Radio propagation is a term used to explain how radio waves behave when they are transmitted, or are propagated from one point on the Earth to another. Radio waves are electromagnetic waves occurring on the Radio frequency portion of the Electromagnetic spectrum. For biologic transmitters see Transmitter substance. A transmitter is an electronic device which usually with the aid of an antenna Wave propagation is any of the ways in which waves travel through a Waveguide. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001

This is an illustration showing how radio signals are split into two components (the ordinary component in red and the extraordinary component in green) when penetrating into the ionosphere. Two separate signals of differing transmitted elevation angles are broadcast from the transmitter at the left toward the receiver (triangle on base of grid) at the right. Click the image for access to a movie of this example showing the three dimensionality of the example.

In free space, all electromagnetic waves (radio, light, X-rays, etc) obey the inverse-square law which states that the power density of an electromagnetic wave is proportional to the inverse of the square of r (where r is the distance [radius] from the source) or:

Doubling the distance 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. In Classical physics, free space is a concept of Electromagnetic theory, corresponding to a theoretically "perfect" Vacuum, and sometimes Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. In Physics, an inverse-square law is any Physical law stating that some physical Quantity or strength is inversely proportional

The far-field magnitudes of the electric and magnetic field components of electromagnetic radiation are equal, and their field strengths are inversely proportional to distance. In Physics, the space surrounding an Electric charge or in the presence of a time-varying Magnetic field has a property called an electric field (that can In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges The power density per surface unit is proportional to the product of the two field strengths, which are expressed in linear units. Thus, doubling the propagation path distance from the transmitter reduces their received field strengths over a free-space path by one-half.

Electromagnetic wave propagation is also affected by several other factors determined by its path from point to point. This path can be a direct line of sight path or an over-the-horizon path aided by refraction in the ionosphere. The horizon ( Ancient Greek ὁ ὁρίζων, /ho horídzôn/ from ὁρίζειν, "to limit" is the apparent line that separates Refraction is the change in direction of a Wave due to a change in its Speed. The ionosphere is the uppermost part of the atmosphere, distinguished because it is Ionized by solar radiation Factors influencing ionospheric radio signal propagation can include sporadic-E, spread-F, solar flares, geomagnetic storms, ionospheric layer tilts, and solar proton events. Sporadic E or Es is an unusual form of Radio propagation utilizing characteristics of the earth's Ionosphere. A solar flare is a violent explosion in a star's (like the Sun 's atmosphere releasing as much Energy as 6 × 1025 Joules Solar flares A geomagnetic storm or solar storm is a temporary disturbance of the Earth 's Magnetosphere caused by a disturbance in Space weather. A Solar proton event occurs when Protons emitted by the Sun become accelerated to very high energies either close to the Sun during a Solar flare or in interplanetary

Lower frequencies (between 30 and 3,000 kHz) have the property of following the curvature of the earth via groundwave propagation in the majority of occurrences. In Physics, surface wave can refer to a Mechanical wave that propagates along the interface between differing media usually two fluids with different densities The interaction of radio waves with the ionized regions of the atmosphere makes radio propagation more complex to predict and analyze than in free space (see image at right). Ionospheric radio propagation has a strong connection to space weather. Space weather is the concept of changing environmental conditions in Outer space.

Since radio propagation is somewhat unpredictable, such services as emergency locator transmitters, in-flight communication with ocean-crossing aircraft, and some television broadcasting have been moved to satellite transmitters. Television ( TV) is a widely used Telecommunication medium for sending ( Broadcasting) and receiving moving Images, either monochromatic This article is about artificial satellites For natural satellites also known as moons see Natural satellite. A satellite link, though expensive, can offer highly predictable and stable line of sight coverage of a given area (see Google Maps for a "real-world" application). Google Maps (for a time named Google Local) is a free Web mapping service application and technology provided by Google that powers many map-based services

A sudden ionospheric disturbance or shortwave fadeout is observed when the x-rays associated with a solar flare ionizes the ionospheric D-region. A sudden ionospheric disturbance (SID is an abnormally high ionization/ plasma density in the D region of the Ionosphere caused by a Solar A solar flare is a violent explosion in a star's (like the Sun 's atmosphere releasing as much Energy as 6 × 1025 Joules Solar flares Enhanced ionization in that region increases the absorption of radio signals passing through it. During the strongest solar x-ray flares, complete absorption of virtually all ionospherically propagated radio signals in the sunlit hemisphere can occur. These solar flares can disrupt HF radio propagation and affect GPS accuracy. High frequency (HF radio frequencies are between 3 and 30 MHz. Basic concept of GPS operation A GPS receiver calculates its position by carefully timing the signals sent by the constellation of GPS Satellites high above the Earth

Radio waves at different frequencies propagate in different ways.

