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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 frequency, or RF, refers to that portion of the electromagnetic spectrum in which electromagnetic waves can be generated by alternating current fed to an antenna. ... In communications and information processing, a transmitter (sometimes abbreviated XMTR) is an object (source) which sends information to an observer (receiver). ... Wave propagation refers to the ways waves travel through a medium (waveguide). ... This article is about Earth as a planet. ...

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: In physics, free space is a concept of electromagnetic theory, corresponding roughly to the vacuum, the baseline state of the electromagnetic field, or the replacement for the electromagnetic aether. ... Electromagnetic radiation is a propagating wave in space with electric and magnetic components. ... This diagram shows how the law works. ...

$rho_P propto frac{1}{r^2}$

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.

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. Doubling the propagation path distance from the transmitter reduces their received field strengths by one-half. The reduction of each of these fields by one-half is the result of the power density reduction to one-quarter over that doubled path length. Magnetic field lines shown by iron filings In physics, the space surrounding moving electric charges, changing electric fields and magnetic dipoles contains a magnetic field. ...

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. When viewing a scene, as in optics, photography, or even hunting, the line of sight is the straight line between the observer and the target. ... Horizon. ... The straw seems to be broken, due to refraction of light as it emerges into the air. ... Relationship of the atmosphere and ionosphere The ionosphere is the uppermost part of the atmosphere, distinguished because it is ionized by solar radiation. ...

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. 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. Ionospheric radio propagation has a strong connection to space weather. Groundwave is the propagation of radio waves close to the surface of the Earth. ... Aurora australis observed by Discovery, May 1991. ...

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. 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). For other uses, see Satellite (disambiguation). ... 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 including the Google Maps website, Google Ride Finder and embedded maps on third-party websites via the Google Maps API. It offers street maps...

A sudden ionospheric disturbance is often the result of large solar flares directed at Earth. These solar flares can disrupt HF radio propagation and affect GPS accuracy. A sudden ionospheric disturbance (SID) is an abnormally high plasma density in the ionosphere caused by an occasional sudden solar flare, which often interrupts or interferes with telecommunications systems. ... A solar flare is a violent explosion in the Suns atmosphere with an energy equivalent to tens of millions of hydrogen bombs. ... High frequency (HF) radio frequencies are between 3 and 30 MHz. ... GPS redirects here. ...

Radio waves at different frequencies propagate in different ways.

### Antenna

The beginning and end of a communication circuit is the antenna. The antenna can provide gain and directivity on both transmit and receive. 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. A Yagi-Uda beam antenna Short Wave Curtain Antenna (Moosbrunn, Austria) A building rooftop supporting numerous dish and sectored mobile telecommunications antennas (Doncaster, Victoria, Australia) An antenna is a transducer designed to transmit or receive radio waves which are a class of electromagnetic waves. ... In electronics, gain is usually taken as the mean ratio of the signal output of a system to the signal input of the system. ... In telecommunications, transmission is the act of transmitting electrical messages (and the associated phenomena of radiant energy that passes through media). ... In radio terminology, a receiver is an electronic circuit that receives a radio signal from an antenna and decodes the signal for use as sound, pictures, navigational-position information, etc. ...

Radio frequencies and their primary mode of propagation
Band Frequency Wavelength Propagation via
VLF Very Low Frequency 3 – 30 kHz 100 – 10 km Guided between the earth and the ionosphere.
LF Low Frequency 30 – 300 kHz 10 – 1 km Guided between the earth and the D layer of the ionosphere.

Surface waves. Very low frequency or VLF refers to radio frequencies (RF) in the range of 3 to 30 kHz. ... A kilohertz (kHz) is a unit of frequency equal to 1,000 hertz (1,000 cycles per second). ... Low Frequency or LF refers to Radio Frequencies (RF) in the range of 30â€“300 kHz. ... A kilohertz (kHz) is a unit of frequency equal to 1,000 hertz (1,000 cycles per second). ... Relationship of the atmosphere and ionosphere The ionosphere is the uppermost part of the atmosphere, distinguished because it is ionized by solar radiation. ...

MF Medium Frequency 300 – 3000 kHz 1000 – 100 m Surface waves.

E, F layer ionospheric refraction at night, when D layer absorption weakens. Medium frequency (MF) refers to radio frequencies (RF) in the range of 300 kHz to 3000 kHz. ... A kilohertz (kHz) is a unit of frequency equal to 1,000 hertz (1,000 cycles per second). ... The F region of the ionosphere is home to the F layer of ionization, also called the Appleton layer , after the English physicist Edward Appleton. ...

HF High Frequency (Short Wave) 3 – 30 MHz 100 – 10 m E layer ionospheric refraction.

