Antenna polarisation
Category: Antennas
Polarisation is an important factor for antennas. Both antennas and electromagnetic waves are said to have a polarisation. For the electromagnetic wave it is effectively the plane in which the electric vibrates. This is important when looking at antennas because they are sensitive to polarisation, and generally only receive or transmit a signal with a particular polarisation. For most antennas it is very easy to determine the polarisation. It is simply in the same plane as the elements of the antenna. So a vertical antenna (i.e. one with vertical elements) will receive vertically polarised signals best and similarly a horizontal antenna will receive horizontally polarised signals.
An electromagnetic wave
It is important to match the polarisation of the antenna to that of the incoming signal. In this way the maximum signal is obtained. If the antenna polarisation does not match that of the signal there is a corresponding decrease in the level of the signal. It is reduced by a factor of cosine of the angle between the polarisation of the antenna and the signal.
Accordingly the polarisation of the antennas located in free space is very important, and obviously they should be in exactly the same plane to provide the optimum signal. If they were at right angles to one another (i.e. cross-polarised) then in theory no signal would be received.
For 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 remains broadly the same.
Polarisation catagories
Vertical and horizontal are the simplest forms of polarisation and they both fall into a category known as linear polarisation. However it is also possible to use circular polarisation. This has a number of benefits for areas such as satellite applications where it helps overcome the effects of propagation anomalies, ground reflections and the effects of the spin that occur on many satellites. Circular polarisation is a little more difficult to visualise than linear polarisation. However it can be imagined by visualising a signal propagating from an antenna that is rotating. The tip of the electric field vector will then be seen to trace out a helix or corkscrew as it travels away from the antenna. Circular polarisation can be seen to be either right or left handed dependent upon the direction of rotation as seen from the transmitter.
Another form of polarisation is known as elliptical polarisation. It occurs when there is a mix of linear and circular polarisation. This can be visualised as before by the tip of the electric field vector tracing out an elliptically shaped corkscrew.
However it is possible for linearly polarised antennas to receive circularly polarised signals and vice versa. The strength will be equal whether the linearly polarised antenna is mounted vertically, horizontally or in any other plane but directed towards the arriving signal. There will be some degradation because the signal level will be 3 dB less than if a circularly polarised antenna of the same sense was used. The same situation exists when a circularly polarised antenna receives a linearly polarised signal.
Applications
Different types of polarisation are used in different applications to enable their advantages to be used. Linear polarisation is by far the most widely used. Vertical polarisation is often used for mobile or point to point applications. This is because many vertical antennas have an omni-directional radiation pattern and it means that the antennas do not have to be re-orientated as positions are changed if for example a moving vehicle. For other applications the polarisation is often determined by antenna considerations. Some large multi-element antenna arrays can be mounted in a horizontal plane more easily than in the vertical plane. This is because the antenna elements are at right angles to the vertical tower of pole on which they are mounted and therefore by using an antenna with horizontal elements there is less physical and electrical interference between the two. This determines the standard polarisation in many cases.
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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