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Mysterious take over of TX signal

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Posts: 7
Jan 19th, 2009 at 1:33am  
I previously posted  "Low rx reading at hub, very high tx reading" Dec, 16, 2008. I have returned to Costa Rica and still have the same problem. I gave the installer the suggestions made  by forum members, speciflically someone or something in the local area interfering on the same spectrum as the TDMA upstrean carrier. He contacted the hub and they said it was probably Radar over riding my tx. I live in a rural area and the Radar scenerio does not fit. When the interference over rides my TX the signal is excellant and I am connected and there are constant packets being sent during the day, mostly tx.
My question is what type of interference could create this problem? Is there any type of instrument that can detect where the signal is coming from? Is there some way to block or screen the interference? The installer is mystifiedand the NOC does not seem to help. Any help is desperately needed. Thanks
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Eric Johnston
Senior Member

Posts: 2109
Reply #1 - Jan 19th, 2009 at 11:58am  
Radar interference near to a remote site may interfere with reception at the remote site.  The symptom would be errors in the reception of the outbound carrier and intermittent loss of receive lock at the remote site.   When loss of receive lock occurs at a remote site then remote site transmit path will be disabled autmatically, but the primary symptom is the degradation of the remote site receive path.  Local radar, wireless or other noise does not interfere with the remote site uplink transmission.

In C band there are now local wireless systems which have been permitted in some places.  These interfere with remote site C band reception.

Interference can occur into the return TDMA path to the hub.  This is occurs at the input to the satellite (NOT at your remote site).  It is normally due to other earth stations bad transmitting, poor cross pol adjustment, wrong burst timing, tx stuck on, sidelobe interference etc.  The hub should see this on their spectrum analyser and investigate.

If your reception is good, i.e. green RX LED on all of the time, no errors being recorded on receive, then the problem is with your transmit system.  Check cables. Get the hub to make your site transmit a CW carrier on a test frequency and adjust the BUC power down and then up till the -1 dB compression point is determined. The hub can then put your TX carrier on a clean uplink slot (as they can verify on their analyser) and then test your modulated carrier.

Best regards, Eric.
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Ex Member
Ex Member

Reply #2 - Jan 19th, 2009 at 1:04pm  
Is it possible for you or your installer to talk with your Satellite Internet Provider's Network Operations Center (NOC) and find out exactly what kind of errors they are seeing.   It is somewhat important in assessing your situation.  

Please ask them if they are observing any of the following on your remote (as viewed with *iMonitor at the hub):

1. SCPC Rx Errors
2. HUB CRC errors
3. Downstream C/N low
4. TDM Lost = X
5. Ask them what your Upstream SNR is and what the network UCP is set to for your inroute group....i.e. 9dB C/N TDMA Nominal.

*If you do not know, iMonitor is a network monitoring tool used by Hub Operators to maintain situational awareness on all networks administered by the hub.

As Eric mentioned, it would be within your best interests to have your (mystified) installer request a CW carrier from the an effort to determine your 1dB compression point (point of BUC saturation).  That is important in determining whether or not you have the right equipment (RFT) for the link you are trying to sustain with the Hub.

Good luck with it.

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« Last Edit: Jan 20th, 2009 at 4:01am by N/A »  
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