F1, F2 layer ionospheric refraction. High frequency (HF) radio frequencies are between 3 and 30 MHz. ... A solid-state, analog shortwave receiver Shortwave radio operates between the frequencies of 3 MHz (3,000 kHz) and 30 MHz (30,000 kHz) [1] and came to be referred to as such in the early days of radio because the wavelengths associated with this frequency range were shorter than... MegaHertz (MHz) is the name given to one million (106) Hertz, a measure of frequency. ... The Kennelly-Heaviside Layer is also known as the E region or just as Heaviside Layer (after Oliver Heaviside). ...

VHF Very High Frequency 30 – 300 MHz 10 – 1 m Infrequent E ionospheric refraction. Extremely rare F1,F2 layer ionospheric refraction during high sunspot activity up to 80 MHz. Generally direct wave.
UHF Ultra High Frequency 300 – 3000 MHz 100 – 10 cm Direct wave.
SHF Super High Frequency 3 – 30 GHz 10 – 1 cm Direct wave.
EHF Extremely High Frequency 30 – 300 GHz 10 – 1 mm Direct wave limited by absorption.
 Radio spectrum ELF SLF ULF VLF LF MF HF VHF UHF SHF EHF 3 Hz 30 Hz 300 Hz 3 kHz 30 kHz 300 kHz 3 MHz 30 MHz 300 MHz 3 GHz 30 GHz 30 Hz 300 Hz 3 kHz 30 kHz 300 kHz 3 MHz 30 MHz 300 MHz 3 GHz 30 GHz 300 GHz

The above table listed as radio spectrum is incorrect in its identification of frequency bands for ULF, ELF and AF (audio frequency) ULF = 3 to 30 Hz, ELF = 30 to 300 Hz, Audio Frequency = 300 to 3000 Hz, VLF = 3000 to 30000 Hz, LF = 30 kHz to 300 kHz, MF = 300 kHz to 3 MHz, HF = 3 to 30 MHz, VLF = 30 to 300 MHz, UHF = 300 to 3000 MHz SHF = 3 to 30 GHz EHF = 30 to 300 HGz

### Surface Modes

Main article: Surface wave

The mode is commonly called the "Ground wave". This can cause confusion since the Direct mode is also sometimes called the "Ground wave". // In physics, a surface wave can refer to a mechanical wave that propagates along the interface between differing media, usually two fluids with different densities. ... Ground wave: In radio transmission, a surface wave that propagates close to the surface of the Earth. ...

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. Since the ground is not a perfect electrical conductor, ground waves are attenuated [need reference to properties and effects of attenuation] 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 electrodynamics, polarization (also spelled polarisation) is the property of electromagnetic waves, such as light, that describes the direction of their transverse electric field. ... Low Frequency or LF refers to Radio Frequencies (RF) in the range of 30â€“300 kHz. ... Very low frequency or VLF refers to radio frequencies (RF) in the range of 3 to 30 kHz. ...

Today LF and VLF are mostly used for military communications, especially with ships and submarines. Early commercial and professional radio services relied exclusively on long wave, low frequencies and ground-wave propagation. 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. Amateur experimentation was then confined only to authorized frequency segments in the range. Military communications are links between battlefield units, including connections to a higher command or home country. ... Longwave radio frequencies are those below 500 kHz, which correspond to wavelengths longer than 600 meters. ... Mediumwave radio transmissions (sometimes called Medium frequency or MF) are those between the frequencies of 300 kHz and 3000 kHz. ... Shortwave radio operates between the frequencies of 3,000 kHz and 30 MHz (30,000 kHz) and came to be referred to as such in the early days of radio because the wavelengths associated with this frequency range were shorter than those commonly in use at that time. ...

### Direct Modes (Line-of-sight)

Line-of-sight is the direct propagation of radio waves between antennas that are visible to each other. This is probably the most common of the radio propagation modes at VHF and higher frequencies. 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. When viewing a scene, as in optics, photography, or even hunting, the line of sight is the straight line between the observer and the target. ... When viewing a scene, as in optics, photography, or even hunting, the line of sight is the straight line between the observer and the target. ... Very high frequency (VHF) is the radio frequency range from 30 MHz to 300 MHz. ...

Ground plane 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 law for ground-plane limited radiation. [Need reference to inverse-fourth-power law + ground plane. Drawings may clarify] In telecommunication, a ground plane is an electrically conductive surface that serves as the near-field reflection point for an antenna, or as a reference ground in a circuit. ... The reflection of a bridge in Indianapolis, Indianas Central Canal. ...

