Editing Radio propagation (section)

{{Short description|Behaviour of travelling radio waves}}{{for|the journal|Radio Propagation (journal)}}
{{Antennas|characteristics}}{{use dmy dates|date=December 2020}}

'''Radio propagation''' is the behavior of [[radio wave]]s as they travel, or are [[wave propagation|propagated]], from one point to another in [[vacuum]], or into various parts of the [[atmosphere]].<ref name=Westman-1968>
{{cite book
 |editor1-first=H.P. |editor1-last=Westman
 |display-editors=etal
 |year=1968
 |title=Reference Data for Radio Engineers
 |edition=Fifth 
 |publisher=Howard W. Sams and Co.
 |isbn=0-672-20678-1 |lccn=43-14665
}}</ref>{{rp|page=26‑1}} 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>
{{cite book
 |first1=Demetrius T. |last1=Paris
 |first2=F. Kenneth   |last2=Hurd
 |name-list-style=and
 |year=1969
 |title=Basic Electromagnetic Theory
 |at=Chapter&nbsp;8
 |publisher=McGraw Hill
 |place=New York, NY
 |isbn=0-07-048470-8
}}</ref> Understanding the effects of varying conditions on radio propagation has many practical applications, from choosing frequencies for [[amateur radio]] communications, international [[shortwave]] [[Broadcasting|broadcasters]], to designing reliable [[Mobile phone|mobile telephone]] systems, to [[radio navigation]], to operation of [[radar]] systems.

Several different types of propagation are used in practical radio transmission systems. ''[[Line-of-sight propagation]]'' means radio waves which travel in a straight line from the transmitting antenna to the receiving antenna. Line of sight transmission is used for medium-distance radio transmission, such as [[cell phone]]s, [[cordless phone]]s, [[walkie-talkie]]s, [[wireless network]]s, [[FM radio]], [[television broadcasting]], [[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.{{efn|At microwave frequencies, moisture in the atmosphere ([[rain fade]]) can degrade transmission.}}

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

At [[medium wave]] and [[shortwave]] frequencies ([[medium frequency|MF]] and [[high frequency|HF]] bands), radio waves can refract from the [[ionosphere]], a layer of [[Charged particle|charged particles]] ([[Ion|ions]]) high in the atmosphere. This means that medium and short radio waves transmitted at an angle into the sky can be refracted back to Earth at great distances beyond the horizon – even transcontinental distances. This is called ''[[skywave]] propagation''. It is used by [[amateur radio]] operators to communicate with operators in distant countries, and by [[List of shortwave radio broadcasters|shortwave broadcast stations]] to transmit internationally.{{efn|Skywave communication is variable: It depends on conditions in the [[ionosphere]]. Long distance shortwave transmission is most reliable at night and during the winter. Since the advent of [[communication satellite]]s in the 1960s, many long range communication needs that previously used skywaves now use satellites and [[submarine communications cable|submerged cables]], to avoid dependence on the erratic performance of skywave communications.}}

In addition, there are several less common radio propagation mechanisms, such as ''[[tropospheric scattering]]'' (troposcatter), ''[[Atmospheric duct|tropospheric ducting]]'' (ducting) at VHF frequencies and ''[[near vertical incidence skywave]]'' (NVIS) which are used when HF communications are desired within a few hundred miles.