Antenna

The beginning and end of a communication circuit is the antenna. An antenna is a Transducer designed to transmit or Receive electromagnetic waves In other words antennas convert electromagnetic waves into The antenna can provide gain and directivity on both transmit and receive. In Electronics, gain is a measure of the ability of a circuit (often an Amplifier) to increase the power or Amplitude of a In Telecommunications transmission is the process of sending propagating and receiving an analogue or digital information signal over a physical point-to-point or This article is about a radio receiver for other uses see Radio (disambiguation. The take-off angle of the antenna is based on the type of antenna, the height of the antenna above ground, and the terrain below and in front of the antenna. The take-off angle will determine the angle of incidence on the ionosphere, which will affect where the signal will be refracted by the ionosphere.

Modes

Surface modes

Main article: Surface wave

The mode is commonly called the "ground wave". In Physics, surface wave can refer to a Mechanical wave that propagates along the interface between differing media usually two fluids with different densities In Physics, surface wave can refer to a Mechanical wave that propagates along the interface between differing media usually two fluids with different densities This can cause confusion since the direct mode is also sometimes called the "ground wave".

In this mode the radio wave propagates by interacting with the semi-conductive surface of the earth. The wave "clings" to the surface and thus follows the curvature of the earth. Vertical polarization is used to alleviate short circuiting the electric field through the conductivity of the ground. Polarization ( ''Brit'' polarisation) is a property of Waves that describes the orientation of their oscillations Since the ground is not a perfect electrical conductor, ground waves are attenuated rapidly as they follow the earth’s surface. Attenuation is proportional to the frequency making this mode mainly useful for LF and VLF frequencies. In Physics, attenuation (in some context also called extinction) is the gradual loss in intensity of any kind of Flux through a medium Low Frequency or LF refers to Radio Frequencies (RF in the range of 30 kHz&ndash300 kHz Very low frequency or VLF refers to radio frequencies (RF in the range of 3 KHz to 30 KHz.

Today LF and VLF are mostly used for time signals, and for military communications, especially with ships and submarines. A time signal is a visible audible mechanical or electronic signal used as a reference to determine the time of day Military communications, or Signals ( not to be confused with military Line of communications (LOC is a field of Military activities tactics Early commercial and professional radio services relied exclusively on long wave, low frequencies and ground-wave propagation. The longwave Radio band is a range of frequencies used for AM broadcasting, which extends from 148 To prevent interference with these services, amateur and experimental transmitters were restricted to the higher (HF) frequencies, felt to be useless since their ground-wave range was limited. Upon discovery of the other propagation modes possible at medium wave and short wave frequencies, the advantages of HF for commercial and military purposes became apparent. Medium Wave (MW is a part of the Medium frequency (MF radio band used mainly for AM broadcasting. Shortwave Radio operates between the frequencies of 3000 KHz (3 Amateur experimentation was then confined only to authorized frequency segments in the range.

Direct modes (line-of-sight)

Line-of-sight is the direct propagation of radio waves between antennas that are visible to each other. Line-of-sight propagation refers to Electro-magnetic radiation including light emissions traveling in a straight line This is probably the most common of the radio propagation modes at VHF and higher frequencies. Very high frequency (VHF is the Radio frequency range from 30 MHz to 300 MHz. Because radio signals can travel through many non-metallic objects, radio can be picked up through walls. This is still line-of-sight propagation. Examples would include propagation between a satellite and a ground antenna or reception of television signals from a local TV transmitter.

