Editing Transmission medium (section)

==== Radio ====
{{main|Radio propagation}}

''Radio propagation'' is the behavior of [[radio wave]]s as they travel, or are [[wave propagation|propagated]], from one point to another, or into various parts of the [[atmosphere]].<ref>H. P. Westman et al., (ed), ''Reference Data for Radio Engineers, Fifth Edition'', 1968, Howard W. Sams and Co., {{ISBN|0-672-20678-1}}, Library of Congress Card No. 43-14665 page 26-1</ref> As a form of [[electromagnetic radiation]], like light waves, radio waves are affected by the phenomena of [[reflection (physics)|reflection]], [[refraction]], [[diffraction]], [[absorption (electromagnetic radiation)|absorption]], [[polarization (waves)|polarization]], and [[scattering]].<ref>Demetrius T Paris and F. Kenneth Hurd, ''Basic Electromagnetic Theory'', McGraw Hill, New York 1969 {{ISBN|0-07-048470-8}}, Chapter 8</ref>   Understanding the effects of varying conditions on radio propagation has many practical applications, from choosing frequencies for international [[shortwave]] [[Broadcasting|broadcasters]], to designing reliable [[mobile telephone]] systems, to [[radio navigation]], to operation of [[radar]] systems.

Different types of propagation are used in practical radio transmission systems. [[Line-of-sight propagation]] means radio waves that travel in a straight line from the transmitting antenna to the receiving antenna. Line of sight transmission is used to medium-range radio transmission such as [[cell phone]]s, [[cordless phone]]s, [[walkie-talkie]]s, [[wireless network]]s, [[FM radio]] and [[television broadcasting]] and [[radar]], and [[satellite communication]], such as [[satellite television]]. Line-of-sight transmission on the surface of the Earth is limited to the distance to the visual horizon, which depends on the height of transmitting and receiving antennas. It is the only propagation method possible at [[microwave]] frequencies and above. At microwave frequencies, moisture in the atmosphere ([[rain fade]]) can degrade transmission.

At lower frequencies in the [[medium frequency|MF]], [[low frequency|LF]], and [[VLF]] bands, due to [[diffraction]] radio waves can bend over obstacles like hills, and travel beyond the horizon as [[surface wave]]s which follow the contour of the Earth. These are called [[ground wave]]s.  [[AM broadcasting]] stations use ground waves to cover their listening areas. As the frequency gets lower, the attenuation with distance decreases, so [[very low frequency]] (VLF) and [[extremely low frequency]] (ELF) ground waves can be used to communicate worldwide. VLF and ELF waves can penetrate significant distances through water and earth, and these frequencies are used for mine communication and military communication with submerged submarines.

At [[medium wave]] and [[shortwave]] frequencies ([[medium frequency|MF]] and [[high frequency|HF]] bands) radio waves can refract from a layer of [[charged particle]]s ([[ion]]s) high in the atmosphere, called the [[ionosphere]]. This means that radio waves transmitted at an angle into the sky can be reflected back to Earth beyond the horizon, at great distances, even transcontinental distances. This is called [[skywave]] propagation. It is used by [[amateur radio]] operators to talk to other countries and shortwave broadcasting stations that broadcast internationally. Skywave communication is variable, dependent on conditions in the upper atmosphere; it is most reliable at night and in the winter. Due to its unreliability, since the advent of [[communication satellite]]s in the 1960s, many long-range communication that previously used skywaves now use satellites.

In addition, there are several less common radio propagation mechanisms, such as [[tropospheric scattering]] (troposcatter) and [[near vertical incidence skywave]] (NVIS) which are used in specialized communication systems.