Polarisation of an Electromagnetic Wave
The polarisation gives the describes the vector of electric field strength, with respect to the earth's surface in the far-field region. It is defined by the radiating antenna, but it can be altered by interactions during its propagation. By definition, the plane of polarisation is spread by the electric field vector and the Poynting-vector (direction of propagation). The strength of the magnetic field oscillates in a plane perpendicular to the direction of propagation.
By using different forms of polarisation, a distinction can be achieved between oscillations at the same frequency. If for example, only antenna is used for both transmitting and receiving the same frequency, by using different types of polarisation, the transmission and reception directions can be separated. This form of decoupling is possible because an antenna only responds (transmits or receives) to the share of the energy in the electromagnetic field that corresponds to their individual direction of polarisation.