<P> In radio communication, skywave or skip refers to the propagation of radio waves reflected or refracted back toward Earth from the ionosphere, an electrically charged layer of the upper atmosphere . Since it is not limited by the curvature of the Earth, skywave propagation can be used to communicate beyond the horizon, at intercontinental distances . It is mostly used in the shortwave frequency bands . </P> <P> As a result of skywave propagation, a signal from a distant AM broadcasting station, a shortwave station, or--during sporadic E propagation conditions (principally during the summer months in both hemispheres) a distant VHF FM or TV station can sometimes be received as clearly as local stations . Most long - distance shortwave (high frequency) radio communication--between 3 and 30 MHz--is a result of skywave propagation . Since the early 1920s amateur radio operators (or "hams"), limited to lower transmitter power than broadcast stations, have taken advantage of skywave for long distance (or "DX") communication . </P>

The radio waves which are received after reflection from ionosphere are called