An electromagnetic wave diffraction device for fixing onto an outer wall face comprising a plurality of electrically conductive resonant elements having an L-shaped profile fixed parallel on the outer face. Each element comprises a first and second wall secured at right angles to one another along a common edge. The first wall is fixed at a right angle to the outer face by a fixing edge parallel to the common edge. The second wall has a free edge parallel to the common edge. The free edges of all elements are parallel and arranged on the same side relative to the common edge of the corresponding element. A weather protection arrangement for reinforcing the protection of a capacitive area generated in a space between the outer face and the second wall, in the form of a water impermeable dielectric material panel, is fixed to the outer face and covers the elements.