SatSig topic:

Fss chart and cross pol

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Jan 28^th, 2009 at 10:53am

Dears

I need help, I got a message from my supplier it says my modems on my line card need cross pol, and send me a FSS chart showing me that my carrier is making an interference with others because of modems high power. Any one can help me by writing his Email to understand that and discuss it with him

thx

Jan 28^th, 2009 at 11:21am

If the situation is serious you should immediately turn off all remote sites that are working with that line card. Stopping the interference to others is more important than ceasing your service temporarily.

If it is just one bad site out of many, watch the return link cross-pol spectrum as you may be able to see the modulated interference bursts. Look in the gap, if any, between the traffic carriers on the opposite polarisation. If the interference bursts are frequent and repetitive set the analyser to zero span and you may be able to turn off one site at a time till the bad spikes disappear. it helps to artifically generate even traffic levels from all remote sites. Try setting a ping -t running on each site.

Make a list of all the sites on the return link together with the cross pol values measured when you lined them up.

For each site in turn, test it by moving the return link frequency to a cross-pol test frequency and measuring the cross pol level. Do this for every site on the list and write all the results down. As you test them, reactivate sites with acceptable cross-pol, exceeding 28 dB.

Now you have a list of bad sites remaining. Contact each one and get them to adjust their polarisation angle while you monitor their CW carrier on the cross-pol frequency. Reactivate sites once they pass.

To avoid the problem in future be more strict with the installers and make them adjust the polarisation to the centre of the narrow null more carefully.

Forward the spectrum plot (fss chart) to me eric@satsig.net for more advice.

Best regards, Eric.

Jan 28^th, 2009 at 6:39pm

Thanks for your email with the spectrum picture. I don't understand now who is supposed to be interfering with whom. It may well be that it is you that is suffering the inferference, not the other way round. You say your system was seriously affected for some time on two separate occasions.

I can see 7 carriers marked that appear on both polarisations at approximately the same level. This suggests to me that they all come from the same uplink site and that that uplink site has its polarisation wrong by about 45 deg and is sending equal level signals into both co-pol and cross-pol transponders. The network operator should contact the uplink hub involved and get them to check their polarisation angle. Maybe they have transmitters on both polarisations and have accidentally switched the tx carriers to both simultaneously. The 7 carriers appear continuous (not TDMA) as they show up clearly with 50mS sweep and max hold does not appear to be on.

It is not clear what services are being interfered with. Maybe there are low level TDMA return link services supposed to be operating where these 7 carriers are located (on one polarisation).

If your TDMA return links are in the above spectrum the individual traffic bursts would look like tiny spikes and need to be recorded in max hold for a while to build up the spectrum envelope. See below.

I suggest you ask the AOC for clarification as to which carriers appear to be causing interference and which carriers appear to be suffering interference. You need to clearly understand which is which to investigate further.

It is very strange that the 7 carriers appear on both polarisations and that there is no hint of actual cross pol interfered with carriers. I can't see any sign of wanted carriers in the slots affected.

Best regards, Eric.