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cause of oscillating SNR?

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Posts: 2
Nov 16th, 2009 at 7:48pm  
any thoughts about what would cause a signal to "pulse" or oscillate  in and out on a well mounted, solidly locked dish?

Watching the SNR in the modem, it can swing from 5.0 to 8.1 SNR in 30 seconds, though I occasionally see wider variations.

This is on a Ku dish with an RX of 11093Mhz.  The dish is located on a military base in Afghanistan, so all ranges of interference are a possibility.  Is there a shot that replacing the LNB, possibly with a PLL LNB will help clean up?

On another dish, on the same base, pointing at a different satellite, I can watch the signal come together on the scope for about 6 seconds, and then turn to noise for another 6 seconds, listening at 11295.  The regularity suggests radar type interference to my inexperienced mind.

Any thoughts or suggestions are welcome!

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Eric Johnston
Senior Member

Posts: 2109
Reply #1 - Nov 17th, 2009 at 11:04am  
If you are operating at low elevation angle (satellite down near the horizon) then tropospheric scintillation may be the reason.  The signal levels go up and down due to the refractive effect of the boundary between layers of moist and dry atmosphere on top of each other.  It is the same effect that makes stars near the horizon twinkle, varying up and down in brightness.

Minimum recommended elevation angles are 5 deg at C band (4/6 GHz) and 10 deg at Ku band (11/14 GHz). Higher angles minimise both scintillation and the path length through rain.

Best regards, Eric.
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« Last Edit: Nov 17th, 2009 at 5:20pm by Admin1 »  
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