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I would like to know how good or bad behave a metallic wall in stopping the propagation of an microwave signal.

To be practical, let's take the example of a GSM relay antenna. If I set up the perpendicular metallic wall between me and the signal sender, will the wall stops the signal like a plank does it in front of a light,

enter image description here

or more like a plank in a mountain river with only a very small protected area just behind the plank ?

enter image description here

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The second diagram is closer -see Knife Edge Effect

If the GSM antennae is further away the angles won't be as obtuse as shown - so the "protected area" will be even smaller.

Apart from that the metal wall will be less effective than shown in the second diagram as there are probably reflections from other surrounding buildings, walls and other objects.

Knife-Edge effect

See Radio Propagation Models and VHF/UHF/Microwave Radio Propagation

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