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Strange iDirect Problem C band on NSS7

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imepals
Ex Member


May 4th, 2011 at 12:28pm  
The VSAT dish, a 2.4m offset C-band antenna dish (Patriot) on NSS7 (22WL) dropped off the network 2 months ago after more than a year in operation.

Several attempts to re-align this antenna have been unsuccessful.

I have the following equipment on site:
1. iDirect 3100 modem
2. TXFCC-240CSCN † † †2.4M C-Band †Antenna † † †Patriot
3. NJT5669 † † †5W BUC † † †NJR
4. NJS8476S † † †PLL LNB † † †NJR
5. IFL RG6 cable (30m)

The highest Rx signal strength on iSite is 8.3v. This signal strength fluctuates between 0, 0.3, 2.3 and 8.3v in no particular pattern.

I have done the following in trying to isolate what could cause this problem where i cannot get more than 8.3v on iSite:

1. † † †Swapped the entire feed assembly (BUC, LNB, Feed-OMT/Polarizer/Filter/TXRX) with another feed from a working site. it worked perfectly at the 2nd site but the feed from the second site does gives the same volts on iSite at the faulty site)
2. † † †Loaded the option file from this site with issues to another modem at another site (about 1k apart) and had both a Rx and Tx lock.
3. † † †Used cables from a working site to connect the modem at this faulty site but it didn't get more than 8.3v.
4. † † †Swapped the modem from a working site with the modem of the faulty site. None of the modems got more than 8.3v on iSite but both modem had a Rx and Tx lock at the working site.
5. † † †I have used a spectrum analyser and got a clear channel but cannot get the modem to Rx lock.

The site geo statistics for the VSAT location is N 6.445 and E 3.41151.

I am not really sure at this point why I canít get more than 8.3v on iSite. Any further data that maybe useful to resolve this will be provided.

Your input will be of great help.
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« Last Edit: Aug 18th, 2011 at 2:29pm by Admin1 »  
 
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USN - Retired
Ex Member


Reply #1 - May 4th, 2011 at 12:58pm  
Have you considered local EMI/RFI? When intercepting the same signal - and assuming identical configurations at both sites - how do the C/N ratios compare between the two locations?

//greg//
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imepals
Ex Member


Reply #2 - May 4th, 2011 at 3:00pm  
For the EMI/RFI interference, there would have been an indication from the spectrum analyser. I would not have had a clear chhannel on the spectrum.

I don't have a log of the C/N of the faulty site before it went down.
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USN - Retired
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Reply #3 - May 4th, 2011 at 8:36pm  
Quote:
For the EMI/RFI interference, there would have been an indication from the spectrum analyser. I would not have had a clear chhannel on the spectrum.

I don't have a log of the C/N of the faulty site before it went down.
Well that's a shortcoming, but not insurmountable. My point was to compare the level of the noise floor at both sites. If the noise floor is higher at the problem site than at the working site, it's fair to suspect that local EMI/RFI has reduced the C/N at the problem site. It would further explain why swapping in known good equipment failed to remedy the situation.

//greg//
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imepals
Ex Member


Reply #4 - May 5th, 2011 at 2:17pm  
Thanks for your feed back. What would be the remedy if EMI/RFI is suspected?
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Eric Johnston
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Posts: 2107
Reply #5 - May 5th, 2011 at 4:32pm  
Diagnose the interference using spectrum analyser. It may help locate the source if you take the feed/LNB off the dish and hold and point the feed all around towards possible sources, such as radio masts, while monitoring the spectrum analyser.

Consider:

Moving the dish down and hiding it from the interference somehow.
see: http://www.satsig.net/satellite/reducing-interference-satellite-tv.htm

If the interference is not at the same frequency as your wanted signal then a filter might suppress the interferer sufficiently for the LNB to work normally. †Filters can be expensive so don't buy till you really understand the problem. †see:
http://www.microwavefilter.com/c-bandfilters.htm
http://www.microwavefilter.com/tvrointerference.htm

Assuming your wanted carrier in the range 3.7-4.2 GHz, some people have had success by using old type LNBs for 3.7 - 4.2 GHz, rather than the more modern wideband LNBs 3.4 - 4.2 GHz. Local WiMAX interference below 3.7 GHz can saturate the LNB. †If you have co-frequency interference then filters won't work. You need to move the dish a long way or go and visit the source and ask them to change frequency.
wxw
Best regards, Eric.
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« Last Edit: May 6th, 2011 at 10:00am by Admin1 »  
 
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