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Dish Alignment and Polarization Contradiction

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sun-tracker
Ex Member


May 21st, 2012 at 8:24am  
Hello,

Located near Kandahar, Afghanistan.

I currently have 1 HX50 VSAT set up (I fell in on it when I arrived here, so it was already set up and functioning).

I am attempting to set up a second system to link them together via the BW-offered Draytek router. It is called the "Dual 2000" system.

I set up the second dish yesterday but not-surprisingly ran into problems once I started the dish-pointing procedure.

(Question: Does it matter how close or far apart the 2 dishes are from each other? I currently have the new dish situated approximately 15 meters in front of and 1 meter left of the old/current dish. They are both pointed slightly to the right and so both have a clear line of sight to the horizon. I presume there is no issues with that -- please confirm.)

When I used the BW dish-alignment page, I entered in the exact coordinates of the dish and it gave me the following information:
[quote]
Satellite: W3a: 7°

Dish elevation (deg): 17.9°
Dish azimuth (deg east relative to true north): 252.46°
Dish azimuth (deg east relative to magnetic north): 250.23°
Polarisation angle (deg): -54.38°
Counter clockwise rotation whilst standing in front of the feed assembly, facing the dish.
[/quote]

(Question: Is the w3a satellite a "Eutelstat" satellite? I've seen mentioned in a few places that I have to add 3.5 degrees to the polarization?)

Most of my confusion comes from the technical manuals which arrived in the boxes, and how it seems to contradict what the BW Youtube video shows.
I believe I am supposed to have a vertical receive polarization, because the current functioning HX50 states vertical receive in the modem's advanced settings page. Therefore I rotated the assembly so the LNB is vertical, and affixed the cone so that the single mark is up and lined up with the LNB, and the fat lump is down towards the feed support tube. Now, am I supposed to rotate the entire dish (using the scale on the back of the dish) to -54.38 degrees? Or do I leave the dish at 0 and rotate the feed horn assembly to -54.38 degrees? The manual states that the feed horn assembly should only rotate +/- 7 degrees within the bracket, but in the BW youtube video the bracket is not present and the guy is rotating the horn assembly way more than 7 degrees! Argh!

BW has supplied me with a MaxPeak SAM-lite2 meter. Here is what I find odd: the current functioning dish has a signal strength of 91 and a rate code which fluctuates between 1/2 and 2/3. I have been using it for the past 4 months with no issues, and can see download rates of up to 130 KiB/s. When I disconnect the cables from that system, and plug the meter into the LNB, it never finds the w3a satellite!! The meter has the most up to date meter-file from December 23, 2011, and I've tried multiple cables to connect it to the LNB. What could be the reason for the meter not finding the satellite, but the modem finds it immediately? The meter only ever gets up to about 70 dBuV, and never actually finds w3a. How could this be?

Thanks for any assistance you can provide. I can take pictures of the setup if you think it will help.
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sun-tracker
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Reply #1 - May 21st, 2012 at 1:36pm  
OK -- here is what I tried today:

I used an inclanometer to set the elevation to 17.9 degrees per the automated pointing tool.

Using the following instructions from your HX50 VSAT install page:

[quote]
Polarisation setting:
This is a two step process - set to nominal and then apply the adjustment amount (between +/- 90 deg).

First set the nominal receive polarisation.   If Horizontal, then the feed arm will be central at the bottom, near the ground (set like the picture above).  If you want Vertical nominal receive polarisation then turn the whole dish by 90 deg either way, so that the feed arm is directly at one side or the other. If the intended adjustment amount is + positive then there is some preference that the initial 90 deg adjustment is anticlockwise (as viewed from behind the dish).   If the intended adjustment amount is - negative then there is some preference that the initial 90 deg adjustment is clockwise (as viewed from behind the dish).  

Having set the nominal polarisation, apply the adjustment amount.  If the amount is + positive then turn the dish by that amount clockwise (as viewed from behind the dish).  If you started with the feed arm at the bottom centre then the read the scale directly. If you are starting with he feed arm out at one side then note that it is the amount of adjustment movement that matters. The scale numbers will read backwards from 90, so a 30 deg movement will result in scale reading of 60.
[/quote]

I set up the assembly for a horizontal polarity, and rotated the entire dish the initial 90 degrees clockwise when facing towards the satellite to set nominal vertical polarization. I then rotated counter-clockwise 54.38 degrees FROM the 90 degree position.

