Antennas transmit or receive information by radiating electromagnetic energy. Therefore, energy and power are key parameters associated with these electromagnetic waves, and it is essential to discuss them. An electromagnetic wave consists of both an electric field and a magnetic field.
At any instant, the wave can be described by these two vectors. The figure below illustrates the electric and magnetic field components of an electromagnetic wave.
In an electromagnetic wave, the electric field is perpendicular to the direction of propagation, and the magnetic field is also perpendicular to the direction of propagation, while the electric and magnetic fields are perpendicular to each other.
Poynting Vector
The Poynting vector describes the energy of an electromagnetic wave per unit time per unit area at any given instant. It was first derived by John Henry Poynting in 1884 and is named after him.
Definition: The Poynting vector gives the rate of energy transfer per unit area.
Alternatively: The energy carried by a wave per unit time per unit area is given by the Poynting vector.
The Poynting vector is denoted by S.
Unit
The SI unit of the Poynting vector is watts per square meter (W/m²).
Mathematical Expression
The instantaneous Poynting vector, which describes the power associated with an electromagnetic wave, is defined as:
where E is the electric field intensity vector and H is the magnetic field intensity vector.
Derivation of the Poynting Vector
To better understand the Poynting vector, let us derive its expression step by step.
Consider an electromagnetic wave perpendicularly crossing an area A that is orthogonal to the direction of propagation (taken as the X-axis). During an infinitesimal time interval dt, the wave travels a distance dx:
where S is the Poynting vector. The above equation gives the energy per unit time per unit area at any instant – this is the physical meaning of the Poynting vector.
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Post time: May-15-2026
