A construction for shielding an item against interference from a microwave radio frequency radiation is disclosed, having a wall structure adapted to minimize radio frequency interference in the vicinity of an antenna. The wall is composed of a plurality of modular wall panels, each having a layer of electrically conductive materials that substantially reflects interfering electromagnetic microwave signals. The wall structure includes a top construction creating a thin, horizontal gap in the conductive material at the wall's top that acts to control diffraction over the wall. In the preferred embodiment, the layer of electrically conductive material is a mesh of expanded metal embedded in the wall modules. The gap comprises the space between the top of the mesh and a pair of vertically adjustable metal strips mounted on the wall. In the preferred embodiment, the wall panels are precast concrete that may be horizontally positioned with a joint allowing up to 180. degree.