I loosened the azimuth bolts and plugged in the SAM-lite2 meter. I began sweeping back and forth and through various elevations and azimuths, the best signal I could get was 74 dBuV. If I clicked left or right on the meter to select different satellites, they also showed 74 dBuV.... so clearly I'm not actually tuning into the w3a satellite...

I thought I had figured it out through all of my reading, but now I am more confused! I'm starting to think that this SAM-lite2 is completely useless....
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USN - Retired
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Reply #2 - May 21st, 2012 at 2:34pm  
Assuming adequate frequency diversity, dish spacing is not critical. 15 feet with 2w transmitters is safe. But you must select operating frequencies carefully to avoid mutual interference. W3A is indeed a Eutelsat, but Eic has the inclination error listed at a negligible 0.1 degree. The list is here: http://www.satsig.net/sslist.htm

What jumps out at me, is no mention of satellite parameters for the second dish. Are you sure both dishes are even assigned to the same satellite? The operational system you inherited will list the current satellite parameters for that specific account. They will not be shared by the 2nd dish, that will pretty much guarantee mutual interference. So let's back up and take a look at the satellite parameters for the 2nd dish, and go from there.

Oh, and a Rate Code that vacillates between 1/2 and 2/3 suggests a pointing error. Given a Signal Strength of 91, it's almost certainly the POL angle. You should be able to fine tune that all the way up to 4/5. But let's limit this to one problem at a time for now.

//greg//
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Eric Johnston
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Reply #3 - May 21st, 2012 at 3:01pm  
Quote:
Does it matter how close or far apart the 2 dishes are from each other?

Simply make sure that each dish has a clear view forwards to the satellite in the sky.  Think of the beam as a cylinder, the same diameter at the dish.

W3A is a "Eutelsat" satellite. Its polarisation is believed to be +3.5 deg clockwise, as viewed facing toward the satellite.

The receive polarisation direction for the Hughes 1501882-0002 Universal LNB is the direction of the LNB connector.

If your system works perfectly with the HX50 modem and when you replace the modem with a  meter the meter fails to say "satellite found", the fault is with the meter configuration.

The meter is in two parts.
First a simple broadband power meter.  I will give a reading on any and all satellites and allow peaking up.
Second the meter may be programmed to look for a particular carrier with a L band frequency and symbol rate.  If it sees such a carrier is says "satellite found" or  "locked" or similar. This is intended to confirm that you have found the wanted satellites.
The problem is that the meter has no idea what type LNB you are using or what polarisation you have set mechanically. The universal LNB can be told to change local oscillator frequency using a 22kHz tone, either from the modem or from the meter.

The mode matched feed will only rotate +/-7 deg within the bracket. That is intentional. The 505 should remain at all times directly away from the feed support arm.
You can attach the LNB at any 30deg increment angle you like at the 12 hole flange but you might as well leave it as it comes since all polarisation settings and adjustments can be made at the rear of the dish on the giant scale.  Just start with the feed support arm either at a side or at the bottom.
Only use the +/-5 deg on the feed itself if asked to make a final tweeking by the hub at commissioning. They can measure your cross pol using a CW carrier and talk you into the narrow cross-pol null.

Best regards, Eric.
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sun-tracker
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Reply #4 - May 21st, 2012 at 3:18pm  
Greg -- interesting point about using the same satellite. I had not thought of that. That wouldn't explain the fact that my satellite meter cannot detect w3a, though, right? Or the fact that the meter cannot find w3a when I hook it up to the LNB on the currently-working dish??

I will have to ask BW if I'm supposed to be aiming at a different dish.

Eric -- is there something else I need to do to configure the SAM-lite2? I'll look at the downloads page on the MaxPeak website.

Please confirm that the method I used to set the polarization is correct: cone assembly has the LNB sideways (pointed left when facing the satellite), with the single-tick and 505 pointed UP away from the support tube. I then rotated 90 degrees clockwise (when facing the satellite), and then counted backwards FROM 90 degrees the 54.38 degrees.

Should I just drop the meter and go straight to using the HX50 to align?

