Shortwave bands are frequency allocations for use within the shortwave radio spectrum the upper MF band and all of the HF band. They are the primary medium for applications such as maritime communications, international broadcasting and worldwide amateur radio activity because they take advantage of ionospheric skip propagation to send data around the world. The bands are conventionally stated in wavelength , measured in metres. Propagation behavior on the shortwave bands depends on the time of day, the season and the level of solar activity. The most recent WRC took place in At WRC in , the following bands were allocated for international broadcasting :.