Polarisation of electromagnetic waves
- and their importance in radio wave propagation
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.
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