LNB Skew settings

[nopfusch]

Hi,
may I post 2 threads here, regarding one item: LNB skew setting?

When I read lungsat or SATCOdx settings, they are for Dishes/LNBs situated in the northern hemisphere.
And the Skew setting for the LNB? They have none but with digging around I discovered exactly opposing answers.

With the aid of the Dish-pointer (great-) program I did the following:
I took "our" satellite and recorded the following data from the dish-pointer:
City........................Satellite: Intelsat7/10 .........................City
Location Longt. ................Location Longt. .................Location Longt.
Istanbul 28.9° .....................68.5° .......................Hongkong 114.1°
Latit. 41.04° ......................................................Latit. 22.4°
LNB skew -36.2 ...................................................LNB skew 60.0°
southern hemisphere:
Germiston South Africa 28.16° .............................Perth 115.8°
Latit. -26.16° ......................................................Latit. -39.95°
LNB skew 52.8° ..................................................LNB skew -49.7°

The difference between Germiston and the Satellite is approx. 40°. So I added to the longitude of Intelsat 7/10 (old PAS 7/10) also 40°. But in the south at this longitude is the Indian Ocean so I went to Perth and in the North is Hongkong, which gives then 50° difference in Longitude between the Satellite and Cities. Not exactly symmetrical to Germiston/Istanbul but for this exercise OK.
Conclusion: the LNB skew depends if the relevant location is before or after the Longitude of the Satellite, when looking from 0°. But with the dish-pointer that problem is now solved.

In a feed horn of another SATDXer, the "local" Satellite (PAS 7/10) can only be viewed in the extreme skew setting either 0° or plus and minus
90° in C- and Ku-Band. The feed horn is exactly aligned to be in line with the dish pivoting axis (Polar axis) and there is no more possible settings. As the equipment is 2ND hand, it might have been tampered with, although it doesn't look like.
I removed the feed horn and looked into the same. The feed horn has the diverter plate in, to receive circular polarized signals. Now the pick-up probes for the C-Band (which rotate when changing the skew) are just looking past the diverter plate . That could maybe explain the C-Band. But the pick-up probes for the Ku-Band is separated and before the diverter plates. And no matter what I tried, It comes back to this setting 90-0-
90 degrees.
And at the Location here, it should be 52° but for which polarity?
I assume it is in the center of both polarities but then: 52°
+ 45° is out of range (can't be >90°). Where is my or the mistake?
Thanks.

The dish pointer and other prog's overdoing the sizes of dishes to receive that Satellite here: 120 cm! a 40 cm dish delivers good pic's.
And the SATco Receivable program says it needs here a 900 cm (9 meters!) dish to receive the C-Badr. My friend with a 3.2m dish has excellent pic's with a quality of > 75% !

 

[Robbo]

And at the Location here, it should be 52° but for which polarity?
I assume it is in the center of both polarities but then: 52°
+ 45° is out of range (can't be >90°). Where is my or the mistake?
Thanks.

The skew angle is set with reference to either polarity, it doesn't matter.
i.e for vertical polarisation the angle is measured from vertical, and for horizontal the angle is from horizontal.

Sorry, I had trouble understanding the rest of your post

I generally just use the figure from dishpointer, then wiggle it a little if it needs it.

 

[nopfusch]

Robbo71 I understand that, when you have a normal LNB. Here the whole LNB is turned and remains in that position, as the dish remains always at the same position as well.
The feedhorn is fixed to the dish and moves (turns) as the dish moves and the skew setting is then obviously different for the polarities at any other azimuth, as we having here a mechanical adjustment (skew motor), versus an electrical switching from e.g. 13V to 18 V (H / V), where the LNB remains in the same mechanical (e.g. 52 degree) position.
There MUST be an adjustment other than zero and 90 degrees. The 52 degrees is correct when a normal LNB is used. So I should get 52-90 = -38 degrees for the
other polarity, but no chance. Where is my mistake?

The other statements were rather a comment and for newcomers rather discouraging information on these web sites, to use a 9m dish when there is a 3m dish more than adequate. Or the info from the satellite's owner are wrong.

