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VSAT technology and installation >> Dish pointing and alignment >> Skew question

Message started by tearepa on Sep 7th, 2012 at 7:29am

Title: Skew question
Post by tearepa on Sep 7th, 2012 at 7:29am
I'm a bit confused over a polarisation matter. Heres the scenario.

I commisioned two vsats the other day.
One Looking East. The other looking West. Both in the Southern Hemi.

I got the pol skew angle from the Service Provider. But what I thought I set right for the East looking dish didn't make sense.

I would like to send an illustration of what I mean, its a little complicated to explain. How do I upload one?

From what I know about pol, If in the southern Hemi, turn CCW for an east looking dish (looking into reflector), and CW for west. Then +- 90 if you need to.

Title: Re: Skew question
Post by Admin1 on May 13th, 2020 at 12:08pm
ignore this text

Title: Re: Skew question
Post by Eric Johnston on Sep 8th, 2012 at 9:27am
To put picture in your message at this forum you may:
1. Upload the image to your own server or to a public image server.
2. When writing your message do this {img}https://www.{/img}
Use square brackets, not squiggly ones.
Alternatively send your image to me by email and say which message it is to go with.

Regarding applying the polarisation adjustment amount, if you are in the southern hemisphere:
For satellites to the north west the polarisation adjustment amount is anticlockwise, as viewed facing towards the satellite in the sky.
For satellites to the north east the polarisation adjustment amount is clockwise, as viewed facing towards the satellite in the sky.
For the satellite due north the polarisation adjustment amount is zero.

If you are near the equator, the polarisation adjustment amount required may be large, approaching +/-90 deg.
+ means clockwise, while facing towards the satellite in the sky.


The west facing antenna looks good to me. The east facing antenna needs the LNB lifting up a bit more so as to make the total movement amount 58 deg from the starting position which is with the LNB sticking out sideways = vertical receive name.

If you are setting up vertical receive polarisation then start with the LNB arm aimed sideways **.

This is vertical polarisation LNB waveguide:

If applying the adjustment amount causes the LNB to hit metal then start on the other side.

I prefer to always consider the view towards the satellite in the sky. It is then quite obvious that, in the southern hemisphere, for satellite to the north west you turn the feed anticlockwise and clockwise for a satellite in the north east.

Start with the correct polarisation name.

Then apply the adjustment amount.

Note that the scale readings may start at 0, 90, 180 and go forwards or backwards. Ignore the numbers and simply count the required amount along the scale. If no scale, use an inclinometer placed sideways across the BUC/LNB and again count the amount of movement, not necessarily the scale reading.

What are your pointing angles as per ?

** this does not apply to white Hughes HX Universal LNB PN PN 1501882-0002 integrated with OMT, where the receive polarisation is the direction of the LNB cable connector.

Best regards, Eric.

Title: Re: Skew question
Post by Forum Admin on Sep 11th, 2012 at 10:24am

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.

Title: Re: Skew question
Post by tearepa on Aug 22nd, 2013 at 12:51pm
I just want to return to this topic for a sec. and try to understand something more.

I have a skew of +65, calculated from Satsig (and the same from others). I'm South of the equator so I'm pointing East. Now I'm consistently getting max Vertical Xpol at about +25. The diagram shows what I mean. Theoretically I should be a bit more CCW at +65. What do you think?

Note: Andrew feed with xpol omt. Lnbs on both feed ports receive only. DVBS carrier using MER function on Signal Strength Meter to peak.

Title: Re: Skew question
Post by Eric Johnston on Aug 22nd, 2013 at 1:21pm
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.

Title: Re: Skew question
Post by tearepa on Sep 14th, 2018 at 7:31pm
It's been a while since I've been on but good to be back. I've been scratching my head over this one until now, apparently the beam of this particular craft is offset (skewed) by 45 deg so at the remote the pol adjustment has to skew a further 45 deg to match. Its also been pointed out that the prodelin waveguide has a notch machined into the inside of the throat to account for this, although I didn't get a chance to open it to see. It's a real doozy for those in the field presetting the typical pol with no luck tuning off a moderate eirp.
Presumably to align the bird’s antennas to the coverage area/region?

Title: Re: Skew question
Post by Admin1 on Sep 15th, 2018 at 9:10pm
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.

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