The MSF radio signal has a very long wavelength (5 km). You may know that FM radio and mobile telephones, which operate at wavelengths around 1 m, can be used even inside a steel-framed building or under a gantry or power cable. These signals penetrate because their wavelength is small, or comparable to the dimensions of the metalwork. However even these signals can be stopped by a metal mesh in the walls and windows to give deliberate screening. Because of the much larger wavelength of the Rugby signal the steel frame of a building, or an overhead gantry or power cable, can act as a screening mesh and greatly reduce the signal strength nearby.