View source for Radio propagation

{{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.

==Frequency dependence==
At different frequencies, radio waves travel through the atmosphere by different mechanisms or modes:<ref name="Seybold">
{{cite book
 | last1  = Seybold  | first1 = John S. 
 | date   = 2005
 | title  = Introduction to RF Propagation
 | pages  = 3–10
 | publisher = John Wiley and Sons
 | language = 
 | url    = https://books.google.com/books?id=4LtmjGNwOPIC&pg=PA6
 | isbn   = 0471743682
}}
</ref>
{{clear}}
{| class="wikitable" style="margin:0.5em auto"
|+ Radio frequencies and their primary mode of propagation
|-
! colspan=2 | Band
! Frequency
! Wavelength
! Propagation via
|-
| [[Extremely low frequency|ELF]]
| Extremely Low Frequency
| 3–30 [[hertz|Hz]]
| 100,000–10,000&nbsp;km
| Guided between the Earth and the [[D layer]] of the ionosphere.
|-
| [[Super low frequency|SLF]]
| Super Low Frequency
| 30–300 [[hertz|Hz]]
| 10,000–1,000&nbsp;km
| Guided between the Earth and the [[ionosphere]].
|-
| [[Ultra low frequency|ULF]]
| Ultra Low Frequency
| 0.3–3&nbsp;[[kilohertz|kHz]]<br/>(300–3,000&nbsp;Hz)
| 1,000–100&nbsp;km
| Guided between the Earth and the [[ionosphere]].
|-
| [[Very low frequency|VLF]]
| Very Low Frequency
| 3–30&nbsp;[[kilohertz|kHz]]<br/>(3,000–30,000&nbsp;Hz)
| 100–10&nbsp;km
| Guided between the Earth and the [[ionosphere]].
[[Ground wave|Ground waves]].
|-
| [[Low frequency|LF]]
| Low Frequency
| 30–300&nbsp;[[kilohertz|kHz]]<br/>(30,000–300,000&nbsp;Hz)
| 10–1&nbsp;km
| Guided between the Earth and the ionosphere.
[[Ground wave|Ground waves]].
|-
| [[Medium frequency|MF]]
| Medium Frequency
| 300–3,000&nbsp;[[kilohertz|kHz]]<br/>(300,000–3,000,000&nbsp;Hz)
| 1000–100&nbsp;m
| [[Ground wave|Ground waves]].
E, [[F layer]] ionospheric refraction at night, when D layer absorption weakens.
|-
| [[High frequency|HF]]
| High Frequency ([[shortwave|Short Wave]])
| 3–30&nbsp;[[megahertz|MHz]]<br/>(3,000,000–30,000,000&nbsp;Hz)
| 100–10&nbsp;m
| [[E layer]] ionospheric refraction.
F1, [[F2 propagation|F2]] layer ionospheric refraction.
|-
| [[Very high frequency|VHF]]
| Very High Frequency
| 30–300&nbsp;[[megahertz|MHz]]<br/>(30,000,000–<br/>&nbsp; &nbsp; 300,000,000&nbsp;Hz)
| 10–1 m
| [[Line-of-sight propagation]]. 
Infrequent [[Sporadic E propagation|E ionospheric (E<sub>s</sub>) refraction]]. Uncommonly [[F2 propagation|F2]] layer ionospheric refraction during high sunspot activity up to 50&nbsp;MHz and rarely to 80&nbsp;MHz. Sometimes [[tropospheric ducting]] or [[Meteor burst communications|meteor scatter]]
|-
| [[Ultra high frequency|UHF]]
| Ultra High Frequency
| 300–3,000&nbsp;[[megahertz|MHz]]<br/>(300,000,000–<br/>&nbsp; &nbsp; 3,000,000,000&nbsp;Hz)
| 100–10&nbsp;cm
| [[Line-of-sight propagation]]. Sometimes [[tropospheric ducting]].
|-
| [[Super high frequency|SHF]]
| Super High Frequency
| 3–30 [[gigahertz|GHz]]<br/>(3,000,000,000–<br/>&nbsp; &nbsp; 30,000,000,000&nbsp;Hz)
