Ferrite rod aerials (or antennas) are a type of small magnetic loop (SML) antenna[3][4] very common in AM radio broadcast band transistor radios, although they began to be used in vacuum tube ("valve") radios in the 1950s. They are also useful in very low frequency (VLF) receivers,[5] and can sometimes give good results over most of the shortwave frequencies (assuming a suitable ferrite is used). They consist of a coil of wire wound around a ferrite rod core (usually several inches longer than the coil, but sometimes over 3 feet long[6]).