Why AM Stations Must Reduce Power, Change Operations, or Cease Broadcasting at Night

Most AM radio stations are required by the FCC’s rules to reduce their power or cease operating at night in order to avoid interference to other AM stations.  FCC rules governing the daytime and nighttime operation of AM radio stations are a consequence of the laws of physics.  Because of the way in which the relatively long wavelengths (see Footnote 1) of AM radio signals interact with the ionized layers of the ionosphere miles above the earth’s surface, the propagation of AM radio waves changes drastically from daytime to nighttime.  This change in AM radio propagation occurs at sunset due to radical shifts in the ionospheric layers, which persist throughout the night.  During daytime hours when ionospheric reflection does not occur to any great degree, AM signals travel principally by conduction over the surface of the earth.  This is known as “groundwave” propagation.  Useful daytime AM service is generally limited to a radius of no more than about 100 miles (162 km), even for the most powerful stations. 

However, during nighttime hours the AM signals can travel over hundreds of miles by reflection from the ionosphere, a phenomenon called “skywave” propagation.  (Shortwave stations, which operate using AM modulation on several bands between 2.3 MHz and 26.1 MHz, also use this phenomenon to broadcast still greater distances, up to thousands of miles.)  Because of this change in signal propagation from daytime to nighttime, if every AM station kept its daytime operating power at night, massive interference would result. (For a similar description, see Hours of Daytime-Only AM Broadcast Stations, First Report and Order, BC Docket 82-538, 95 FCC 2d 1032 (1983) [ PDF ] and related Click here to read more

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