Ground plane reflection effects are an important factor in VHF line of sight propagation. In Electrical engineering, a ground plane is an electrically conductive surface. Reflection is the change in direction of a Wave front at an interface between two different media so that the wave front returns into the medium from which The interference between the direct beam line-of-sight and the ground reflected beam often leads to an effective inverse-fourth-power law for ground-plane limited radiation. [Need reference to inverse-fourth-power law + ground plane. Drawings may clarify]

Ionospheric modes (skywave)

Skywave propagation, also referred to as skip, is any of the modes that rely on refraction of radio waves in the ionosphere, which is made up of one or more ionized layers in the upper atmosphere. Skywave is the propagation of electromagnetic waves bent (refracted back to the Earth's surface by the Ionosphere. In Radio propagation, skip is when a Radio signal is reflected or Refracted by the atmosphere or Ionosphere, and Refraction is the change in direction of a Wave due to a change in its Speed. The ionosphere is the uppermost part of the atmosphere, distinguished because it is Ionized by solar radiation Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five F2-layer is the most important ionospheric layer for HF propagation, though F1, E, and D-layers also play some role. These layers are directly affected by the sun on a daily cycle, the seasons and the 11-year sunspot cycle determines the utility of these modes. During solar maxima, the whole HF range up to 30 MHz can be used and F2 propagation up to 50 MHz are observed frequently depending upon daily solar flux values. During solar minima, propagation of higher frequencies is generally worse. Solar variations are changes in the amount of Solar radiation emitted by the Sun.

Forecasting of skywave modes is of considerable interest to amateur radio operators and commercial marine and aircraft communications, and also to shortwave broadcasters. Amateur radio, often called ham radio, is both a Hobby and a service in which participants called "hams" use various types of Radio communications An ocean (from Greek, ''Okeanos'' (Oceanus) is a major body of saline water, and a principal component of the Hydrosphere. Shortwave Radio operates between the frequencies of 3000 KHz (3

Meteor scattering

Meteor scattering relies on reflecting radio waves off the intensely ionized columns of air generated by meteors. While this mode is very short duration, often only from a fraction of second to couple of seconds per event, digital Meteor burst communications allows remote stations to communicate to a station that may be hundreds of miles up to over 1,000 miles (1,600 km) away, without the expense required for a satellite link. Meteor burst communications, or MBC for short is a radio Propagation mode that exploits the ionized trails of Meteors during Atmospheric This mode is most generally useful on VHF frequencies between 30 and 250 MHz.

Auroral reflection

Intense columns of Auroral ionization at 100 km altitudes within the auroral oval reflect radio waves, perhaps most notably on HF and VHF. The reflection is angle-sensitive - incident ray vs. magnetic field line of the column must be very close to right-angle. Random motions of electrons spiraling around the field lines create a Doppler-spread that broadens the spectra of the emission to more or less noise-like—depending on how high radio frequency is used. The radio-aurora is observed mostly at high latitudes and rarely extend down to middle latitudes. The occurrences of radio-auroras depends on solar activity (flares, coronal holes, CMEs) and annually the events are more numerous during solar cycle maximas. A solar flare is a violent explosion in a star's (like the Sun 's atmosphere releasing as much Energy as 6 × 1025 Joules Solar flares Coronal holes are areas where the Sun's Corona is darker colder and has lower-density plasma than average A coronal mass ejection (CME is an ejection of material from the solar Corona, usually observed with a white-light Coronagraph. Radio aurora includes the so-called afternoon radio aurora which produces stronger but more distorted signals and after the Harang-minima, the late-night radio aurora (sub-storming phase) returns with variable signal strength and lesser doppler spread. The propagation range for this predominantly back-scatter mode extends up to about 2000 km in east-west plane, but strongest signals are observed most frequently from north at nearby sites on same latitudes.

Rarely, a strong radio-aurora is followed by Auroral-E, which resembles both propagation types in some ways.

Sporadic-E propagation

Sporadic E (Es) propagation can be observed on HF and VHF bands. Sporadic E or Es is an unusual form of Radio propagation utilizing characteristics of the earth's Ionosphere. It must not be confused with ordinary HF E-layer propagation. Sporadic-E at mid-latitudes occurs mostly during summer season, from May to August in the northern hemisphere and from November to February in the southern hemisphere. There is no single cause for this mysterious propagation mode. The reflection takes place in a thin sheet of ionisation around 90 km height. The ionisation patches drift westwards at speeds of few hundred km per hour. There is a weak periodicity noted during the season and typically Es is observed on 1 to 3 successive days and remains absent for a few days to reoccur again. Es do not occur during small hours, the events usually begin at dawn, there is a peak in the afternoon and a second peak in the evening. Es propagation is usually gone by local midnight.

