Radio propagation is the behavior ofradio waves as they travel, or arepropagated, from one point to another invacuum, or into various parts of theatmosphere.^{[1]: 26‑1} As a form ofelectromagnetic radiation, like light waves, radio waves are affected by the phenomena ofreflection,refraction,diffraction,absorption,polarization, andscattering.^[2] Understanding the effects of varying conditions on radio propagation has many practical applications, from choosing frequencies foramateur radio communications, internationalshortwavebroadcasters, to designing reliablemobile telephone systems, toradio navigation, to operation ofradar 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 ascell phones,cordless phones,walkie-talkies,wireless networks,FM radio,television broadcasting,radar, andsatellite communication (such assatellite 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 atmicrowave frequencies and above.^[a]

At lower frequencies in theMF,LF, andVLF 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 calledsurface waves orground wave propagation.AM broadcast and amateur radio stations use ground waves to cover their listening areas. As the frequency gets lower, theattenuation with distance decreases, sovery low frequency (VLF) toextremely 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 militarycommunication with submerged submarines.

Atmedium wave andshortwave frequencies (MF andHF bands), radio waves can refract from theionosphere, a layer ofcharged particles (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 calledskywave propagation. It is used byamateur radio operators to communicate with operators in distant countries, and byshortwave broadcast stations to transmit internationally.^[b]

In addition, there are several less common radio propagation mechanisms, such astropospheric scattering (troposcatter),tropospheric ducting (ducting) at VHF frequencies andnear vertical incidence skywave (NVIS) which are used when HF communications are desired within a few hundred miles.

At different frequencies, radio waves travel through the atmosphere by different mechanisms or modes:^[3]

Infree space, allelectromagnetic waves (radio, light, X-rays, etc.) obey theinverse-square law which states that the power density${\displaystyle \rho }$ of an electromagnetic wave is proportional to the inverse of the square of the distance${\displaystyle r}$ from apoint source^{[1]: 26‑19} or:

${\displaystyle \rho \propto \frac{1}{r^2}.}$

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 thespeed 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 slightrefraction (bending) of waves over distances.

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 atVHF and above, and the only possible mode atmicrowave frequencies and above. On the surface of the Earth, line of sight propagation is limited by thevisual horizon to about 40 miles (64 km). This is the method used bycell phones,^[c]cordless phones,walkie-talkies,wireless networks, point-to-pointmicrowave radio relay links,FM andtelevision broadcasting andradar.Satellite communication uses longer line-of-sight paths; for example homesatellite dishes receive signals from communication satellites 22,000 miles (35,000 km) above the Earth, andground stations can communicate withspacecraft billions of miles from Earth.

Ground planereflection 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(1⁄distance⁴) law for ground-plane limited radiation.^{[citation needed]}

Lower frequency (between 30 and 3,000 kHz)vertically polarized radio waves can travel assurface waves following the contour of the Earth; this is calledground 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 invertical polarization so vertical antennas (monopoles) are required. Since the ground is not a perfect electrical conductor, ground waves areattenuated 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 theMF,LF andVLF bands. Ground waves are used byradio broadcasting stations in the MF and LF bands, and fortime signals andradio navigation systems.

At even lower frequencies, in theVLF toELF bands, anEarth-ionosphere waveguide mechanism allows even longer range transmission. These frequencies are used for securemilitary 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 thelongwave bands and relied exclusively on ground-wave propagation. Frequencies above 3 MHz were regarded as useless and were given to hobbyists (radio amateurs). The discovery around 1920 of the ionospheric reflection orskywave mechanism made themedium wave andshort wave frequencies useful for long-distance communication and they were allocated to commercial and military users.^[9]

(BLOS) is a related term often used in the military to describe radio communications capabilities that link personnel or systems too distant or too fully obscured by terrain for LOS communications. These radios utilize activerepeaters,groundwave propagation,tropospheric scatter links, andionospheric propagation to extend communication ranges from a few kilometers to a few thousand kilometers.

HF propagation conditions can be simulated usingradio propagation models, such asthe Voice of America Coverage Analysis Program, and realtime measurements can be done usingchirp transmitters. For radio amateurs theWSPR mode provides maps with real time propagation conditions between a network of transmitters and receivers.^[10] 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.^[11]

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

InAM broadcasting, the dramatic ionospheric changes that occur overnight in the mediumwave band^[12] drive a uniquebroadcast license scheme in the United States, with entirely differenttransmitter power output levels anddirectional antenna patterns to cope with skywave propagation at night. Very few stations are allowed to run without modifications during dark hours, typically only those onclear channels inNorth America.^[13] Many stations have no authorization to run at all outside of daylight hours.

ForFM broadcasting (and the few remaining low-bandTV stations), weather is the primary cause for changes in VHF propagation, along with some diurnal changes when the sky is mostly withoutcloud cover.^[14] 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 causesdew,frost, orfog, 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 anothermedia market – usually a neighboring one, but sometimes ones from a few hundred kilometers (miles) away.Ice storms are also the result of inversions, but these normally cause more scattered omnidirection propagation, resulting mainly in interference, often amongweather 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 km (600 miles) away.

Non-broadcast signals are also affected.Mobile phone signals are in the UHF band, ranging from 700 to over 2600 MHz, a range which makes them even more prone to weather-induced propagation changes. Inurban (and to some extentsuburban) areas with a highpopulation density, this is partly offset by the use of smaller cells, which use lowereffective radiated power andbeam tilt to reduce interference, and therefore increasefrequency reuse and user capacity. However, since this would not be very cost-effective in morerural 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 thatcellular networks handle cell-to-cellhandoffs, whencross-border signals are involved, unexpected charges for internationalroaming may occur despite not having left the country at all. This often occurs between southernSan Diego and northernTijuana at the western end of theU.S./Mexico border, and between easternDetroit and westernWindsor along theU.S./Canada border. Since signals can travel unobstructed over abody of water far larger than theDetroit River, and cool water temperatures also cause inversions in surface air, this "fringe roaming" sometimes occurs across theGreat Lakes, and between islands in theCaribbean. Signals can skip from theDominican Republic to a mountainside inPuerto Rico and vice versa, or between the U.S. and BritishVirgin Islands, among others. While unintended cross-border roaming is often automatically removed bymobile phone company billing systems, inter-island roaming is typically not.

Aradio propagation model, also known as theradio wave propagation model or theradio frequency propagation model, is anempiricalmathematicalformulation for the characterization ofradio wave propagation as afunction offrequency,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 thepath loss along a link or the effective coverage area of atransmitter.

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 oncomputing 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 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

Wikimedia Commons has media related toRadio propagation.

• Solar widget Propagation widget based on NOAA data. Also available as WordPress plugin.
• ARRL Propagation Page TheAmerican Radio Relay League page on radio propagation.
• HF Radio and Ionospheric Prediction Service - Australia
• NASA Space Weather Action Center
• Online Propagation Tools, HF Solar Data, and HF Propagation Tutorials
• HF ionospheric propagation several pages

• Radio frequency propagation

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