| 10–1&nbsp;cm
| [[Line-of-sight propagation]]. Sometimes [[Radio propagation#Rain scattering|rain scatter]].
|-
| [[Extremely high frequency|EHF]]
| Extremely High Frequency
| 30–300&nbsp;[[gigahertz|GHz]]<br/>(30,000,000,000–<br/>&nbsp; &nbsp; 300,000,000,000&nbsp;Hz)
| 10–1&nbsp;mm
| [[Line-of-sight propagation]], limited by atmospheric absorption to a few kilometers (miles)
|-
| [[Tremendously high frequency|THF]]
| Tremendously High frequency
| 0.3–3&nbsp;[[Terahertz (unit)|THz]]<br/>(300,000,000,000–<br/>&nbsp; &nbsp; 3,000,000,000,000&nbsp;Hz)
| 1–0.1&nbsp;mm
| [[Line-of-sight propagation]], limited by atmospheric absorption to a few meters.<ref name=Coutaz>{{cite book
 | last1  = Coutaz  | first1 = Jean-Louis
 | last2  = Garet   | first2 = Frederic
 | last3  = Wallace | first3 = Vincent P.
 | year   = 2018
 | title  = Principles of Terahertz Time-Domain Spectroscopy: An introductory textbook
 | publisher = CRC Press  |place = Boca Raton, FL
 | page   = 18
 | isbn   = 9781351356367
 | url    = https://books.google.com/books?id=zah8DwAAQBAJ&pg=PA18
}}
</ref><ref name=Siegel>
{{cite web
  | last = Siegel | first = Peter
  | year = 2002
  | title = Studying the Energy of the Universe
  | series = Education materials
  | website = [[National Aeronautics and Space Administration]] (nasa.gov)
  | url = https://www.nasa.gov/audience/foreducators/k-4/features/Peter_Siegel.html
  | access-date = 19 May 2021
}}
</ref>
|- style="background-color:lightgrey;"
| [[Far infrared|FIR]]
| Far infrared light<br/>(overlaps radio)
| 0.3–20&nbsp;[[Terahertz (unit)|THz]]<br/>(300,000,000,000–<br/>&nbsp; &nbsp; 20,000,000,000,000&nbsp;Hz)
| 1,000–150&nbsp;μm<ref name=Byrnes>
{{cite book
  |last=Byrnes |first=James
  |year=2009
  |title=Unexploded Ordnance Detection and Mitigation
  |publisher=Springer
  |isbn=978-1-4020-9252-7
  |pages=[https://archive.org/details/unexplodedordnan00abry/page/n29 21]–22
  |url=https://archive.org/details/unexplodedordnan00abry |url-access=limited
}}
</ref><ref name=Glagoleva>
{{cite journal
  |first=A. |last=Glagoleva-Arkadiewa |author-link=Alexandra Glagoleva-Arkadieva 
  |year=1924
  |title=Short electromagnetic waves of wave-length up to 82&nbsp;Microns
  |journal=[[Nature (journal)|Nature]]
  |volume=2844 |issue=113
  |doi=10.1038/113640a0
|doi-access=free}}
</ref><ref name=caltech>
{{cite web
  |title=Near, mid and far-infrared
  |series=Caltech Infrared Processing and Analysis Center
  |publisher=[[California Institute of Technology]]
  |url=http://www.ipac.caltech.edu/outreach/Edu/Regions/irregions.html
  |access-date=2013-01-28 |url-status=dead
  |archive-url=https://archive.today/20120529/http://www.ipac.caltech.edu/Outreach/Edu/Regions/irregions.html
  |archive-date=2012-05-29
}}
</ref>
| [[Line-of-sight propagation]], mostly limited by atmospheric absorption to a few meters.<ref name=Byrnes/><ref name=caltech/>
|}