Maximum observed frequency (MOF) for Es is found to be lurking around 30 MHz on most days during the summer season, but sometimes MOF may shoot up to 100 MHz or even more in ten minutes to decline slowly during the next few hours. The peak-phase includes oscillation of MOF with periodicity of approximately 5. . . 10 minutes, possibly related to gravity waves. In Fluid dynamics, gravity waves are waves generated in a Fluid medium or at the interface between two media (e The propagation range for Es single-hop is typically 1000 to 2000 km, but with multi-hop, double range is observed. The signals are very strong but also with slow deep fading.

Thomas F. Giella, a noted retired Meteorologist, Space Plasma Physicist and Amateur Radio Operator, KN4LF cites the following from his professional research. Meteorology (from Greek grc μετέωρος metéōros, "high in the sky" and grc -λογία -logia) is the Interdisciplinary

Just as the E layer is the main refraction medium for medium frequency (300–3000 kHz) signal propagation within approximately 5000 km (3000 mi), so is a Sporadic-E (Es) cloud. Sporadic-E (Es) clouds occur at approximately 100 km (60 mi) in altitude and generally move from ESE to WNW. Like Stratosphere level warming and Troposphere level temperature and moisture discontinuities, Sporadic-E (Es) clouds can depending on the circumstances absorb, block or refract medium, high and very high frequency RF signals in an unpredictable manner. The stratosphere is the second major layer of Earth's atmosphere, just above the Troposphere, and below the Mesosphere. The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 75% of the atmosphere's mass and almost all of its Water vapor and Radio frequency ( RF) is a Frequency or rate of Oscillation within the range of about 3 Hz to 300 GHz

The main source for "high latitude" Sporadic E (Es) clouds is geomagnetic storming induced radio aurora activity. Earth 's magnetic field (and the surface magnetic field) is approximately a Magnetic dipole, with one pole near the North pole (see

The main source for "mid latitude" Sporadic-E (Es) clouds is wind shear produced by internal buoyancy/gravity waves (IBGW's), that create traveling ionosphere disturbances (TID's), most of which are produced by severe thunderstorm cell complexes with overshooting tops that penetrate into the Stratosphere. Wind shear, sometimes referred to as windshear or Wind gradient, is a difference in Wind speed and direction over a relatively Another tie in between Sporadic-E (Es) and a severe thunderstorm is the Elve. Lightning is an atmospheric discharge of Electricity, which typically occurs during Thunderstorms and sometimes during volcanic eruptions or

The main sources for "low latitude" Sporadic-E (Es) clouds is wind shear produced by internal buoyancy/gravity waves (IBGW's), that create traveling ionosphere disturbances, most of which are produced by severe thunderstorm cell complexes tied to tropical cyclones. High electron content in the Equatorial Ring Current also plays a role.

The forecasting of Sporadic-E (Es) clouds has long been considered to be impossible. However it is possible to identify certain troposphere level meteorological conditions that can lead to the formation of Sporadic E (Es) clouds. One is as mentioned above the severe thunderstorm cell complex.

Sporadic-E (Es) clouds have been observed to initially occur within approximately 150 km (90 mi) to the right of a severe thunderstorm cell complex in the northern hemisphere, with the opposite being observed in the southern hemisphere. To complicate matters is the fact that Sporadic-E (Es) clouds that initially form to the right of a severe thunderstorm complex in the northern hemisphere, then move from ESE-WNW and end up to the left of the severe thunderstorm complex in the northern hemisphere. So one has to look for Sporadic-E (Es) clouds on either side of a severe thunderstorm cell complex. Things get even more complicated when two severe thunderstorm cell complexes exist approximately 1000–2000 miles apart.

Not all thunderstorm cell complexes reach severe levels and not all severe thunderstorm cell complexes produce Sporadic-E (Es). This is where knowledge in tropospheric physics and weather analyses/forecasting is necessary.

Some of the key elements in identifying which severe thunderstorm cell complexes have the potential to produce Sporadic-E (Es) via wind shear, from internal buoyancy/gravity waves, that produce traveling ionosphere disturbances include:

1. ) Negative tilted mid and upper level long wave troughs.

