An infinitesimal element excited with an alternating current is called the Hertz dipole. In practice a linear antenna can be approximated by a Hertz dipole if the linear antenna is of size much smaller the wavelength.

The analysis of Hertz dipole is important as any complicated radiating structure can be decomposed into Hertz dipoles.A Hertz dipole is shown in Fig.

The Hertz dipole is oriented along the z-axis, has length and current .

Since the Hertz dipole is small the vector potential at point P is almost same as the Green's function multiplied by the volume integral of the current density.