An antenna (or hydrophone) is a linear device that forms the interface between free propagation of electromagnetic waves (or pressure waves) and guided propagation of electromagnetic signals. An antenna can be used either to transmit an electromagnetic signal or to receive an electromagnetic signal. During transmission, the function of the antenna is to concentrate the electromagnetic wave into a beam that points in the desired spatial direction. During reception, the function of the antenna is to collect the incident signal and deliver it to the receiver. An important theorem of antenna theory, known as the reciprocity theorem, allows us to deal with the antenna either as a transmitting device or as a receiving device, depending on which is more convenient for a particular discussion.

The only aspect of antennas that we shall study is the propagation and diffraction of waves traveling between the antenna aperture and points far away. During transmission, an antenna creates a time-varying and spatially distributed signal across its aperture to form a wave that will propagate as desired through free space. The spatial distribution of the signal across the antenna aperture is called the illumination function. The distribution in the far field of the waveform amplitude and phase over the spherical coordinate angles is called the antenna radiation pattern or the antenna pattern. The relationship between the antenna pattern and the aperture illumination function can be described with the aid of the two-dimensional Fourier transform.