2. ) Approximate 150 knot (170 mph, 280 km/h) jet stream jet maxes that produce divergence and therefore create a sucking vacuum effect above thunderstorm cells, that assist thunderstorm cells in reaching and penetrating the tropopause into the stratosphere. Jet streams are fast flowing relatively narrow air currents found at the Tropopause, the transition between the Troposphere (where temperature decreases The tropopause is in the atmosphere between the Troposphere and the Stratosphere.

3. ) 500 mb (50 kPa) temperatures of −20 °C or colder, which produce numerous positive and negative lightning bolts and inter-related Sprites and Elves. Upper-atmospheric lightning is an obsolete term that is no longer used by researchers to refer to a family of electrical breakdown phenomena that occurs well above the altitudes of normal

4. ) Approximate 150–175 knot (170–200 mph) updrafts within thunderstorm cells complexes that create overshooting tops that penetrate the Tropopause into the Stratosphere (See definition #20 on Stratospheric Warming), launching upwardly propagating internal buoyancy/gravity waves, which create traveling ionosphere disturbances and then wind shear.

Tropospheric modes

Tropospheric scattering

At VHF and higher frequencies, small variation (turbulence) in the density of the atmosphere at a height of around 6 miles (10 km) can scatter some of the normally line-of-sight beam of radio frequency energy back toward the ground, allowing over-the-horizon communication between stations as far as 500 miles (800 km) apart. Tropospheric scatter (or troposcatter) is the scattering of distant TV and FM radio stations by the Troposphere so that they travel farther than Very high frequency (VHF is the Radio frequency range from 30 MHz to 300 MHz.

Tropospheric ducting and enhancement or refraction via inversion layer

Sudden changes in the atmosphere's vertical moisture content and temperature profiles can on random occasions make microwave and UHF & VHF signals propagate hundreds of kilometers up to about 2,000 kilometers (1,300 mi)—and for ducting mode even farther—beyond the normal radio-horizon. TV DX and FM DX are two terms customarily grouped together that refer to long-distance reception of TV and FM Radio stations respectively Microwaves are electromagnetic waves with Wavelengths ranging from 1 mm to 1 m or frequencies between 0 Very high frequency (VHF is the Radio frequency range from 30 MHz to 300 MHz. The inversion layer is mostly observed over high pressure regions, but there are several tropospheric weather conditions which create these randomly occurring propagation modes. In meteorology an inversion is a deviation from the normal change of an atmospheric property with altitude Inversion layer's altitude for non-ducting is typically found between 100 meters (300 ft) to about 1 kilometer (3,000 ft) and for ducting about 500 meters to 3 kilometers (1,600 to 10,000 ft), and the duration of the events are typically from several hours up to several days. Higher frequencies experience the most dramatic increase of signal strengths, while on low-VHF and HF the effect is negligible. Propagation path attenuation may be below free-space loss. Some of the lesser inversion types related to warm ground and cooler air moisture content occur regularly at certain times of the year and time of day.

Rain scattering

Rain scattering is purely a microwave propagation mode and is best observed around 10 GHz, but extends down to a few gigahertz—the limit being the size of the scattering particle size vs. The hertz (symbol Hz) is a measure of Frequency, informally defined as the number of events occurring per Second. wavelength. In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency. This mode scatters signals mostly forwards and backwards when using horizontal polarization and side-scattering with vertical polarization. Polarization ( ''Brit'' polarisation) is a property of Waves that describes the orientation of their oscillations Polarization ( ''Brit'' polarisation) is a property of Waves that describes the orientation of their oscillations Forward-scattering typically yields propagation ranges of 800 km. Scattering from snowflakes and ice pellets also occurs, but scattering from ice without watery surface is less effective. The most common application for this phenomenon is microwave rain radar, but rain scatter propagation can be a nuisance causing unwanted signals to intermittently propagate where they are not anticipated or desired. Similar reflections may also occur from insects though at lower altitudes and shorter range. Rain also causes attenuation of point-to-point and satellite microwave links. Attenuation values up to 30 dB have been observed on 30 GHz during heavy tropical rain.

Aeroplane scattering

Aeroplane scattering (or most often reflection) is observed on VHF through microwaves and besides back-scattering, yields momentary propagation up to 500 km even in a mountain-type terrain. The most common back-scatter application is air-traffic radar and bistatic forward-scatter guided-missile and aeroplane detecting trip-wire radar and the US space radar.

