Chapter 9 addresses the concept of wake-up radios for IoT and describes the implementation of a wake-up radio system with addressing capabilities for wireless sensor network application.

Chapter 11 concludes the book with an elegant demonstration of wireless power transfer and backscatter communication applied to a practical problem by proposing and prototyping a remote control system that operates without batteries or other local power source in the remote control unit.

Chapter 2 describes the fundamentals, applications, standardization, and operating principles of RFID technology and offers a glimpse into the design considerations and architectures of modern UHF RFID readers.

Chapter 6 explores low-cost and low-complexity techniques for the design of an ISO 18000-63-compliant RFID reader and presents an experimental prototype to validate the proposed concepts.

Chapter 3 discusses the fundamentals of backscatter radio communications, analyzes the RFID backscatter channel, its major limitations and mitigation approaches, and presents recent advances including novel RFID quadrature backscatter modulation techniques.

Chapter 1 walks the reader through the fascinating history and evolution of RFID technology from the early days of radio transmissions in the nineteenth century to today’s internet of things.

In the last few chapters we have examined the propagation of electromagnetic waves; freely propagating waves in Chapter 8, waves guided along transmission lines in Chapter 9, and waves guided within waveguides in Chapter 10. But we paid no attention in these discussions to the generation of these waves. In this chapter our goal is to remedy this shortcoming. As we will show, an oscillating current in an open-ended wire can produce an electromagnetic wave. We will examine the distribution of the radiated power, the total radiated power, the efficiency of the power generation, the polarization of the wave, and the input impedance of a few simple radiating systems. We will start by examining a short, or elemental, dipole antenna, and then expand this to longer, more efficient, antennas. We will also look at the field distribution and power density produced by an array of antennas, and show how the distribution varies with the relative phase of the radiators.

We have now reached the end of our journey exploring the fundamentals and simple applications of electromagnetics. We are surrounded by applications of these concepts in our daily lives. A partial list includes electric motors and generators, microwave ovens, remote controls for our television or garage door opener, magnetic resonance imaging, broadcast, satellite, or cable television, high-speed chip-to-chip communications on printed circuits, and many, many more. While we have not dealt much here with the specific engineering principles of many of these devices, we have tried to lay the fundamental concepts on which they are based.

To this point in our discussions, we have dealt solely with static fields. We started with static electric fields, in which all charges are stationary. The electric fields produced by these charges are stationary as well. With electric fields, we developed the notion of the electric potential, the energy stored by electric fields, and the capacitance of a configuration of conductors. We then moved on to introduce static magnetic fields, which are produced by stationary currents. For magnetic fields, we have also introduced potential functions, one a vector function, the other a scalar, but we have not yet discussed the energy stored by a magnetic field, or the inductance of a configuration of current-carrying wires. We will, of course, treat these important topics, but before we do so, we find it useful to take a first look at some time-varying effects. In particular, we will develop a law known as Faraday’s Law, which is the basis for circuit elements such as inductors and transformers, as well as electrical generators and many other useful devices. After we have mastered Faraday’s Law, we will be in a much better position to discuss the energy stored in magnetic fields and inductances, and so we will return to these topics at that time.

Having examined many useful and interesting properties of first static electric fields, then static magnetic fields, and most recently the combination of electric and magnetic fields through the introduction of time-varying effects, we have reached a turning point in our studies. Specifically, we will introduce what is perhaps the most revolutionary concept in electromagnetism: propagation of electromagnetic waves. Electromagnetic waves can carry information and energy, and their properties are described in full using Maxwell’s Equations. We will explore these properties in detail in this and the following chapters.