### Ionospheric Modes (Sky-Wave)

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. 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. It has been suggested that this article or section be merged with Ionosphere. ... In radio propagation, skip is when a radio signal is reflected or refracted by the atmosphere or ionosphere, and returns to Earth in an unexpectedly far-away place. ... The straw seems to be broken, due to refraction of light as it emerges into the air. ... Relationship of the atmosphere and ionosphere The ionosphere is the uppermost part of the atmosphere, distinguished because it is ionized by solar radiation. ... Air redirects here. ... 400 year sunspot history A sunspot is a region on the Suns surface (photosphere) that is marked by a lower temperature than its surroundings, and intense magnetic activity. ... Last 30 years of solar variability Solar variations are changes in the amount of radiant energy emitted by our Sun. ...

#### Meteor scattering

Meteor scattering relies on reflecting radio waves off the intensely ionized columns 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. Photo of a burst of meteors with extended exposure time A meteor is the visible path of a meteoroid that enters the Earths (or another bodys) atmosphere, commonly called a shooting star or falling star. ... Meteor scatter propogation as used by SNOTEL Meteor burst communications, or MBC for short, is a radio propagation mode that exploits the ionized trails of meteors during atmospheric entry to establish brief communications paths between radio stations up to 2200 kilometers (1400 miles) apart. ...

#### Auroral reflection

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

Sporadic E (Es) propagation can be observed on HF and VHF bands. 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. TV DX and FM DX are two terms, customarily grouped together, that mean distant reception of TV and FM radio stations, respectively. ...

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. The propagation range for Es single-hop is typically 1000...2000 km, but with multi-hop, double range is observed. The signals are very strong but also with slow deep fading. Ocean wave Wave clouds over Theresa, Wisconsin, USA Atmospheric gravity waves as seen from space. ...

Thomas F. Giella, a noted retired Meteorologist, Space Plasma Physicist and Amateur Radio Operator, KN4LF cites the following from his professional research. Meteorology is the scientific study of the atmosphere that focuses on weather processes and forecasting. ...

Just as the E layer is the main refraction medium for medium frequency (300-3000 kc) signal propagation within approximately 5000 km/3100 mi, so is a Sporadic-E (Es) cloud. Sporadic-E (Es) clouds occur at approximately 100 km/60 miles 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. Atmosphere diagram showing stratosphere. ... Atmosphere diagram showing the mesosphere and other layers. ... It has been suggested that this article or section be merged with Radio waves. ...

The main source for "high latitude" Sporadic E (Es) clouds is geomagnetic storming induced radio aurora activity. The cause of Earths magnetic field (the surface magnetic field) is not known for certain, but is possibly explained by dynamo theory. ...

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. Another tie in between Sporadic-E (Es) and a severe thunderstorm is the Elve. For the Marvel Comics character, see Windshear (comics). ... A thunderstorm, also called an electrical storm or lightning storm, is a form of weather characterized by the presence of lightning and its attendant thunder produced from a cumulonimbus cloud. ... For alternate meanings, see Lightning (disambiguation). ...

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. Coincidentally I have a B.S. in Meteorology and an M.S. in Space Plasma Physics and am qualified to identify which severe thunderstorm cell complexes are most likely to produce Sporadic-E (Es) clouds.

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 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 in the atmosphere at around 12 km above the surface of the Earth, just under the tropopause. ... The tropopause is between the troposphere and the stratosphere. ...

3.) 500 mb temperatures of -20 deg. C or colder, which produce numerous positive and negative lightning bolts and inter-related Sprites and Elves. Upper-atmospheric lightning is an early term sometimes invoked by researchers to refer to a family of electrical breakdown phenomena that occurs well above the altitudes of normal lightning. ...

4.) Approximate 150-175 knot/172-200 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 the line of sight. ... Very high frequency (VHF) is the radio frequency range from 30 MHz to 300 MHz. ...

#### 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. For example, this mode is used to send radio signals over a mountain range when a line-of-sight path is not available. 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. In electromagnetic wave propagation, the knife-edge effect is a redirection by diffraction of a portion of the incident radiation that strikes a well-defined obstacle such as a mountain range or the edge of a building. ... When viewing a scene, as in optics, photography, or even hunting, the line of sight is the straight line between the observer and the target. ...

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. Cellular redirects here. ... In telecommunication, multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths. ...

#### Absorption

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

• Boithais, Lucien, "Radio Wave Propagation". McGraw-Hill Book Company, New York. 1987. ISBN 0-07-006433-4

Results from FactBites:

 Radio propagation - Wikipedia, the free encyclopedia (981 words) 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. High frequency propagation on Earth is not only affected by the inverse-square model, but by a number of other factors determined by its path from point to point. 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.
 11Meter_Propagation (1313 words) For successful radio communications, it is desirable to have a good understanding of the characteristics of radio waves and how they are influenced by solar and geomagnetic fields as they travel through the earth's atmosphere. Radio communications is effected by the variable conditions that exist in the earth's atmosphere, and this can be divided into three main regions. The amount by which a radio wave is reflected and bounced, depends on the intensity of the ionization and also the wavelength or frequency used.
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