Disclaim: I am a plumber, not a builder. That being said, I have seen foundations for a number of various installations. The most important thing you need to know is the frost line for the area you are working in. Here in Michigan, the frost line is 42" below grade. The reason you need to be below the frost line is that with changing temperatures, the ground can shift and move your footing.
Based on what I have seen for transition satellite dishes (from a radio station), I would suggest you have a concrete pad 11' x 11' or 4m x 4m. The depth should exceed the frost line by at least 6". Here is Michigan we would use 50" depth.
Yes, that is A LOT of concrete. But the thing will not move!