Lightning scattering

Lightning scattering has sometimes been observed on VHF and UHF over distance of about 500 km. The hot lightning channel scatters radiowaves for a fraction of a second. The RF noise burst from the lightning makes the initial part of the open channel unusable and the ionisation disappears soon because of combination at low latitude high atmospheric pressure. This mode has no practical use.

Other effects

Diffraction

Knife-Edge diffraction is the propagation mode where radio waves are bent around sharp edges. In Electromagnetic wave propagation, the knife-edge effect or edge diffraction is a redirection by Diffraction of a portion of the incident For example, this mode is used to send radio signals over a mountain range when a line-of-sight path is not available. Line-of-sight propagation refers to Electro-magnetic radiation including light emissions traveling in a straight line However, the angle cannot be too sharp or the signal will not diffract. The diffraction mode requires increased signal strength, so higher power or better antennas will be needed than for an equivalent line-of-sight path.

Diffraction depends on the relationship between the wavelength and the size of the obstacle. In other words, the size of the obstacle in wavelengths. Lower frequencies diffract around large smooth obstacles such as hills more easily. For example, in many cases where VHF (or higher frequency) communication is not possible due to shadowing by a hill, one finds that it is still possible to communicate using the upper part of the HF band where the surface wave is of little use.

Diffraction phenomena by small obstacles are also important at high frequencies. Signals for urban cellular telephony tend to be dominated by ground-plane effects as they travel over the rooftops of the urban environment. They then diffract over roof edges into the street, where multipath propagation, absorption and diffraction phenomena dominate. In Wireless Telecommunications multipath is the propagation phenomenon that results in Radio signals reaching the receiving

Absorption

Low-frequency radio waves travel easily through brick and stone and VLF even penetrates sea-water. As the frequency rises, absorption effects become more important. At microwave or higher frequencies, absorption by molecular resonance in the atmosphere (mostly water, H₂O and oxygen, O₂) is a major factor in radio propagation. Microwaves are electromagnetic waves with Wavelengths ranging from 1 mm to 1 m or frequencies between 0 For example, in the 58–60 GHz band, there is a major absorption peak which makes this band useless for long-distance use. This phenomenon was first discovered during radar research during World War II. Radar is a system that uses electromagnetic waves to identify the range altitude direction or speed of both moving and fixed objects such as Aircraft, ships World War II, or the Second World War, (often abbreviated WWII) was a global military conflict which involved a majority of the world's nations, including Beyond around 400 GHz, the Earth's atmosphere blocks some segments of spectra while still passes some—this is true up to UV light, which is blocked by ozone, but visible light and some of the NIR is transmitted.

Heavy rain and snow also affect microwave reception.

References

• Larry D. Wolfgang et. al, (ed), The ARRL Handbook for Radio Amateurs, Sixty-Eighth Edition , (1991), ARRL, Newington CT USA ISBN 0-87259-168-9

Further reading

• Boithais, Lucien, "Radio Wave Propagation". McGraw-Hill Book Company, New York. 1987. ISBN 0-07-006433-4
• Radio Propagation is also a popular radio show on Austin Based Radio Station KAOS Radio www. kaosradioaustin. org It broadcasts worldwide every friday from 11p. m. -1a. m. (CST) The 2 hour show is hosted by Austin D. J. Ordnance and plays Metal, Hardcore, Industrial, as well as local Central Texas Bands.

See also

Main article: List of radio propagation terms

• Amateur radio bands
• ARRL
• Cellular telephony
• Diversity scheme
• E-skip
• Fading
• FM DX
• Fresnel zone
• Radio propagation model
• RAIM
• Radio frequency
• Radio horizon
• Rayleigh fading
• Ray tracing (physics)
• Schumann resonance
• Skip (radio)
• Skip zone
• Skywave
• Time signal
• TV-FM DX
• White Alice Communications System
• Inversion (meteorology)

