Editing Radio propagation (section)

==Surface modes (groundwave)==
{{Main|Ground wave}}
[[File:Groud Wave Propagation.svg|alt=Ground Wave Propagation|thumb|171x171px|Ground wave propagation]]
Lower frequency (between 30 and 3,000&nbsp;kHz) [[vertical polarization|vertically polarized]] radio waves can travel as [[surface wave]]s following the contour of the Earth; this is called ''ground wave'' propagation. 

In this mode the radio wave propagates by interacting with the conductive surface of the Earth. The wave "clings" to the surface and thus follows the curvature of the Earth, so ground waves can travel over mountains and beyond the horizon. Ground waves propagate in [[polarization (waves)|vertical polarization]] so vertical antennas ([[monopole antenna|monopoles]]) are required. Since the ground is not a perfect electrical conductor, ground waves are [[attenuation (electromagnetic radiation)|attenuated]] as they follow the Earth's surface. Attenuation is proportional to frequency, so ground waves are the main mode of propagation at lower frequencies, in the [[medium frequency|MF]], [[low frequency|LF]] and [[very low frequency|VLF]] bands. Ground waves are used by [[radio broadcasting]] stations in the MF and LF bands, and for [[time signal]]s and [[radio navigation]] systems.

At even lower frequencies, in the [[very low frequency|VLF]] to [[extremely low frequency|ELF]] bands, an [[Earth-ionosphere waveguide]] mechanism allows even longer range transmission. These frequencies are used for secure [[military communications]]. They can also penetrate to a significant depth into seawater, and so are used for one-way military communication to submerged submarines.

Early long-distance radio communication ([[wireless telegraphy]]) before the mid-1920s used low frequencies in the [[longwave]] bands and relied exclusively on ground-wave propagation. Frequencies above 3&nbsp;MHz were regarded as useless and were given to hobbyists ([[radio amateur]]s). The discovery around 1920 of the ionospheric reflection or [[skywave]] mechanism made the [[medium wave]] and [[short wave]] frequencies useful for long-distance communication and they were allocated to commercial and military users.<ref>
{{cite book
 |first = Clinton B. |last=DeSoto
 |year  = 1936
 |title = 200&nbsp;meters & Down - The Story of Amateur Radio
 |pages = 132–146
 |publisher = The [[American Radio Relay League]]
 |place = Newington, CT
 |isbn = 0-87259-001-1
}}</ref>