Sign off

Medium-wave AM

Medium wave radio is a special case due to its unusual propagation characteristics; it can bounce hundreds of miles by reflecting from the upper atmosphere at night, but during the day these same layers absorb signal instead of reflecting. A few powerful regional clear-channel stations have an extensive secondary coverage area which is protected by having smaller local co-channel stations in distant communities sign off shortly before sunset. A frequency on which a broadcaster has to drastically reduce power or sign off entirely at sunset was traditionally the least desirable assignment, which would usually go to small or new-entrant stations when all of the more favourable slots were already allocated.