External links

• DXing.info - Propagation links
• Solar Cycle 24 and VHF Aurora Website (www.solarcycle24.com)
• Ionospheric Prediction Service - Australia
• HF Radio Propagation Software for Firefox - Propfire Firefox plug for monitoring radio propagation, website utility to display HF radio propagation status and article on understanding HF radio propagation forecasting
• RadioWORKS A radio wave propagation and antenna length calculator
• SWDXER ¨The SWDXER¨ - with general SWL information and radio antenna tips. This is a list of Radio propagation terms A A index - A-index - Aa index - Active prominence - Active prominence region The American Radio Relay League (ARRL is the largest membership association of Amateur radio enthusiasts in the USA. In Telecommunications, a diversity scheme refers to a method for improving the reliability of a message signal by utilizing two or more communication channels with TV DX and FM DX are two terms customarily grouped together that refer to long-distance reception of TV and FM Radio stations respectively This is about the phenomenon of loss of signal in telecommunications TV DX and FM DX are two terms customarily grouped together that refer to long-distance reception of TV and FM Radio stations respectively In Optics and Radio communications, a Fresnel zone (pronounced FRA-nel Zone) named for physicist Augustin-Jean Fresnel, is one of A radio propagation model, also known as the Radio Wave Propagation Model or the Radio Frequency Propagation Model, is an Empirical Mathematical RAIM is the abbreviation for Receiver Autonomous Integrity Monitoring, a technology developed to assess the integrity of Global Positioning System (GPS signals in Radio frequency ( RF) is a Frequency or rate of Oscillation within the range of about 3 Hz to 300 GHz In Telecommunication, radio horizon is the locus of points at which direct rays from an antenna are tangential to the surface of the Earth Rayleigh fading is a statistical model for the effect of a propagation environment on a Radio signal such as that used by Wireless In physics ray tracing is a method for calculating the path of Waves or Particles through a system with regions of varying propagation Velocity, absorption The Schumann resonances (SR are a set of spectrum peaks in the Extremely low frequency (ELF portion of the Earth 's Electromagnetic field spectrum In Radio propagation, skip is when a Radio signal is reflected or Refracted by the atmosphere or Ionosphere, and Skywave is the propagation of electromagnetic waves bent (refracted back to the Earth's surface by the Ionosphere. A time signal is a visible audible mechanical or electronic signal used as a reference to determine the time of day TV DX and FM DX are two terms customarily grouped together that refer to long-distance reception of TV and FM Radio stations respectively The White Alice Communications System (WACS was a United States Air Force Telecommunication link system constructed in Alaska during the cold war. In meteorology an inversion is a deviation from the normal change of an atmospheric property with altitude
• Space Weather and Radio Propagation Resource Center Live data and images of space weather and radio propagation. Space weather is the concept of changing environmental conditions in Outer space.
• ARRL Propagation Page The American Radio Relay League page on radio propagation. The American Radio Relay League (ARRL is the largest membership association of Amateur radio enthusiasts in the USA.
• The Basics of Radio Wave Propagation A resource by Edwin C. Jones (AE4TM), MD, PhD, Department of Physics and Astronomy, University of Tennessee. Doctor of Medicine ( MD or MD, from the Latin Medicinæ Doctor meaning "Teacher of Medicine" is a doctoral "PhD" redirects here for other uses see PhD (disambiguation. Physics (Greek Physis - φύσις in everyday terms is the Science of Matter and its motion. Astronomy (from the Greek words astron (ἄστρον "star" and nomos (νόμος "law" is the scientific study The University of Tennessee (also known as UT) sometimes called the University of Tennessee Knoxville ( UT Knoxville, or UTK) is the flagship
• KN4LF 160 Meter Propagation Theory Notes. A website dedicated to layman level explanations of "seemingly" mysterious 160 meter (MF/HF) propagation occurrences.

The following external references provide practical examples of radio propagation concepts as demonstrated using software built on the VOACAP model.

• High Frequency radio propagation de-mystified.
• Is High Frequency radio propagation reciprocal?
• How does noise affect radio signals?

The following external link is designed for use by cell phones and mobile devices that can display content using Wireless Markup Language and the Wireless Application Protocol:

• WAP/WML Space Weather and Radio Propagation Resources Space weather and radio propagation resources. A mobile device (also known as cellphone device, handheld device, handheld computer, "Palmtop" or simply handheld) is a pocket-sized Wireless Markup Language, based on XML, is a Markup language intended for devices that implement the Wireless Application Protocol (WAP specification WAP is an open international Standard for Application layer network communications in a Wireless communication environment Space weather is the concept of changing environmental conditions in Outer space.