Thanks.
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Reply #5 - May 21st, 2012 at 6:29pm  
Quote:
Eric -- is there something else I need to do to configure the SAM-lite2? I'll look at the downloads page on the MaxPeak website.

Unless you know for certain the L band frequency of a suitably large DVB-S TV carrier that is on the same polarisation as your wanted signal you are probably wasting your time with the 'satellite identifier portion of your meter'.  We don't know for certain the local oscillator frequencies of the Universal LNB or which way the 22kHz tone switches it.  Your meter will still work fine for detecting and peaking up on each satellite along the orbit.

Set the second dish at the same angles as the first good dish, particularly elevation and polarisation angle.  Then swing the antenna sideways and peak up on the satellite.  You are unlikely to be more than 1 satellite high or low along the diagonal orbit line (up left to down right).

You can operate two VSATs to the same satellite even if the VSATs are side by side. If the HX modems are powered from different AC mains generators, or from different phases of a 230/415 volts supply then space the antennas sufficiently far apart so that no one can touch the metal of both simultaneously.

The HX modem serves fine as a meter if you can locate the PC screen near the antenna.  Scale 0 to 29 is basic wideband noise, peaking up on anything.  Scale 30 to 99 means signal quality on the wanted satellite and wanted carrier.  If you get 29 start looking at the HX config, or try 90 deg other polarisation.

Best regards, Eric.
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sun-tracker
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Reply #6 - May 21st, 2012 at 7:06pm  
OK. Apparently my sat meter needs a frequency list update, so perhaps I'll toy around with the HX50's pointing utilities and see if I get anything.

It turns out I'm supposed to be aiming for W6, not w3a!
BW also says I should have both dishes aimed at W6, and not two seperate satellites. I would guess that Greg is right if it was 2 dishes operating under the same modem, but since I have 2 seperate HX50s, they should uniquely identify themselves to the satellite I would think.

I suspect that though my first dish does work, it's not ideally calibrated on its polarization either. The dish itself is at 0 degrees rotation and the feed assembly is what's rotated. Despite BW recommending to me to leave the dish at 0, I'm inclined to listen to you guys and the manuals which came with the dish, as it makes more sense to rotate the dish (since it is assymetric)!

-Austin
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Reply #7 - May 21st, 2012 at 7:12pm  
Eric Johnston wrote on May 21st, 2012 at 3:01pm:
W3A is a "Eutelsat" satellite. Its polarisation is believed to be +3.5 deg clockwise,

You might want to update your list Eric. It says 0.1 degrees. Matter of fact, all the listed values are positive. I'm pretty sure that inclination errors can be negative too, which in turn dictate either clockwise or anti-clockwise POL correction values.

//greg//
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Eric Johnston
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Reply #8 - May 21st, 2012 at 10:54pm  
The W3A satellite is being maintained in its orbit stationkeeping box of approx +/- 0.1 deg, so that fixed pointing small earth station antennas may be used.  Its inclination is always increasing at approx 0.8 deg per year due to sun and moon, and to keep it below 0.1 deg, station keeping operations are done frequently.  Inclination is a positive number, but you can think of it, at times, as negative, for example if you launch the satellite into an initial orbit with a 'negative' inclination of say 0.4 deg such that after 6 months its orbit inclination reduces to zero, having used no fuel for 6 months. You then start stationkeeping for 15 years, then allow it to go into inclined orbit mode, with only E-W stationkeeping for say a further 5 years, by which time the inclination has increased to about 4 deg, which needs tracking ground antennas.

The polarisation of virtually all satellites is either parallel with the equator or the earth's axis.  A few satellites have their polarisation tilted.  Some Eutelsat satellites may have +3.5 clockwise and Astra may have 7 deg clockwise. Quite which satellite have the anomaly is not known. It was done early on to reduce cross-pol interference due to the flat underside of raindrops that would be less of a problem if the polarisation was parallel to or at right angles to the ground in the middle of the coverage area.

When you set up linear polarisation be prepared to make a final adjustment of a few deg either way according to what the teleport hub tell you to do. They can see your CW carrier in the cross pol transponder and help you verbally into the exact deep null, so that you do not cause interference to others or suffer interference from others, now or in the future.

Best regards, Eric.
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