==Free space propagation==
{{further|Free-space path loss}}

In [[free space]], all [[electromagnetic wave]]s (radio, light, X-rays, etc.) obey the [[inverse-square law]] which states that the power density <math>\rho\,</math> of an electromagnetic wave is proportional to the inverse of the square of the distance <math>r\,</math> from a [[point source]]<ref name=Westman-1968/>{{rp|page=26‑19}} or:

:<math>\rho \propto \frac{1}{r^2}~.</math>

At typical communication distances from a transmitter, the transmitting antenna usually can be approximated by a point source. Doubling the distance of a receiver from a transmitter means that the power density of the radiated wave at that new location is reduced to one-quarter of its previous value.

The power density per surface unit is proportional to the product of the electric and magnetic field strengths. Thus, doubling the propagation path distance from the transmitter reduces each of these received field strengths over a free-space path by one-half.

Radio waves in vacuum travel at the [[speed of light]]. The Earth's atmosphere is thin enough that radio waves in the atmosphere travel very close to the speed of light, but variations in density and temperature can cause some slight [[refraction]] (bending) of waves over distances.

==Direct modes (line-of-sight)==
{{Main|Line-of-sight propagation}}
[[Line-of-sight propagation|Line-of-sight]] refers to radio waves which travel directly in a line from the transmitting antenna to the receiving antenna, often also called direct-wave. It does not necessarily require a cleared sight path; at lower frequencies radio waves can pass through buildings, foliage and other obstructions. This is the most common propagation mode at [[VHF]] and above, and the only possible mode at [[microwave]] frequencies and above. On the surface of the Earth, line of sight propagation is limited by the [[horizon|visual horizon]] to about {{convert|40|mi|km}}. This is the method used by [[cell phone]]s,{{efn|Cellular networks function even without a single clear line-of-sight by relaying signals along multiple line-of-sight paths through cell towers.}} [[cordless phone]]s, [[walkie-talkie]]s, [[wireless network]]s, point-to-point [[microwave radio relay]] links, [[FM broadcasting|FM]] and [[television broadcasting]] and [[radar]]. [[Satellite communication]] uses longer line-of-sight paths; for example home [[satellite dish]]es receive signals from communication satellites {{convert|22,000|mi|km}} above the Earth, and [[satellite ground station|ground stations]] can communicate with [[spacecraft]] billions of miles from Earth.

[[Ground plane]] [[Reflection (physics)|reflection]] effects are an important factor in VHF line-of-sight propagation. The interference between the direct beam line-of-sight and the ground reflected beam often leads to an effective inverse-fourth-power {{nowrap|({{frac|1|distance<sup>4</sup>}})}} law for ground-plane limited radiation.{{citation needed|date=December 2020|reason=Need reference to inverse-fourth-power law + ground plane. Drawings may clarify.}}

==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>

==Non-line-of-sight modes==
{{excerpt|Non-line-of-sight propagation}}

==Measuring HF propagation==
HF propagation conditions can be simulated using [[radio propagation model]]s, such as [[VOACAP|the Voice of America Coverage Analysis Program]], and realtime measurements can be done using [[Chirp Transmitter|chirp transmitters]]. For radio amateurs the [[WSPR (amateur radio software)|WSPR mode]] provides maps with real time propagation conditions between a network of transmitters and receivers.<ref>
{{cite web
 |title=WSPR propagation conditions
 |type=map
 |website=wsprnet.org
 |url=http://wsprnet.org/drupal/wsprnet/map
 |access-date=2020-12-04
}}</ref> Even without special beacons the realtime propagation conditions can be measured: A worldwide network of receivers decodes morse code signals on amateur radio frequencies in realtime and provides sophisticated search functions and propagation maps for every station received.<ref>
{{cite web
 |title=Network of CW signal decoders for realtime analysis
 |website=Reverse Beacon Network
 |url=http://www.reversebeacon.net/
 |access-date=2020-12-04
}}</ref>

==Practical effects==
The average person can notice the effects of changes in radio propagation in several ways.

In [[AM broadcasting]], the dramatic ionospheric changes that occur overnight in the mediumwave band drive a unique [[broadcast license]] scheme in the United States, with entirely different [[transmitter power output]] levels and [[directional antenna]] patterns to cope with skywave propagation at night. Very few stations are allowed to run without modifications during dark hours, typically only those on [[clear-channel station|clear channels]] in [[NARBA|North America]].<ref>
{{cite report
 |title=Why AM stations must reduce power, change operations, or cease broadcasting at night
 |date=2015-12-11
 |language=en
 |publisher=U.S. Federal Communications Commission
 |url=https://www.fcc.gov/media/radio/am-stations-at-night
 |access-date=2017-02-11
}}</ref> Many stations have no authorization to run at all outside of daylight hours.

