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Polarisation

Polarisation is the phenomenon where the direction of the vibration of waves of electromagnetic radiation is restricted by a constraint, or set of constraints.

Electromagnetic radiation consists of both electrical and magnetic lines of force ('fields') at right angles to each other. When the electrical field is perpendicular to the ground, the polarisation is said to be vertical. Similarly, when the electrical field is parallel to the earth, the polarisation is horizontal.

Antennas radiate energy in the form of an expanding sphere with 'wave fronts', which travel at right-angles to the radiation's propagation path. Initially, these wave fronts progress symmetrically, however the further they travel the less spherical the radiation becomes. This eventually forms 'plane waves'.

A signal becomes polarised, at distances far enough from an antenna to form these plane waves, in the same direction as the antenna's elements (relative to the ground). If the radiating elements of an antenna are perpendicular to the ground, then the signal will become vertically polarised.

With normal whip antennas or yagi antennas, the elements are all on one plane ('linear'), and changing the polarisation is as simple as aligning their elements to the ground respectively. The same goes for some directional antennas (eg. parabolic antennas, depending on how they're made.

Omni-directional antennas obviously break this rule because of the way they're constructed. Most horizontally polarised omni's are expensive and look much fatter than they're vertical counterparts.

Both horizontal and vertical polarisation is called 'linear polarisation'. Many other forms of linear polarisation are possible, as any angle can be used. Apart from horizontal and vertical, 45 degree polarisation is also used in some situations by amateurs to feign circular polarisation (at a great loss of signal, though).

Horizontal and Vertical are the most common linear polarisations because of the lack of confusion, and conformity. If a vertically polarised antenna is upside-down, it remains in phase. However, if 45-degree polarised antennas are used, it depends on perspective - one may actually be polarised at 315-degrees.

Where antennas are of differing polarisation, heavy amounts of signal attenuation can occur, often greater than -20dB. This is called 'Polarisation Discrimination'. For this reason, radio links between two sites must use the same polarisation. In some instances, polarisation discrimination can be used as a method of reusing frequencies.

Vertical polarisation is very common. Antennas are easy to make (think about FM-radio whip antennas), and it works well spread over long distances which are close to the ground.

Horizontal polarisation is said to be helpful for point to point links over distances which may have a large amount of water vapour, such as over bushy areas. Horizontal polarisation is also believed to assist weak signals in general, especially because man-made RF noise is less likely to be horizontally polarised.

Circular polarisation is also possible. Circular polarisation is done by antennas with crossed-linear elements, like helical antennas. They produce a beam which works rather like a cork-screw. That is, the wave front appears to rotate by 90-degrees to effectively switch constantly between horizontal and vertical polarisation.

Any linear polarised antennas can communicate effectively with a circularly polarised antenna, and of course vice-versa. However, this convenience does not come cheap. Having a linear polarised antenna communicating to a circular polarised antenna will incur a -3dB signal attenuation. This half-power loss is due to the wave-front being incorrectly polarised, or 'cross-polarised', half the time.

Antennas which are circular polarised can sometimes help in situations where signal reflections warp the signal enough to change its polarisation, such as a long haul over a lake.

Elliptical polarisation is produced where the waves rotate continually, somewhat at random. This gradual shift in polarisation, known as Faraday rotation, is subject to fading due to cross-polarisation. In space, the concepts of vertical and horizontal polarisation are meaningless, which results in elliptical polarisation.

Most circular polarised antennas are used for satellite communications, to correct for elliptical polarisation. Circular polarised antennas overcome Faraday rotation.

Harmonic antennas are polarised in the direction of the antenna's axis, and are subject to becoming elliptical in most directions.


Version 1 (current) modified Tue, 03 Jul 2007 23:12:18 +1000
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