1.1 DEFINITION OF PROPAGATION
Information can be transmitted in many ways. The use of electromagnetic waves for this purpose is attractive, in part, because direct physical connections such as wires or cables are not required. This advantage gave rise to the terms “wireless telegraphy” and “wireless telephony” that were commonly used for radio in the early part of the past century and have returned to popular usage with the widespread development of “wireless” systems for personal communications in recent decades. Electromagnetic waves are utilized in many engineering systems: long-range point-to-point communications, cellular communications, radio and television broadcasting, radar, global navigation satellite systems such as the Global Positioning System (GPS), and so on. The same considerations make electromagnetic energy useful in “sensors”, that is, systems that obtain information about regions from which transmitted energy is reflected. Electromagnetic sensors can be used for detecting hidden objects and people, oil and gas exploration, aircraft control, anticollision detection and warning systems, for measuring electron concentrations in the Earth's upper atmosphere (and in planetary atmospheres in general), the wave state of the sea, the moisture content of the lower atmosphere, soils, and vegetation, and in many other applications.
In most cases, it is possible to divide the complete system, at least conceptually, into three parts. The first is the transmitter, ...