SatSig topic:

Good signal with wrong polarization ?

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Mar 29^th, 2006 at 7:37am

Hey all ,
I'm back with more questions Wink

Can you please tell me if it is possible for me to get the right signal from the satellite (18 volts) , and even been able to lock and surf the net - while having wrong polarization ? (instructed to move +18, now I can get good reception at -72 ) .

Thank you for the help.

P.S Im using Idirect 3000 series router

Mar 29^th, 2006 at 9:05am

You have obviously found the correct polarisation see:
https://www.satsig.net/africa/idirect-vsat-sudan.htm

There is probably some misunderstanding about polarisation for your Ku band 1.2m dish.

My recommended process is as follows, which always works: First what is the nominal downlink receive polarisation you have been told to use. This will be horizontal or vertical, and is applicable without further adjustment if you are on the same longitude as the satellite. Note that receive polarisation is defined by the pin inside the LNB waveguide or by the narrow sides of the LNB waveguide. Set the feed rotation start position accordingly. It will be either upright or on its side. For mechanical limitation reasons you may wish to start with it downwards or on the opposite side. There are 4 possible start positions.

Polarisation scale readings may be anything but will normally be 0 or 90 or -90 deg, and the scale may read forwards or backwards. Plus and minus signs may be marked. Ignore the +/- signs and note that 0 may mean 90 and vice versa.
From calculation https://www.satsig.net/sf.htm your polarisation should be -72 deg which means rotate the feed ANTIclockwise by 72 deg WHILE FACING TOWARDS THE SATELLITE.

You have not said what was the nominal polarisation (vertical or horizontal) that you have been told to start with, but now you have found the carrier don't worry.

As you rotate the feed through a full 360 deg there will be a region of good signals and region of no signal.

If there is no interference from signals on the opposite polarisation the good maximum will be very broad, possibly as wide as 100 deg, with no clearly defined optimum position angle. If you note the angles where the signal drops out or two accurately equal degraded signal qualities either side (eg around +/- 40 deg from the best region) then you can halve the angle between the two marks.

The hub will ask that you make small rotation adjustments of the feed while your transmit a CW test carrier to get your polarisation adjustment in the exact centre of the very sharp cross pol null, to an accuracy of +/- 1 deg if possible. Make sure you are under the feed arm when you do this, for your safety, and also since any obstruction of the beam will ruin the very sensitive cross-pol measurements.

Best regards, Eric.

Mar 29^th, 2006 at 11:24am

Hi,
I'm quite soo happy to know that I've found the carrier.
and yes I had to move my BUC anticlockwise to get the signal.
My VSAT seems to be working fine, but I still have couple of issues to clear , and I will be grateful if you kindly clear them for me :
1- All through the VSAT installation procedures I've found on the net , the last step was always doing the cross polarization , I've never done that with my service providers ( never been asked to do!), and still I'm able to get to connection and surf the net ! is that okay ?
2- Whenever I power down my router , it takes very long time for my TX and NET indicators to go green , and register to the net ! (20 to 30 mins!) Why? Anything I need to do about this?

Kind regards

Mar 30^th, 2006 at 8:23am

1. If you have rotated the feed 72 deg anticlockwise that will be fine. The problem is if you have in fact turned it 65 or 77 deg. Scales are often very small and hard to read. In such cases wrapping a strip of paper round and marking the length L of the circumferance is an idea. Distance required is then L * 72 /360 mm. Polarisation optimisation while the hub talks to you is best.

If you are at the wrong angle your transmit bursts may interfere and cause errors in other services. On receive you will be vulnerable to incoming cross pol interference. In both cases, it depends on what, if anything, is occupying the opposite polarisation, now or in the future.

2. Ask the hub about the time it takes to start up.

Best regards, Eric.