Skew question

ignore this text

Note in the above table:
The red arrow V is the BUC transmit polarisation.
The blue arrow H is the LNB receive polarisation.

One of each pair of green lines should be deleted from the diagram.   The earth station cannot be transmitting on both polarisations simultaneously.

The view is taken facing into the dish with your back to the satellite in the sky.

The third line of text is best ignored to avoid confusion.  The "pol angle" is the amount of CW or CCW adjustment angle required.
The "pol angle" is the apparent angle between the  local vertical and the satellite pitch axis (parallel with the earth's axis).  To start discussing the satellites polarisation is confusing.  If the satellite transmits vertical (parallel with the earth's axis) it is called vertical name when it arrives at the earth station.  The earth station receive polarisation adjustment amount may be as much as +/-90 deg either way, CW or CCW.
In the last line of text where is says horizontal, this refers to the BUC transmit polarisation.


In this picture above the receive polarisation is shown by the orange line.
The LNB was intially at the side (true nominal vertical receive polarisation) and has been adjusted downwards by 22 deg (wrong amount), see red lines.
The required adjustment amount is 57 deg so the LNB could be pushed downwards further or, alternatively start again on the other side, and push the LNB upwards by an amount of 57 deg as shown in pink.

Since 25 + 65 =90 degrees, I guess there is not problem at all. Either figure can be correct, depending on where is the start position and which way you look at the feed.

My satsig definition is "The amount of adjustment angle required, as viewed while standing behind the dish and facing forwards towards the satellite in the sky, positive is clockwise".

If you are in the southern hemisphere and facing north east then you need to turn the feed by an amount clockwise, as viewed while standing behind the dish and facing forwards towards the satellite in the sky. Remember to start with true vertical receive polarisation, if that is what you want, as you would if wanting vertical polarisation on a satellite high up at due north.

For a VSAT with BUC and LNB on a filter arm, the starting position for vertical receive polarisation is with the LNB side arm sticking out sideways. Either side is vertical receive polarisation. Choose one side if the other results in the LNB hitting metal when the adjustment is applied.

Vertical receive polarisation LNB input waveguide

If you have a spectrum analyser, adjust for minimum xpol interference level. The wanted signal will have a broad ill-defined maximum, the cross pol signal will have a sharp narrow null. Go for the middle of the null, about 2 deg wide.

If co-pol and x-pol are the same level you have 45 deg error.

Best regards, Eric.

Linear polarised satellites normally have their polarisations parallel to or at right angles to the earth's axis and equator.

If you are told a polarisation you need to know if this refers to the uplink or downlink. They are normally opposite and very rarely the same polarisation.

Start with the named nominal polarisation. e.g. Vertical means that the broad faces of the rectangular waveguide will be on top and underneath.

There will be a polarisation adjustment amount required. I define the adjustment as positive = clockwise as seen with you standing behind the dish and facing forwards towards the satellite in the sky.  Others may use a different sense (+/-) and the figures given may refer to scales that go forwards or backwards, starting at 0, 90 or 180. I give only the adjustment amount and its direction. You need to apply the adjustment amount using whatever scale or tick marks provided.

In the southern hemisphere, satellites to the north west need anticlockwise adjustments, satellite to the north east clockwise.

I am only aware of two satellite systems with polarisations not oriented with regard to the earth's axis. Eutelsat with +3.5 deg and Astra with +7 deg, both clockwise. The idea was to get the polarisation near vertical/horizontal relative to the ground near the middle of the european coverages to minimise rain depolarisation.

What satellite and beam is at 45 deg ? If something is at 45 deg it sounds like a circular polarisation system where there is a polariser between the OMT and the horn. The polariser is a tube with vanes, rows of pins or slots along two opposite sides. The OMT must be at +/- 45 deg to the polariser vanes. +/- 45 deg gives right circular or left circular. Read how to set up circular polarisation

Prodelin horns with a fat lump on one side of the throat and notches inside the lump is another matter. The fat lump needs to be always towards the feed support arm. To adjust the polarisation angle, turn the entire dish using a giant scale behind the dish. You may turn the BUC/ORU but without turning the horn, but only in 30 deg increments.

To align the polarisation accurately by measurement only, you need to find two severely degradred but exactly equal levels either side of the very broad co-pol maximum. Then turn to the centre by halving the total side to side amount.  If you have spectrum analyser and can see clearly any cross-pol carrier, you can aim for minimum cross pol which appears as a deep narrow null and may be easier to center.