Explanation of Polarisation Angle.
Vertical polarisation is when the electric field is vertical. When people talk about polarisation they are referring to the electric field vector.
References to 'vertical' and 'horizontal' are frequent since terrestrial antennas are normally vertical or horizontal relative to the ground.
Example 1: Vertical polarisation is produced by a vertical dipole antenna. At an instant the top is positive voltage and the bottom is negative voltage - so the electric field produced is vertical.
|Example 2. Vertical polarisation is produced by the open end of a rectangular waveguide whose narrow dimension is vertical and whose broad face is horizontal. The waveguide is typically energised by a vertical probe (like a single ended quarter-wave half dipole antenna) sticking through the broad face of the waveguide. Inside the waveguide think in terms of voltage between the centre lines of the two broad faces. This is the vertical electric field. Around the two narrow sides, current flows. It does not matter if the pin is at the top, pointing downwards, it is still vertical polarisation.||
To set nominal Vertical receive polarisation the broad faces of the LNB waveguide must be on top and underneath.
|Always start from the nominal
position, vertical or horizontal. Then apply the adjustment.
Tilting the angle
In the case of satellite communications, polarisation angles are tilted over a wide range.
If you are in the northern hemisphere: Face the equator (by looking south) and you can envisage the geostationary orbit as curved line across the sky starting in the south east, rising to say 35 deg elevation at due south and falling in the south west. The polarisation of satellites located to the east will be tilted anticlockwise, satellites to the west clockwise. Only a satellite due south of you will have its polarisation aligned vertical.
If you are in the southern hemisphere: Face the equator (by looking north) and you can envisage the geostationary orbit as curved line across the sky starting in the north east, rising to say 60 deg elevation at due north and falling in the north west. The polarisation of satellites located to the west will be tilted anticlockwise, satellites to the east clockwise. Only a satellite due north of you will have its polarisation aligned vertical.
Important: Think in terms of the view from behind the dish, facing towards the satellite; clockwise is positive.
The calculated angle
If the calculated angle is positive you need to face towards the satellite and tilt the top of the vertical polarisation clockwise so many degrees to the right.
If the calculated angle is negative you need to face towards the satellite and tilt the top of the vertical polarisation anti-clockwise so many degrees to the left.
These diagrams above show your view facing towards the satellite. Your viewpoint is from behind the dish looking forwards towards the satellite.
You can see the feed horn window. The rectangular LNB waveguide is shown. Note the four possible starting positions in the top row.
You then apply an adjustment, between -90, 0 and +90 deg, anticlockwise or clockwise. A -45 deg adjustment is shown as an example. Sometimes the LNB will hit the metal arm when you apply the adjustment; in this case start again with the alternative starting position, on the other side.
A few satellites (notably Astra and Eutelsat) were not constructed with their polarisation angles aligned accurately north-south. In the case of Eutelsat II Ku band and W Ku band you need to add +3.5 degrees to the calculated results, i.e. turn it a bit more clockwise to the right. In the case of Astra satellites add + 7 degrees clockwise rotation, while facing the satellite.
Two polarisations (at right angles to one another) are often used simultaneously to provide two fold frequency reuse and this applies in both the uplink and down link frequency bands.
VSATs normally transmit on one polarisation and receive on the other.
If you rotate the feed by 180 degrees you are back to the same polarisation as when you started. This is useful to know as rotation of the feed may cause metal parts to knock into each other. In these circumstances start your adjustments from the opposite 180 degree position.
Polarisation angle should be optimised to an accuracy of +/- 1 deg to reduce interference on the opposite polarisation to an acceptable level. This is particularly important for transmitting antennas and for receive-only antennas trying to receive digital carriers.
For transmit antennas the hub will normally request the transmission of a low power un-modulated carrier on a carefully selected frequency. They will ask you to rotate the feed around the optimum angle while they observe for the very sharp null in the cross polar signal, using a spectrum analyser with a narrow resolution bandwidth (say 30 Hz). You may well need to turn the feed in steps of 0.5 deg to find the best position.
To optimise polarization angle by practical measurement on receive-only is more difficult since the co-polar wanted signal has a very broad maximum. If you can measure receive quality such as Eb/No, C/N or BER with a fine degree of resolution (e.g. Eb/No in 0.1 dB steps) and can accurately measure the angle of the feed a good method is to degrade the receive quality by a substantial amount, say about 4 dB down. Record the degraded quality accurately and the angle of the feed accurately. Then turn the feed a similar amount on the other side and adjust for exactly the same degraded quality. Record the second angle and set the feed to the calculated mid point. This will normally give an accuracy of +/-1 deg, particularly if you repeat the process several times and average the results from several pairs of angle/quality measurements. This method does not work if there are interfering cross polar signals that are varying in level all the time. Feed rotation angles may be recorded using a strip of paper wrapped around, with the half way point being marked up while it is temporarily removed.
Page started 1998, last amended 8 June 2010
This page is copyright ©1998 Satellite Signals Ltd All rights reserved.