For [[FM broadcasting]] (and the few remaining low-band [[TV station]]s), weather is the primary cause for changes in VHF propagation, along with some diurnal changes when the sky is mostly without [[cloud cover]].<ref>
{{cite web
 |title=VHF/UHF Propagation 
 |publisher=Radio Society of Great Britain
 |website=rsgb.org
 |language=en-GB
 |url=http://rsgb.org/main/get-started-in-amateur-radio/operating-your-new-station/vhfuhf-propagation/
 |access-date=2017-02-11
}}</ref> These changes are most obvious during temperature inversions, such as in the late-night and early-morning hours when it is clear, allowing the ground and the air near it to cool more rapidly. This not only causes [[dew]], [[frost]], or [[fog]], but also causes a slight "drag" on the bottom of the radio waves, bending the signals down such that they can follow the Earth's curvature over the normal radio horizon. The result is typically several stations being heard from another [[media market]] – usually a neighboring one, but sometimes ones from a few hundred kilometers (miles) away.  [[Ice storm]]s are also the result of inversions, but these normally cause more scattered omnidirection propagation, resulting mainly in interference, often among [[weather radio]] stations. In late spring and early summer, a combination of other atmospheric factors can occasionally cause skips that duct high-power signals to places well over 1000&nbsp;km (600 miles) away.

Non-broadcast signals are also affected. [[Mobile phone signal]]s are in the UHF band, ranging from 700 to over 2600&nbsp;MHz, a range which makes them even more prone to weather-induced propagation changes. In [[urban area|urban]] (and to some extent [[suburb]]an) areas with a high [[population density]], this is partly offset by the use of smaller cells, which use lower [[effective radiated power]] and [[beam tilt]] to reduce interference, and therefore increase [[frequency reuse]] and user capacity. However, since this would not be very cost-effective in more [[rural]] areas, these cells are larger and so more likely to cause interference over longer distances when propagation conditions allow.

While this is generally transparent to the user thanks to the way that [[cellular network]]s handle cell-to-cell [[handoff]]s, when [[international boundary|cross-border]] signals are involved, unexpected charges for international [[roaming]] may occur despite not having left the country at all. This often occurs between southern [[San Diego]] and northern [[Tijuana]] at the western end of the [[Mexico–United States border|U.S./Mexico border]], and between eastern [[Detroit]] and western  [[Windsor, Ontario|Windsor]] along the [[Canada–United States border|U.S./Canada border]]. Since signals can travel unobstructed over a [[body of water]] far larger than the [[Detroit River]], and cool water temperatures also cause inversions in surface air, this "fringe roaming" sometimes occurs across the [[Great Lakes]], and between islands in the [[Caribbean]]. Signals can skip from the [[Dominican Republic]] to a mountainside in [[Puerto Rico]] and vice versa, or between the U.S. and British [[Virgin Islands]], among others. While unintended cross-border roaming is often automatically removed by [[mobile phone company]] billing systems, inter-island roaming is typically not.

==Empirical models{{anchor|Models}}==
A '''radio propagation model''', also known as the '''radio wave propagation model''' or the '''radio frequency propagation model''', is an [[empirical]] [[mathematical]] [[formulation]] for the characterization of [[radio wave]] propagation as a [[Function (mathematics)|function]] of [[frequency]], [[distance]] and other conditions. A single model is usually developed to predict the behavior of propagation for all similar links under similar constraints. Created with the goal of formalizing the way radio waves are propagated from one place to another, such models typically predict the [[path loss]] along a link or the effective coverage area of a [[transmitter]].

The inventor of radio communication, [[Guglielmo Marconi]], before 1900 formulated the first crude empirical rule of radio propagation: the maximum  transmission distance varied as the square of the height of the antenna.

As the path loss encountered along any radio link serves as the dominant factor for characterization of propagation for the link, radio propagation models typically focus on realization of the path loss with the auxiliary task of predicting the area of coverage for a transmitter or modeling the distribution of signals over different regions.

Because each individual telecommunication link has to encounter different terrain, path, obstructions, atmospheric conditions and other phenomena, it is intractable to formulate the exact loss for all telecommunication systems in a single mathematical equation. As a result, different models exist for different types of radio links under different conditions. The models rely on [[Reference distance|computing the median path loss]] for a link under a certain probability that the considered conditions will occur.

Radio propagation models are empirical in nature, which means, they are developed based on large collections of data collected for the specific scenario. For any model, the collection of data has to be sufficiently large to provide enough likeliness (or enough scope) to all kind of situations that can happen in that specific scenario. Like all empirical models, radio propagation models do not point out the exact behavior of a link, rather, they predict the most likely behavior the link may exhibit under the specified conditions.

