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Shielding vsat signal from microwave interference

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Ex Member
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Jan 28th, 2009 at 11:56pm  
Hi again this is my 3rd post and hopefully the last one.
The installer and NOC have been telling me my problem is a weak RX signal and that it could only be cured by a stronger RX signal. I have argued for several months that the reception is perfect except during daylight hours when the signal is interupted. Even a so call local expert said it could only be a weak signal or, believe it or not, a worm or trojan. The installer spent several hours yesterday increasing the signal, told me the trouble was in the feedhorn and buc which he replaced. He left at night and the first thing this morning the  same interferance occured.

From information I received from the forum from TDMA Mike and Eric I followed up on their suggestions and researchd the problem. This is in Costa Rica and the state run Internet is terrestrial microwave. I definitely know this is the source of interference. I know the general direction it is coming from. It is very rural and there are higher elevations from line of site to my antenna but I have not been able to locate the tower.
I have told them this repeatedly but they all think it is impossible, until now.

I have a copy of  Eric's article on site shielding and experimented with a shield that partially obstructed the signal but not eough to block it totally. What I need to know is can I use tin roofing for a shield, or possibly 4x8 sheets of concrete fibre siding? The choice of radio absorbing material is very limited here.

Any suggestions would be greatly appreciated.
Thanks, Zia77

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Eric Johnston
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Reply #1 - Jan 29th, 2009 at 8:38am  
Hold a sheet of tin roofing near the antenna, on either side, in front and behind and at various angles. †Try to direct more interference into the dish. †Try reflecting the interference away from the dish. †Take care not to actually obstruct the beam to/from the satellite. †If the signal quality does not change then the problem is not local interference. †If the signal quality varies up and down then it is local interference.

Interference that comes on during the day may be due to some machinery or electrical equipment nearby perhaps. †Try investigating all electrical equipment nearby (say within 50m) that is powered on for the daytime (e.g. fans, air conditioning, electric neon signs etc). †Does the interference occur at weekends ?

Microwave links are normally on 24hr. †Does the interference instantaneously come on and later instantly turn off ? † Or is it a slow gradual change from good quality to failed and back again ? † If very slow it could be that you are pointed at an inclined orbit satellite and it is moving and going out of the beam once or twice each day.

Reference: site shielding

Complete loss of the receive signal during the day may be due to a faulty LNB whose local oscillator stops or jumps in frequency when it warmed by the sun.

Best regards, Eric.
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Reply #2 - Jan 30th, 2009 at 4:50pm  
Hi This is Bentley Walker in the UK , what Satellite are you on and where and you located and what system have you got ?

One solotion could be to use a lower or higher frequency Network this can at a stroke cure your issue , for example if the interference is beating at the same band as your Network then using a lower or higher Band Network can usually move you away from the TI , this is why we have 2 HX Networks in ME especially for this reason , please contact me if you want to know more Anthony@bentleywalker.com
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Reply #3 - Jan 31st, 2009 at 1:04pm  
Hello;
     I would like to ask some questions that might understand the situation more. What is the type of antenna you are using? Single or dual reflector? And if possible to tell us the antenna manufacturer. Did you try using filter on the waveguide? Did you try to check the signal (while your transmit is off)? And besides, what are you/ your installer using for the lineup? Are you checking if you are on correct polarity?
     By the way, I am sorry as I donít have a background on your other posts, so is the (week RX) is in your side or on Teleport/HUB side? If it is at your side and when your installer changed feedhorn and BUC, did you also tried to change the LNB as well?
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Reply #4 - Mar 11th, 2009 at 7:18am  
Hi!!!

I think i am having a similar problem. My service suddenly went down and i had my sat guy check out everything with the guyz at noc. i seem to have a very low rx snr of about 4.9-5.0 and my voltage reading in isite when pointing is around 13.5. The guys at noc and my sat guy insist there is a local inteference.

My dish is a 1.8 mtr dish and i located on a 15ft tower, due to limitations on space. Now my sat guy i suggesting i relocate the dish, but i have very serious space constraints.

Is there any thing possible i can do???? I think we are on intelsat...

thanks alot
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Reply #5 - Mar 11th, 2009 at 11:15am  
If your provider is at "contract power" and your installer is telling you that you are peaked and pol'ed properly then you may may need to consider a larger aperture to see that carrier.  That or consider another provider with a stronger carrier (better link budget) ]. 

Did the installer look at the Ku RF signal on your systems (prior to LO translation?)?  What is observed at 4.9-5.0 above the noise floor in RF? Just wanted to ensure that he is seeing like values for C/N in RF and L-Band.

Naturally 5dB of SNR wont get the job done.  Your transmitter will remain muted below 5.5.  You realisticlly should be better than 9-10dB for error free trasnmissions.
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