Editing Medium frequency (section)

==Propagation==
Radio waves at MF wavelengths propagate via [[ground wave]]s and reflection from the [[ionosphere]] (called [[skywave]]s).<ref name="Seybold">{{cite book |last1=Seybold |first1=John S. |title=Introduction to RF Propagation |publisher=John Wiley and Sons |date=2005 |pages=55–58 |url=https://books.google.com/books?id=4LtmjGNwOPIC&q=cross+polarization+discrimination&pg=PA57 |isbn=0471743682}}</ref> Ground waves travel just above the earth's surface, following the terrain. At these wavelengths, they can bend ([[diffraction|diffract]]) over hills, and travel beyond the [[visual horizon]], although they may be blocked by mountain ranges. Ground waves are progressively absorbed by the Earth, so the signal strength decreases exponentially with distance from the transmitting antenna. Typical MF radio stations can cover a radius of several hundred kilometres/miles from the transmitter, with longer distances over water and damp earth.<ref name="ips.gov.au">{{cite web|title=Ground wave MF and HF propagation |url=https://www.sws.bom.gov.au/Category/Educational/Other%20Topics/Radio%20Communication/Intro%20to%20HF%20Radio.pdf|work=Introduction to HF Propagation|publisher=IPS Radio and Space Services, Sydney Australia|access-date=27 September 2010}}</ref> MF [[Radio station|broadcasting stations]] use ground waves to cover their listening areas.

MF waves can also travel longer distances via [[skywave]] propagation, in which radio waves radiated at an angle into the sky are [[Refraction|refracted]] back to Earth by layers of charged particles ([[ion]]s) in the [[ionosphere]], the [[E layer|E]] and [[F layer]]s. However, at certain times the D layer (at a lower altitude than the refractive E and F layers) can be electronically noisy and absorb MF radio waves, interfering with skywave propagation. This happens when the ionosphere is heavily ionised, such as during the day, in summer and especially at times of high [[Solar variation|solar activity]].

At night, especially in winter months and at times of low solar activity, the ionospheric D layer can virtually disappear. When this happens, MF radio waves can easily be received hundreds or even thousands of miles away as the signal will be refracted by the remaining F layer. This can be very useful for long-distance communication, but can also interfere with local stations. Because of 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 signals of distant stations may reflect off the ionosphere and interfere with the signals of local stations on the same frequency. 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 channels'', and the stations, called ''[[clear-channel station]]s'', are required to broadcast at higher powers of 10 to 50&nbsp;kW.