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Alignment problem with iDirect 3000 series

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Ex Member
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Jul 13th, 2009 at 12:51pm  
Hi every body :
iam trying to repoint my 1.2 m dish toward 68.5 Degree E .and i managed to do it successfuly .but the internet connection is very very slow .and that let me think that there some thing wrong with alignment so i do it agian and again but no improvment.so i start thinking may be it servce provider issue so i call them and they say the problem is from your system like they always say and they tell please check you cables and check your LNB , check your ethernet cable etc.... so i bring new cables and new LNB. and i try again and i can find the carrier very easly (more than 18v on isite 7.1.1.0). and strange thing that my service provider tell me that the carrier voltage should be around 12-17 volt.
so i contact our branch office (we have the same equibments Idirect 3000 series and the same service provider) and they tell me that  internet is fine bear in mind that our main office bandwidth is double the bandwith of our branch office. so i collect some information from our branch office and our main office to compare them. then i think it good idea to post them here may be some of the experts can find out what the wrong with my Vsat.
main office :
rx AGC:31.07
Downstream SNR:10.5
rx Power: 12dBm

branch office:
rx AGC:33.38
Downstream SNR:10.402
rx Power: 13.5dBm
so can some body what those values mean .
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« Last Edit: Jul 13th, 2009 at 3:00pm by Admin1 »  
 
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Eric Johnston
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Reply #1 - Jul 13th, 2009 at 1:23pm  
The SNR Signal to Noise Ratio is the important measure of the downlink receive signal quality.  At both sites you have a good figure.

AGC means Automatic Gain Control.  The receive power level simply needs to be amplified or attenuated to get the result to the nominal level in the receiver demodulator.  These figures result from the power of the signal from the satellite, the gain of the dish, the gain of the LNB and the length of the cable.  Unless receive signal power is extremely low or extremely high it does not matter and the value has no effect on receive signal quality.

I doubt that dish pointing is the issue here.  Ask the hub to investigate further.  

At main office site:  What is the quality of your transmit signal when it arrives at the hub ?  What is the transmit output power from your modem when transmitting under clear sky conditions ?  What is the  transmit power output from your modem needed to saturate your BUC ?

At branch office site: What is the quality of your transmit signal when it arrives at the hub ?  What is the transmit output power from your modem when transmitting under clear sky conditions ?  What is the  transmit power output from your modem needed to saturate your BUC ?

Transmit power under clear sky conditions should be several dB (typically 6dB) below the saturated BUC power at its -1 dB compression point.  During commissioning, the hub staff should measure to find your BUC -1dB gain compression point, and then make a record of this maximum value.  The hub should then never tell your modem to exceed this value, during a rain fade.

Remember that the hub staff may be inexperienced.  Simply commissioning your site by following the correct procedures may fix the problem.

Best regards, Eric.
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« Last Edit: Jul 13th, 2009 at 6:34pm by Admin1 »  
 
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Reply #2 - Jul 14th, 2009 at 8:21am  
Thank you Eric for your quick and prompt response
from your reply i conclude that i had to repoint my antenna again and to do it properly i had to consult my service provider and let them guide me step by step.
if it possible i would like you to tell the steps cause i feary new to this although i manage to do couple installation successfully.
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Eric Johnston
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Reply #3 - Jul 14th, 2009 at 3:52pm  
No.  Wrong conclusion.  I did not think there was anything wrong with your pointing.  Rather I suggested the hub should investigate, starting  with your BUC transmit configuration level settings.  If you want to help it may be useful to print out a copy of the 'options file' from each site and ask for explanation of all the differences.  Apart from BUC power settings there are many other things that could be wrong but these are all matters for the hub to get right.

The hub need to see and measure your transmit signals on their hub spectrum analyser.  I would not touch the dish unless the hub ask you to make an adjustment. e.g. elevation up 2 flats etc. They have precision test equipment and can easily see level changes in your transmit signal (in CW mode) and give you verbal guidance as you move the beam. Work slowly and systematically.

Polarisation:
Calculate the angle using one of the dish pointing calculators http://www.satsig.net/maps/  Add +3.5 deg for some Eutelsat satellites.  Set the angle by starting with your LNB at the top or at the side and then apply the adjustment angle.  Positive is clockwise while viewing towards the satellite.  This Iraq dish pointing calculator shows a picture of how your feed system should look.  The calculator starts in Iraq but will work anywhere if you are not in Iraq.   You need to input the name of your polarisation: vertical or horizontal.

If the polarisation scale is too tiny to read accurately try putting an inclinometer sideways across the BUC or LNB.  Wrapping a paper strip round the throat may work.  Mark and measure the circumferance C mm.  The the distance needed is C x angle/360 mm. Rewrap the paper round the throat and use the pencil marks as a guide.  Your hub may ask you to get under your feed arm and reach up to turn the feed in 1 deg increments till you are in the centre of the cross-pol null. Such adjustments need patience as it takes the hub 15 - 20 seconds to make each cross-pol measurement.

Azimuth:
In azimuth (sideways) the trick is to loosen both nuts by say 2 turns and gently swing the dish many times against each nut alternately, measuring all the time and adjusting one nut till both degraded readings are exactly the same.  Then wind in both nuts by exactly the same number of turns and flats, so as to centre the dish.

Elevation:
In elevation (up-down), there is no backlash.  Mark one flat on the nut with felt tip pen.  Then wind right across the beam peak from a degraded measured value below to the same degraded value above.  Then half count the turns and flats back to the centre.

Note that your receive beam has a broad rounded centre and it is not practical to try for the max value, you need to get exactly half way between the steeper sides of the beam.  The transmit beam is narrower than the receive beam so any error (e.g. 0.5 dB) in the receive beam and your transmit pointing will be well off centre and your transmit quality will fail.

Best regards, Eric.
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« Last Edit: Jul 15th, 2009 at 4:12pm by Admin1 »  
 
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Reply #4 - Jul 15th, 2009 at 9:58am  
thank you very much Eric
i will try what you say and i will tell you the result
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