Different models have been developed to meet the needs of realizing the propagation behavior in different conditions. Types of models for radio propagation include:

;Models for free space attenuation
* [[Free-space path loss]]
* [[Dipole field strength in free space]]
* [[Friis transmission equation]]

;Models for outdoor attenuation
*Terrain models
**[[ITU terrain model]]
**[[Egli model]]
**[[Longley–Rice model|Longley–Rice irregular terrain model (ITM)]]
**[[Two-ray ground-reflection model]]
*City models
**[[Okumura model]]
**[[Hata model]]
**[[COST Hata model]]

;Models for indoor attenuation
*[[ITU model for indoor attenuation]]
*[[Log-distance path loss model]]

==See also==
{{Portal|Radio}}
{{Main|Index of radio propagation articles}}
{{Div col|colwidth=22em}}
* [[Anomalous propagation]]
* [[Channel model]]
* [[Computation of radiowave attenuation in the atmosphere]]
* [[Critical frequency]]
* [[Diversity scheme]]
* [[Earth bulge]]
* [[Earth-ionosphere waveguide]]
* [[Effective Earth radius]]
* [[Electromagnetic radiation]]
* [[F2 propagation]]
* [[Fading]]
* [[Free space]]
* [[Fresnel zone]]
* [[Inversion (meteorology)]]
* [[Kennelly–Heaviside layer]]
* [[Link budget]]
* [[Mobility model]]
* [[Nakagami distribution|Nakagami fading]]
* [[Near and far field]]
* [[Propagation graph]]
* [[Radio atmospherics]]
* [[Radio frequency]]
* [[Radio horizon]]
* [[Radio resource management]]
* [[Ray tracing (physics)]]
* [[Rayleigh fading]]
* [[Schumann resonance]]
* [[Skip (radio)]]
* [[Skip zone]]
* [[Skywave]]
* [[Traffic generation model]]
* [[Tropospheric propagation]]
* [[TV and FM DX]]
* [[Upfade]]
* [[Vertical and horizontal (radio propagation)]]
* [[VOACAP]] – Free professional HF propagation prediction software
{{colend}}

==Footnotes==
{{notelist|1}}

==References==
{{reflist|25em}}

==Further reading==
{{refbegin}}
* {{cite book
 |first=Lucien |last=Boithais
 |year=1987
 |title=Radio Wave Propagation
 |publisher=McGraw-Hill Book Company
 |place=New York, NY
 |isbn=0-07-006433-4
}}
* {{cite book
 |first=Karl |last=Rawer
 |year=1993
 |title=Wave Propagation in the Ionosphere
 |publisher=Kluwer Acad. Publ.
 |place=Dordrecht, NL
 |isbn=0-7923-0775-5
}}
* {{cite book
 |author-first=Emil |author-last=Pocock
 |article=Propagation of Radio Signals
 |at=Chapter&nbsp;19
 |year=2010
 |editor1-first=H. Ward |editor1-last=Silver
 |name-list-style=and
 |editor2-first=Mark J. |editor2-last=Wilson
 |title=The ARRL Handbook for Radio Communications
 |edition=88th
 |publisher=American Radio Relay League
 |place=Newington, CT
 |isbn=978-0-87259-095-3
}}
* {{cite magazine
 |first=Yuri (VE3BMV, K3BU) |last=Blanarovich
 |date=June 1980
 |title=Electromagnetic wave propagation by conduction
 |magazine=CQ Magazine
 |page=44
 |url=http://k3bu.us/propagation.htm
}}
* {{cite book
 |first1=Adbollah |last1=Ghasemi
 |first2=Ali      |last2=Abedi
 |first3=Farshid  |last3=Ghasemi
 |name-list-style=and
 |year=2016
 |title=Propagation Engineering in Wireless Communication
 |edition=2nd
 |isbn=978-3-319-32783-9
}}
{{refend}}

==External links==
{{Commons category|Radio propagation}}
* [http://rigreference.com/solar Solar widget] Propagation widget based on NOAA data. Also available as WordPress plugin.
* [http://www.arrl.org/propagation-of-rf-signals ARRL Propagation Page] The [[ARRL|American Radio Relay League]] page on radio propagation.
* [https://web.archive.org/web/20050527220501/http://www.ips.gov.au/HF_Systems HF Radio and Ionospheric Prediction Service - Australia]
* [http://sunearthday.nasa.gov/swac/data.php NASA Space Weather Action Center]
* [http://www.hamqsl.com/solar.html Online Propagation Tools, HF Solar Data, and HF Propagation Tutorials]
* [https://www.electronics-notes.com/articles/antennas-propagation/ionospheric/hf-propagation-basics.php HF ionospheric propagation] several pages

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