Medium Frequency - Propagation

Propagation

Propagation at MF wavelengths is via ground waves and skywaves. Ground waves follow the curvature of the Earth. At MW wavelengths they can cover a radius of several hundred miles from the transmitter, with longer distances over water and damp earth. MW broadcasting stations use ground waves to cover their listening areas.

MF can also travel longer distances via skywave propagation, in which radio waves radiated at an angle into the sky are reflected (actually refracted) back to Earth by the ionosphere E and F layers. However at certain times the D layer (at a lower altitude than the refractive E and F layers) can be electronically noisy and absorptive of MF 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 activity,

Late 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 on a quiet frequency, but can have the opposite effect in many other cases. For example, due to the limited number of available channels in the MW broadcast band, the same frequencies are re-allocated to different broadcasters provided they transmit several hundred miles apart. On nights of good MF propagation, distant stations may appear superimposed onto local ones causing interference.