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radio communication Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmit ...
, skywave or skip refers to the
propagation Propagation can refer to: * Chain propagation in a chemical reaction mechanism *Crack propagation, the growth of a crack during the fracture of materials * Propaganda, non-objective information used to further an agenda * Reproduction, and other fo ...
of
radio waves Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz ( GHz) and below. At 300 GHz, the corresponding wavelength is 1 mm (s ...
reflected or
refracted In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomeno ...
back toward Earth from the
ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an ...
, an
electrically charged Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respe ...
layer of the upper
atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
. Since it is not limited by the curvature of the Earth, skywave propagation can be used to communicate beyond the
horizon The horizon is the apparent line that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This line divides all viewing directions based on whether i ...
, at intercontinental distances. It is mostly used in the
shortwave Shortwave radio is radio transmission using shortwave (SW) radio frequencies. There is no official definition of the band, but the range always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (100 to 10 me ...
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
bands. As a result of skywave propagation, a signal from a distant
AM broadcasting AM broadcasting is radio broadcasting using amplitude modulation (AM) transmissions. It was the first method developed for making audio radio transmissions, and is still used worldwide, primarily for medium wave (also known as "AM band") transmis ...
station, a
shortwave Shortwave radio is radio transmission using shortwave (SW) radio frequencies. There is no official definition of the band, but the range always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (100 to 10 me ...
station, or – during
sporadic E propagation Sporadic E (usually abbreviated E) is an unusual form of radio propagation using a low level of the Earth's ionosphere that normally does not refract radio waves. Sporadic E propagation reflects signals off relatively small "clouds" in ...
conditions (principally during the summer months in both hemispheres) a distant VHF FM or TV station – can sometimes be received as clearly as local stations. Most long-distance shortwave (
high frequency High frequency (HF) is the ITU designation for the range of radio frequency electromagnetic waves (radio waves) between 3 and 30 megahertz (MHz). It is also known as the decameter band or decameter wave as its wavelengths range from one to ten ...
) radio communication – between 3 and 30 MHz – is a result of skywave propagation. Since the early 1920s
amateur radio operators An amateur radio operator is someone who uses equipment at an amateur radio station to engage in two-way personal communications with other amateur operators on radio frequencies assigned to the amateur radio service. Amateur radio operators hav ...
(or "hams"), limited to lower transmitter power than broadcast stations, have taken advantage of skywave for long-distance (or " DX") communication. Skywave propagation is distinct from: *
tropospheric scatter Tropospheric scatter, also known as troposcatter, is a method of communicating with microwave radio signals over considerable distances – often up to and further depending on frequency of operation, equipment type, terrain, and climate fact ...
, an alternative method of achieving over-the-horizon transmission at higher frequencies, *
groundwave In physics, a surface wave is a mechanical wave that propagates along the interface between differing media. A common example is gravity waves along the surface of liquids, such as ocean waves. Gravity waves can also occur within liquids, at t ...
propagation, where radio waves travel along Earth's surface without being reflected or refracted by the atmosphere – the dominant propagation mode at lower frequencies, *
line-of-sight propagation Line-of-sight propagation is a characteristic of electromagnetic radiation or acoustic wave propagation which means waves travel in a direct path from the source to the receiver. Electromagnetic transmission includes light emissions travelin ...
, in which radio waves travel in a straight line, the dominant mode at higher frequencies.


Local and distant skywave propagation

Skywave transmissions can be used for long-distance communications (DX) by waves directed at a low angle as well as relatively local communications via nearly vertically directed waves ( Near Vertical Incidence Skywaves – NVIS).