 

[stocksie]

In a steerable dish set up the LNB is set square to the dish thus upright
with the dish at zero (North where you are) As the dish is moved east or west the LNB
naturally leans one way or the other thus providing automatic skew to compensate for the longitude difference between receiving site and satellite.
Your system servo controlled aerial should move 90 degrees only when you
select H or V. If you imagine a fixed aerial LNB at the front of your steerable
dish (as nearly all systems are now) then imagine the position required for
the tiny aerial in your system. Early days some satellites did radiate signals
not square with the earths equator and receivers did allow for some small
differences in H and V polarity( plus or minus 10 or so degrees)
The Skew offset figures you have are for use in installing fixed dishes.
The little servo motors and drive system were very prone to water/weather
damage and produced some strange results! Maybe you have a problem there! Easy to check with sight of the aerial.

 

[nopfusch]

Thanks stocksie.
I'm at 28.2 East Longitude and 26.2 South Latitude.
The reference for this dish was and is the Badr-C (Arabsat) because that is nearly the same Longitude as my place.
My question is now should I do that adjustment (set square) when the dish "looking" to true North (zero) or to the apex (Arabsat = zero now)?
The difference is quite big. The Dishpointer can visualize the difference quite good:
compare Germiston with the Meteosat9 (0.0 Grad) and Germiston with the Badr-C.
Thor at .08W gives a blue line on the dishpointer screen about 45 degree inclined to the West.
Badr-C 's line is +/- 3 degree inclined to the West, i.e. nearly 90 degree against the pictures horizontal base line.
Or must the dish be at Badr-C minus the +/- 3 degree** ?
And here** is the Feedhorn set square to the dish (or the Polar [skew] motor is
perfectly vertical aligned) with the skew setting also at zero?
And why is the LNB skew setting for ASTRA 2A-B-D (unfortunately not receivable here) which is exactly above me (true azimuth 0.0 degrees), 7.5 degrees ? Is it because the centre of reception
is Europe ?
I know, I sound stupid but something I miss somewhere ...

 

[Robbo]

As stocksie says, the LNB skew should be set to zero on a motorised dish.

So when the motor is at zero degrees, i.e when you are pointing at 28.2E, the LNB should be vertical.

BadrC is not your true North, it is the closest satellite to it that you can actually pick-up, I presume.

I.E LNB vertical, zero skew at the apex, or top of your arc.


26E should not be at your APEX,it should be just by the side of it.

28.2E should be your APEX.

 

[stocksie]

Generally we in Europe will not know which satellites you can see in J'burg
however BadrC at 26E is near enough due north and at the arc apex for you to set the LNB and skew to upright (zero) then see how good your arc tracking is by looking at satellites E and W of you. We could use a gyro or Google Earth to find due North but we have lots of conveniently positioned satellites to home in on! I live at 4W and use the satellite at 5W as my apex reference. I notice you dont mention reception of any Ku band signals. Be aware that 3.6M dish is going to be very sharply focussed so searches for signals will need to be done very slowly! also some C band mesh dishes dont perform well at 10-12 Ghz. Once you are near to the arc you will need to find
what errors are occuring and in which direction to allow you to trim your arc shape. No going out and flexing that beast!! Is the dish actuator driven? Man sized!! Let us know how you get on and what you see from down there.

 

[nopfusch]

Thanks to both of you.
I'm away now for +/- 2 weeks and then I'll continue.

 

[patmos]

LNB skew will be nearly zero or horizontal. On BADRC it will be -3.72 deg. _http://www.satlex.de/en/azel_calc-params.html?satlo=26.2&user_satlo=&user_satlo_dir=E&location=-26.16%2C28.03&la=-26.16&lo=28.03&country_code=za&diam_w=240&diam_h=240. You can use an inclinometer to adjust exactly. Normally I align it on my top most satellite with nearly horizontal(zero) lNB. I then adjust skew at the most westerly satellite AB3. This is because the elevation is low there. I can just stand before the dish and optimise for best skew (=best signal strength/quality). It will be optimised for the whole arc.

 

[stocksie]

Ref. Patmos input. The scales for setting up LNB angles on smaller dishes do not
allow for small degrees of offset. Normally it is easier to set the LNB to Zero (upright) in line with the dish construction. Horizontal would only be possible with
a circular dish. Setting LNB offset when the dish is towards the east or west of th
arc means that it will now be wrong for the peak of the arc and even further out for the other end of the travel. There should be no need for LNB offset on a steerable dish. There is no ability to set up LNB offset on digital receivers and
I cannot see users having to climb ladders each time they track the arc!!

 

[patmos]

This applies to Prime focus dishes in case you cannot accurately set the skew at true North. It also relates to a motorised prime focus dish. If the skew is finely tuned at any place in the arc it should do well throughout the arc if the declination angle is correct. I've done it for C band big dish ( 12 ft).