This chapter lays the foundation for wide‐band signals. Normally, antennas and signals do not appear in the same book, but timed arrays are designed for realistic time‐varying signals and scenarios. The chapter covers the basics of baseband and passband RF signals. It deals with polarization and bandwidth. The chapter presents two types of signals: a carrier signal that is a high‐frequency sinusoid and a lower frequency information (baseband) signal. White noise is an unwanted random signal that has a flat spectrum over the frequency range of interest. If normal distribution with zero mean describes the noise samples, then the signal is Gaussian white noise. The polarization of the carrier describes how the magnitude and orientation of the electric field vector changes as a function of time. The carrier frequency determines which frequency band the system operates.