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Low Eb/No

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Sep 24th, 2007 at 9:18am  
Hi all, I have these sites with 2.4M dish installed, The satellites are Intelsat 904 and 906. C-band, with Ipsat modems. On both sites, the Eb/No. is no greater than 5.2 db. i and almost 3 other enngineers have been there to fine tune the dish but to no avail. i suspect a footprint issue( maybe the rx power in Lagos (6.45N, 3.47E) is low), problem is that at times, the Eb./No drops, and the modem goes in and out of lock.
pls advice
Daniel
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Reply #1 - Sep 24th, 2007 at 9:41am  
Wheres the site in Lagos??U never can tell there may be a microwave source affecting ur installation.
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Reply #2 - Sep 24th, 2007 at 12:59pm  
Don't conclude yet, there must be an interference like gsm radar or any kind of microwave interference. were you able to get the maximum signal strength in your spectrumeter or does it flunctuate?
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Reply #3 - Sep 24th, 2007 at 10:07pm  
Check for GSM cell sites for microwave interference, but if there isnt any one there, then call your provider and be sure if 2.4m Antenna can actually see Intelsat 904, because my last installation on Intelsat 904 is on 4.9m Antenna.
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Reply #4 - Sep 25th, 2007 at 5:33pm  
Thanks all for your responses. The site is located close to the Lagos state secretariat in Ikeja.The signal flunctuates from 4.2 to 5.2 db on the Modem,as regarding the dish size, i also have two other 2.4M dish installations on the int 904 satellite with Eb/No of 7db.
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Reply #5 - Sep 27th, 2007 at 11:00am  
Install a cband interference filter.If this doesnt work switch ur lnb i.e if u have temp 15k try with lnb of temp 25k.Good luck
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Reply #6 - Oct 2nd, 2007 at 10:53pm  
It's difficult to follow your train of thought, forgive me. Can you compare antennae? Point the antenna that is not peforming to specification to your known good source,which is the satellite resulting in Eb/No of 7dB. As they are also 2.4 meter antennae, you should have a relatively close Eb/No value on all 3 antennae using the same satellite carrier as your reference carrier. If you see your signal reduced by the 2dB, which you seem to be seing check the antenna for deformation, perhaps you're using an old school mesh antenna and comparing it to a state of the art moldedantenna. If the antennae are the same, that's even better. Was the reflector damaged/deformed in some way? Is the focal length the same? A simple tape measure, a piece of string will help you answer these most basic questions. Is the feedhorn damaged in some way? Are you incorrectly using a linear polarized feedhorn, when you should be using a circularly polarized feedhorn? Do you have condensation (moisture) in the feedhorn? Several years ago I flew out on a very perplexing issue that did not add up. I had to see for myself, you know what I mean? I was being told that identical feed assemblies were resulting in a 2dB delta in performance. I visited the site convinced I would find the problem, and I did. A close look at the throat of the feedhorn revealed a very small machining issue during fabrication. The metal sliver protruding into the throat resulted in an almost perfect 2dB attenuator. I removed the metal sliver, and the feed then performed as expected. Often times it's the easy stuff that gets overlooked.

In your case, perhaps what explains the difference in Eb/No is something as simple as the data rate and modulation scheme not matching. Curious, are the 2 satellite carriers that you are comparing the same which is to say the same data rate, FEC 1/2, 3/4, 7/8's or whatever? If not, this will also result in the Eb/No not matching assuming everything else is equal.

Good luck!
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