A substrate integrated waveguide (10) comprises a top conductive layer (14) and a bottom conductive layer (15) provided on either sides a substrate (11). At least one wall (12, 13) of conductive material is provided in the substrate (11) to define, together with the top and bottom layers (14, 15), the waveguide. The at least one wall (12, 13) comprise a multitude of thin conductive wires densely arranged close to each other in the substrate (11) and having respective short ends connected to the top and bottom layers (14, 15). The high number of wires per surface unit in the wall (12, 13) effectively prevent significant amount of power leakage through the wall (12, 13) during operation of the substrate integrated waveguide (10).