With mobile radio, the radio link may be interrupted if there is no direct line of sight (LOS) between the transmitter
and the receiving device. A line of sight may be impaired by higher buildings or ground elevation. This is especially true if the structure is metal, such as a steel frame building. Underground tunnels or bridges of
steel construction also have the same effect, known as radio shadowing or shadowing. Shadowing also occurs in studio use when using wireless microphones, and occurs when the radio link between the transmitter and the radio microphone is impaired by walls or studio structures.