Radiation Patterns
Recall from Chapter 1, “RF Signals and Modulation,” that antenna gain is normally a comparison of one antenna against an isotropic antenna, and is measured in dBi (decibel-isotropic). An isotropic antenna does not actually exist because it is ideal, perfect, and impossible to construct. It is also the simplest, most basic antenna possible, which makes it a good starting place for antenna theory.
An isotropic antenna is shaped like a tiny round point. When an alternating current is applied, an RF signal is produced and the electromagnetic waves are radiated equally in all directions. The energy produced by the antenna takes the form of an ever-expanding sphere. If you were to move all around an isotropic antenna at a fixed distance, ...
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