TABLE OF CONTENTS In the previous chapters, we learned that electromagnetic waves can propagate in free space of an infinite extent and that-these same waves can also propagate along a common transmission line. A question that remains to be answered is whether the same electromagnetic waves can be excited in a finite region and then be launched or radiated into infinite space. In this chapter, we first examine the fundamentals of the radiation of electromagnetic waves, which provides a natural lead-in to the important topic of antennas.
Before examining the radiation properties of an antenna, we should first understand the physical process that causes the radiation of electromagnetic waves. This means that we have to examine possible radiation characteristics of an electric charge from a fundamental viewpoint. There are certain requirements that an electric charge must meet in order for it to radiate electromagnetic waves. These requirements will be presented from an intuitive point of view. Once we understand this general argument, it is only a short step further in the same direction toward developing an understanding of antenna radiation theory. The principle of superposition applies in the linear medium being considered in this text, and the antenna can be considered to comprise a large number of charges. The argument also illustrates the type of calculation that can be written on the backs of old envelopes.
We can understand radiation of electromagnetic waves using Poynting's theorem that the total power P rad radiated from a source is given by...
