The polarisation of electromagnetic waves often has a significant effect on the way in which radio wave propagate. While it is important to match the polarisation of the transmitting and receiving antennas, the choice of polarisation is also important for the signal propagation.
What is polarisation
The polarisation of an electromagnetic wave indicates the plane in which it is vibrating. As electromagnetic waves consist of an electric and a magnetic field vibrating at right angles to each other it is necessary to adopt a convention to determine the polarisation of the signal. For this purpose the plane of the electric field is used.
Vertical and horizontal polarisations are the most straightforward forms and they fall into a category known as linear polarisation. Here the wave can be thought of as vibrating in one plane, i.e. up and down, or side to side. This form of polarisation is the most commonly used, and the most straightforward.
However this is not the only form as it is possible to generate waveforms that have circular polarisation. Circular polarisation can be visualised by imagining a signal propagating from an antenna that is rotating. The tip of the electric field vector can be seen to trace out a helix or corkscrew as it travels away from the antenna. Circular polarisation can be either right or left handed dependent upon the direction of rotation as seen from the transmitting antenna.
It is also possible to obtain elliptical polarisation. This occurs when there is a combination of both linear and circular polarisation. Again this can be visualised by imagining the tip of the electric field tracing out an elliptically shaped corkscrew.
Importance for propagation
For many terrestrial applications it is found that once a signal has been transmitted then its polarisation will remain broadly the same. However reflections from objects in the path can change the polarisation. As the received signal is the sum of the direct signal plus a number of reflected signals the overall polarisation of the signal can change slightly although it usually remains broadly the same. When reflections take place from the ionosphere, then greater changes may occur.
In some applications there are performance differences between horizontal and vertical polarisation. For example medium wave broadcast stations generally use vertical polarisation because ground wave propagation over the earth is considerably better using vertical polarisation, whereas horizontal polarisation shows a marginal improvement for long distance communications using the ionosphere. Circular polarisation is sometimes used for satellite communications as there are some advantages in terms of propagation and in overcoming the fading caused if the satellite is changing its orientation.