Editing Medium wave (section)

== Propagation characteristics ==
{{unreferenced section|date=March 2025}}
Wavelengths in this band are long enough that radio waves are not blocked by buildings and hills and can propagate beyond the horizon following the curvature of the Earth; this is called the [[groundwave]].  Practical groundwave reception of strong transmitters typically extends to {{convert|200|-|300|mi|km}}, with greater distances over terrain with higher [[ground conductivity]], and greatest distances over salt water. The groundwave reaches further on lower medium wave frequencies.

Medium waves can also reflect off charged particle layers in the [[ionosphere]] and return to Earth at much greater distances; this is called the [[skywave]].  At night, especially in winter months and at times of low solar activity, the lower ionospheric [[D layer]] virtually disappears. When this happens, MW radio waves can easily be received many hundreds or even thousands of miles away as the signal will be reflected by the higher [[F layer]]. This can allow very long-distance broadcasting, but can also interfere with distant local stations.  Due to the limited number of available channels in the MW broadcast band, the same frequencies are re-allocated to different broadcasting stations several hundred miles apart.  On nights of good skywave propagation, the skywave signals of a distant station may interfere with the signals of local stations on the same frequency.  In North America, the [[North American Regional Broadcasting Agreement]] (NARBA) sets aside certain channels for nighttime use over extended service areas via skywave by a few specially licensed AM broadcasting stations.  These channels are called [[Clear-channel station|clear channels]], and they are required to broadcast at higher powers of 10 to 50&nbsp;kW.