pair of transmitting and receiving antennas. It is observed that the overall signal from the OWPA can be received with two PWPAs at an angle as OWPA has a dispersive beam. Moreover, the receiving antenna with a large aperture and plane wave horn antenna (PWHA) in the line of sight (LOS) range can also be used to receive the overall signal from OWPA. The received signals in the PWPA–PWPA, OWPA–OWPA, OWPA–PWPA–PWPA, and OWPA–PWHA transmitting and receiving pairs are thoroughly compared and studied. Measured and simulated results for transmission are -30 dB for 0 dB input signal in the OWPA–PWPA–PWPA and OWPA–PWHA cases, which are reasonably justified within the sensitivity/dynamic range of short-distance communication and radar sensing receivers.

Dielectric and conducting liquids with varying electromagnetic properties can offer novel alternatives for building tunable microwave passive components and antennas. Injecting these fluidics in or around microwave substrates alters their overall electrical characteristics, enabling circuit reconfigurability. Alternatively, changing the shapes and dimensions of conductors by using liquid metals can achieve similar

reconfigurability. Chapter 2 provides an overview of different liquids and their electromagnetic properties as well as discusses the principles behind the reconfigurability of the electrical characteristics of typical guiding structures based on mode shape variation in the presence of fluids. It also describes an N-bit programmable impedance tuner in 3D LTCC technology based on these principles.

Chapter 3 describes multiscale auxiliary sources mainly used to solve complex electromagnetic problems, especially those that insert localized elements into circuits. Several equivalent relations (field-circuit) are established to simplify and make more accurate electromagnetic calculations by changing some characteristics of the localized elements known by their field representation as "voltage-current" representation and vice versa. Various examples are illustrated to show the effects of auxiliary sources in planar circuits containing localized elements (dipoles, diodes, transistors) in the millimeter and terahertz bands. An example of a graphene or gold dipole is demonstrated in this approach. Another typical example of a diode integrated with a radiating structure is also simulated.

Chapter 4 generalizes a recently proposed method of moment (MoM)-based approach to waveguide port excitation (WPE) problems on arbitrary conducting and composite geometries. This approach combines the canonical aperture coupling approach with the EFIE-PMCHWT formulation for composite structures. Each WPE problem in this approach is divided into equivalent sub-problems for internal and external regions solved using the MoM. Internal WPE problems are solved using waveguide modal expansion in the port plane, while external problems are solved using the equivalence principle to reduce these problems to the systems of algebraic equations for unknown electric and magnetic currents. The developed approach is validated on radiation and coupling problems for coaxial ports by comparing simulated results with those obtained by other approaches and measurements. An excellent agreement between the simulated and measured results is demonstrated. Finally, this approach is applied to practical EMC problems for microwave antennas fed by coaxial ports.

Chapter 5 deals with the modeling of microwave heating of a water drop. A drop model is reduced to its electric dipoles, masses, and charges. These components are then constructed using COMSOL Multiphysics and MATLAB software. The considered model proposes a microscopic point of view on microwave heating, which transforms electrical energy into heat.

Chapter 6 covers filter structures using defected ground structures (DGS). It discusses the limitations of electromagnetic band gap (EBG) structures and the development of DGS from the EBG structure.

DGS is an area of increasing interest in EBG technology. The chapter also discusses the features and physics of the well-known dumbbell DGS structures. It presents sew investigations on the choice of geometrical shapes for the DGS structure as an element for the proposed filters. All the proposed DGS structures used to implement different types of filters (lowpass, bandpass, and bandstop) are validated.

Tunable filters enable dynamic spectrum access for wireless systems, and tunable bandpass filters with constant bandwidth (BW) are most favorable for practical

applications. Chapter 7 investigates the synthesis and realization techniques for tunable filters using the coupling matrix with variable entries. It presents the synthesis method and the matrix extraction procedures for the constant-BW bandpass filter and gives typical numerical examples. The chapter also discusses the relationship between the theoretical matrix and the physical circuits and then presents a planar tunable filter design to verify this relationship. Furthermore, the general approach to designing the constant-BW filters using the element variable coupling matrix is concluded. The planar circuit and 3D structure realizations are offered to demonstrate the synthesis design approach practically.