Low-angle skywaves

The ionosphere is a region of the upper
atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
, from about 80 km to 1000 km in altitude, where neutral air is
ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
ized by solar
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...
s and
cosmic ray Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...
s. When
high-frequency High frequency (HF) is the ITU designation for the range of radio frequency electromagnetic waves (radio waves) between 3 and 30 megahertz (MHz). It is also known as the decameter band or decameter wave as its wavelengths range from one to te ...
signals enter the ionosphere at a low angle they are bent back towards the earth by the ionized layer. If the peak
ionization Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule i ...
is strong enough for the chosen frequency, a wave will exit the bottom of the layer earthwards – as if obliquely reflected from a mirror. Earth's surface (ground or water) then reflects the descending wave back up again towards the ionosphere. When operating at frequencies just below the
MUF muf is a collaborative of artists, architects and urban designers based in London, England, specialising in the design of the urban public realm to facilitate appropriation by users. Projects include Kings Crescent Estate. In 2011 muf worked ...
, losses can be quite small, so the radio signal may effectively "bounce" or "skip" between the earth and ionosphere two or more times (multi-hop propagation), even following the curvature of the earth. Consequently, even signals of only a few Watts can sometimes be received many thousands of miles away. This is what enables
shortwave Shortwave radio is radio transmission using shortwave (SW) radio frequencies. There is no official definition of the band, but the range always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (100 to 10 me ...
broadcasts to travel all over the world. If the ionization is not great enough, the wave only curves slightly downwards, and subsequently upwards as the ionization peak is passed so that it exits the top of the layer only slightly displaced. The wave is then lost in space. To prevent this, a lower frequency must be chosen. With a single "hop", path distances up to 3500 km may be reached. Longer transmissions can occur with two or more hops.


Near-vertical skywaves

Skywaves directed almost vertically are referred to as ''near-vertical-incidence skywaves'' (''NVIS''). At some frequencies, generally in the lower
shortwave Shortwave radio is radio transmission using shortwave (SW) radio frequencies. There is no official definition of the band, but the range always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (100 to 10 me ...
region, the high angle skywaves will be reflected directly back towards the ground. When the wave returns to ground it is spread out over a wide area, allowing communications within several hundred miles of the transmitting antenna. NVIS enables local plus regional communications, even from low-lying valleys, to a large area, for example, an entire state or small country. Coverage of a similar area via a line-of-sight VHF transmitter would require a very high mountaintop location. NVIS is thus useful for statewide networks, such as those needed for emergency communications. In short wave broadcasting, NVIS is very useful for regional broadcasts that are targeted to an area that extends out from the transmitter location to a few hundred miles, such as would be the case in a country or language group to be reached from within the borders of that country. This will be much more economical than using multiple FM (VHF) or AM broadcast transmitters. Suitable antennas are designed to produce a strong lobe at high angles. When short range skywave is undesirable, as when an AM broadcaster wishes to avoid interference between the ground wave and sky wave, anti-fading antennas are used to suppress the waves being propagated at the higher angles.


Intermediate distance coverage

For every distance, from local to maximum distance transmission, (DX), there is an optimum "take off" angle for the antenna, as shown here. For example, using the F layer during the night, to best reach a receiver 500 miles away, an antenna should be chosen that has a strong lobe at 40 degrees elevation. One can also see that for the longest distances, a lobe at low angles (below 10 degrees) is best. For NVIS, angles above 45 degrees are optimum. Suitable antennas for long distance would be a high Yagi or a rhombic; for NVIS, a dipole or array of dipoles about .2 wavelengths above ground; and for intermediate distances, a dipole or Yagi at about .5 wavelengths above ground. Vertical patterns for each type of antenna are used to select the proper antenna.


Fading

At any distance sky waves will fade. The layer of ionospheric
plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral), a green translucent silica mineral * Quark–gluon plasma, a state of matter in quantum chromodynamics Biology * Blood pla ...
with sufficient ionization (the reflective surface) is not fixed, but undulates like the surface of the ocean. Varying reflection efficiency from this changing surface can cause the reflected signal strength to change, causing "'' fading''" in shortwave broadcasts. Even more serious fading can occur when signals arrive via two or more paths, for example when both single-hop and double-hop waves interfere with other, or when a skywave signal and a ground-wave signal arrive at about the same strength. This is the most common source of fading with nighttime AM broadcast signals. Fading is always present with sky wave signals, and except for digital signals such as
DRM DRM may refer to: Government, military and politics * Defense reform movement, U.S. campaign inspired by Col. John Boyd * Democratic Republic of Madagascar, a former socialist state (1975–1992) on Madagascar * Direction du renseignement milita ...
seriously limit the fidelity of shortwave broadcasts.


