Why does rain and snow affect signal

[rellstar]

Sorry if this is really a stupid question but why exactly does heavy rain and snow affect the signal from a satellite. Is it the actual cloud blocking the signal or is it rain on the dish / LNB making is harder to process the signal.


UPDATE: Sorry, should have done a search first. I have my answer from another thread.

 

[moosegoose]

it is mostly the rain and snow on the dish which change the surface and perfomance of the dish.

 

[PaulR]

I disagree. It is mostly the water vapour and water particles (rain) in the atmosphere which absorb the signal. It is a similar effect to that of a microwave oven. The radio waves are absorbed by water molecules and raise its energy state (temperature).

You'd be hard pressed to to measure the temperature rise from a satellite (hundreds of watts over thousands of miles) compared to a microwave oven (kilowatts over inches) but it's there.

 

[dxsat]

This explains it:
http://www.geo-orbit.org/sizepgs/Noise.html
In the text is the fact that rain fade is greater the lower the angle of the satellite in the sky, so Sweden it will be worse than in Spain for Astra 2.

 

[moosegoose]

how can u diagree if theres snow on the dish and its not snowing .it can only be snow on the dish the same 4 rain .or are u saying snow and rain on dishes has no effect on the dishes perfomance .i think it could be one or the other or a combination of the two.

 

[Analoguesat]

Water & water vapour is an excellent absorber for the microwave frequencies radiated by the satellites (around 12GHz) Hence a thundercloud heavily laden with rain but out to your south east can interrupt the signals even if its not actually raining at your own location.

Snow on the dish is just frozen water so it will absorb signals bouncing off the dish.

Remember the satellites are not transmitting high power signals. The signals radiated from the birds have the effective power of a lightbulb, so it doesnt take much to knock out the signal

 

[PaulR]

are u saying snow and rain on dishes has no effect on the dishes perfomance.

No, what I said is that it's mostly the water in the atmosphere that causes the loss.

Obviously if you have some snow on the dish it will affect the signal but it will be miniscule. Snow covering the LNB feedhorn will actually have more of an effect as the signal will have been concentrated in this area but LNBs usually have their face nearly vertical so snow doesn't stick for very long.

 

[moosegoose]

ok paul thanks i never stop learning