cause of oscillating SNR?

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cpugrud

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afghanistan

Nov 16^th, 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!

Chris

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Eric Johnston

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Reply #1 - Nov 17^th, 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.

« Last Edit: Nov 17^th, 2009 at 5:20pm by Admin1 »

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