Chapter 8 presents novel compact metamaterial-based bandpass filters with improved stopbands. These filters have a compact size and a wider upper stopband resulting from the bandstop resonator characteristics. It evaluates several filters' design methodologies and performances using broadside-coupled complementary splitring resonators (BC-CSRR) and edge-coupled complementary split-ring resonators (EC-CSRR) techniques. A comprehensive method to evaluate negative permittivity and permeability for designing the proposed metamaterial structure is also described.

With the remarkable progress in using the Internet of Things (IoT) and 5G, there is a demand for higher performance such as miniaturization, broadband/multi-band, low loss, and high integration for several microwave circuits. Chapter 9 treats microwave power dividers/combiners used in amplifiers, mixers, phase shifters, antenna feeding circuits, and so on. Here, the treated circuits are inductance-capacitance LC-ladder circuits and an absorption resistor. The chapter shows that multi-band (dual-band and tri-band) and broadband can be achieved by changing the number of stages of the LC-ladder circuit. In addition, the effectiveness of this design method is demonstrated by electromagnetic simulations and prototype experiments.

Chapter 10 describes recent research activities from the perspective of microwave technologies in medicine and biology. It brings new ideas about the possibilities of using microwaves in thermotherapy, particularly for hyperthermia in cancer treatment. It discusses the development of new hyperthermia applicators (e.g., on technologies like metamaterials, evanescent modes in waveguides and other types of transmission structures, etc.) and the use of microwaves in medical diagnostics. For example, microwave differential tomography, ultra-wideband UWB radar, and microwave radiometers will soon play an important role. Finally, the chapter highlights the experimental equipment necessary for research on the biological effects of electromagnetic fields.

Wireless power transmission (WPT) can provide an alternative for wireless power in implantable medical devices (IMDs), such as implantable pacemakers, optogenetic devices, and bio-impedance sensors. Chapter 11 comprehensively reviews recent WPT studies for emerging biomedical applications. It also outlines the specific key components for those applications and examines operating principles and system design.

Microwave imaging has long been proposed as an effective means for biomedical applications, such as breast cancer detection and therapy monitoring. Numerous numerical simulations have been presented demonstrating feasibility, but the translation to actual physical and clinical implementations is lacking. A team at Dartmouth

has taken somewhat counterintuitive but fundamentals-based approaches to the problem, primarily addressing the confounding multipath signal corruption problem and exploiting core concepts from the parameter estimation community. The team has configured a unique system design that is a synergism of hardware and software worlds. Chapter 12 shares the team's approaches in the context of competing strategies and suggests reasons for why these techniques work, especially in 2D. Finally, the chapter presents data from actual neoadjuvant chemotherapy exams that confirm that the technique can image the tumor and visualize its progression during treatment.

The exponential growth of publications on additive manufacturing (AM) technologies in the microwave field proves the increasing interest of research institutions and industries in these techniques. Some advantages of AM with respect to conventional machining are weight reduction, design flexibility, and integration of different functionalities (electromagnetic, thermal, and structural) in a single part. Chapter 13 begins with an overview of the AM processes, focusing on material properties and post-processing. Then, it presents an extensive survey on microwave-guided components fabricated by AM processes published in the literature. It also discusses the most employed AM technologies for manufacturing RF waveguide components.

One of the greatest challenges for the European Union is the "Circular Economy Package." To achieve this goal, materials at the end of their life cycle must be recycled using a sustainable process. Chapter 14 examines the process of employing microwaves to recycle plastics to preserve their energy and chemical content. It illustrates microwave-assisted pyrolysis (MAP), which is an industrial novelty in plastic recycling. As a thermochemical treatment, pyrolysis represents a significant opportunity as long as it leads to the recovery of both the energy and chemical content of mixed, contaminated, or deteriorated plastics.

> **Ahmed Kishk** Department of Electrical and Computer Engineering, Concordia University, Montreal, Canada

> > **Kim Ho Yeap** Department of Electronic Engineering, Universiti Tunku Abdul Rahman, Malaysia

Section 1

Physics of Electromagnetics

Section 1
