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Skew degrees labeled 30-150

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Posts: 1
Aug 31st, 2016 at 6:00pm  
I have a DishHD that the SKEW scale is from 30-150 degrees. I have aligned the dish with 61.5 sat functioning. However to align satellite 72 I have to set the skew at -21.2. A local dish installer says I should set the skew at about 80 degrees for my scale.

This dish I use for camping but my other dish at home is labeled with the 29 degree scales showing!

Is there a formula for changing the -21.2 degree to the 30-150 degree scale?
Why do the 2 dishes have different scales?
They both seem to be the same dishes?

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Posts: 1195
Reply #1 - Aug 31st, 2016 at 6:48pm  
The various calculators used to determine satellite polarisation angle vary in their results according to where is the zero reference and in which direction the numbers on the scale go: positive or negative, clockwise or anticlockwise and from where the scale is viewed.

My convention is as follows:

Start with the LNB set upright in its yoke. There will hopefully be a polarisation scale mark at the top centre. This is likely to be marked zero, +90 or -90.

A universal satellite TV type LNB will now receive vertical polarisation with +13 volts supply and horizontal polarisation with +19 volts supply. The words vertical and horizontal (at this time) also refer to the local horizon.

By my convention the polarisation adjustment amount required here is zero. Horizontal name is actually horizontal and vertical name is truly vertical.

Stand behind the dish and face forwards towards the satellite in the sky.

If you are in the northern hemisphere and face due south you will be looking at the highest point of the geostationary satellite arc.

If you point at a satellite to the east, you will turn the dish to the left and the elevation angle will decrease and the further east you go the more the polarisation will tilt to the left - anticlockwise - as viewed from behind the dish and facing forwards towards the satellite in the sky. In my convention, this is a negative adjustment amount.

If you point at a satellite to the west, you will turn the dish to the right and the elevation angle will decrease and the further west you go the more the polarisation will tilt to the right i.e. clockwise. In my convention this is a positive adjustment amount.

If you concentrate on setting the starting position to the nominal polarisation name that you are told to use and then apply the required adjustment amount you will not go wrong. Depending on your scale numbers the actual reading may be quite different, going forwards or backwards from 0, 90 or -90. It is only the amount of adjustment that matters. Just count the tick marks along the scale from the starting position.

Exceptions: Some Eutelsat satellites have their polarisation offset 3.5 deg clockwise. Some Astra satellite have thier polarisation offset 7 deg clockwise. This means you need to manually add 3.5 or 7 deg clockwise to the calculated figure.

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« Last Edit: Mar 11th, 2020 at 10:10pm by Admin1 »  
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