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XPOL test issue on Yahsat1

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Mar 30th, 2014 at 6:56am  
We pointed 1.8m Andrew circular antenna to Yahsat1a with 11db snr and -18 tx power, we run the CPI test with the satellite NOC it passed with 29.15db but the rx reduced to -7 and tx power increased to the saturation point of -10. We did everything possible like changing feed horn, check polarization etc Kindly help to tackle this issue we have been on it for past 3 days. Thanks
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Eric Johnston
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Reply #1 - Mar 31st, 2014 at 1:35pm  
If your Rx has dropped from 11 dB to -7 dB you have probably:

Changed the Rx tuning.
Mispointed the antenna.
Changed the polarisation.

Start again, trying to get the Rx=11 dB, which sounds good.
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Eric Johnston
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Reply #2 - Mar 31st, 2014 at 7:44pm  
If it's iDirect maybe the hub downloaded a new options file with wrong tuning info or wrong LNB assumptions.

Talk to the hub making sure they know the hardware you are using.

If circular polarisation and you have disassembled the feed-polariser assembly, have you put it back together correctly. The Polariser goes at + or - 45 deg to the LNB-BUC assembly. One way will give you 100% your wanted carrier. The other way will be the opposite polarisation. Half way will be linear polarisation and 100% cross pol interference if there is a carrier on the opposite polarisation.
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Reply #3 - Apr 23rd, 2014 at 10:06am  
we have been activating links on yahsat 1A since a long time now and we had the same problem as you many times.
We realized that each time, the problem was happening with the same brand of Dish (Channel Master bought by Andrew and then bought by Skywareglobal) witch is the same brand and type of antenna.

Even with the best pointing of the world those antennas are generally not able to do better than -18dB isolation.
After many talked with a lot of wholesalers, integrators and Yahsat operator, it appears that it is normal because those antennas were engineered to achieve -18dB and not more when using the circular polarizer.

The problem is that some new satellite operators are requiring -30dB isolation even for C-band Circular when Intelsat was happy with -18dB until now for example...

We solved the problem by using another antenna brand and now this problem doesn't happen anymore. We were offered another solution by a Skyware wholesaler that was to buy a high precision Circular Polarizer @4000$ just for the polarizer but that doesn't make sense.

If you need a 1.8m or 2.4m antenna that works with Yahsat (But the problem is usually the same with Amos 5) we wouldbe happy to help you because we have some in stock.

You can contact us on info@dsdtelecom.com or contact@dsdafrique.com, we are French and can speak English.
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Reply #4 - Apr 24th, 2014 at 8:12pm  
More thoughts.

You say it passed the CPI test with 29.15 dB. That is good.

You say that you originally had 11db snr and -18 tx power. Both sound good.

After the CPI test it went wrong. I do not understand your -7 dB for the Rx. That does not make sense.

The Tx power increased from -18 to -10 and thus  saturation ?  Is this an iDirect system and was the hub commanding your modem to increase its output power ?

Please check you have a transmit reject filter fitted prior to the LNB, in case your tx power is overloading/damaging your LNB.

Check that the coax outer braids are connected at all 4 ends, in case the BUC power is returning down the LNB cable.

Has the hub reconfigured your modem wrong after the CPI test ?  Try to get back to the original 11db snr and -18 tx power.

If it is iDirect, the hub should find out your modem output level that causes saturation and then set a lower "never exceed" value in the hub database so that the hub never tells your modem to output such a high power that it causes your BUC to saturate.

Best regards, Eric.
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