Other considerations

VHF signals with frequencies above about 30 MHz usually penetrate the ionosphere and are not returned to the Earth's surface.
E-skip Sporadic E (usually abbreviated E) is an unusual form of radio propagation using a low level of the Earth's ionosphere that normally does not refract radio waves. Sporadic E propagation reflects signals off relatively small "clouds" i ...
is a notable exception, where VHF signals including FM broadcast and VHF TV signals are frequently reflected to the Earth during late spring and early summer. E-skip rarely affects UHF frequencies, except for very rare occurrences below 500 MHz.
Frequencies Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
below approximately 10 MHz (wavelengths longer than 30 meters), including broadcasts in the
mediumwave Medium wave (MW) is the part of the medium frequency (MF) radio band used mainly for AM radio broadcasting. The spectrum provides about 120 channels with more limited sound quality than FM stations on the FM broadcast band. During the dayti ...
and
shortwave Shortwave radio is radio transmission using shortwave (SW) radio frequencies. There is no official definition of the band, but the range always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (100 to 10 me ...
bands (and to some extent
longwave In radio, longwave, long wave or long-wave, and commonly abbreviated LW, refers to parts of the radio spectrum with wavelengths longer than what was originally called the medium-wave broadcasting band. The term is historic, dating from the e ...
), propagate most efficiently by skywave at night. Frequencies above 10 MHz (wavelengths shorter than 30 meters) typically propagate most efficiently during the day. Frequencies lower than 3 kHz have a wavelength longer than the distance between the Earth and the ionosphere. The
maximum usable frequency In radio transmission maximum usable frequency (MUF) is the highest radio frequency that can be used for transmission between two points via reflection from the ionosphere (skywave or "skip" propagation) at a specified time, independent of transm ...
for skywave propagation is strongly influenced by
sunspot Sunspots are phenomena on the Sun's photosphere that appear as temporary spots that are darker than the surrounding areas. They are regions of reduced surface temperature caused by concentrations of magnetic flux that inhibit convection. Sun ...
number. Skywave propagation is usually degraded – sometimes seriously – during
geomagnetic storm A geomagnetic storm, also known as a magnetic storm, is a temporary disturbance of the Earth's magnetosphere caused by a solar wind shock wave and/or cloud of magnetic field that interacts with the Earth's magnetic field. The disturbance that d ...
s. Skywave propagation on the sunlit side of the Earth can be entirely disrupted during sudden ionospheric disturbances. Because the lower-altitude layers (the E-layer in particular) of the
ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an ...
largely disappear at night, the refractive layer of the ionosphere is much higher above the surface of the Earth at night. This leads to an increase in the "skip" or "hop" distance of the skywave at night.


History of discovery

Amateur radio operators An amateur radio operator is someone who uses equipment at an amateur radio station to engage in two-way personal communications with other amateur operators on radio frequencies assigned to the amateur radio service. Amateur radio operators hav ...
are credited with the discovery of skywave propagation on the shortwave bands. Early long-distance services used
surface wave In physics, a surface wave is a mechanical wave that propagates along the Interface (chemistry), interface between differing media. A common example is gravity waves along the surface of liquids, such as ocean waves. Gravity waves can also occu ...
propagation at very low frequencies, which are attenuated along the path. Longer distances and higher frequencies using this method meant more signal attenuation. This, and the difficulties of generating and detecting higher frequencies, made discovery of shortwave propagation difficult for commercial services. Radio amateurs conducted the first successful transatlantic tests in December 1921, operating in the 200 meter
mediumwave Medium wave (MW) is the part of the medium frequency (MF) radio band used mainly for AM radio broadcasting. The spectrum provides about 120 channels with more limited sound quality than FM stations on the FM broadcast band. During the dayti ...
band (1500 kHz)—the shortest wavelength then available to amateurs. In 1922 hundreds of North American amateurs were heard in Europe at 200 meters and at least 30 North American amateurs heard amateur signals from Europe. The first two-way communications between North American and Hawaiian amateurs began in 1922 at 200 meters. Extreme
interference Interference is the act of interfering, invading, or poaching. Interference may also refer to: Communications * Interference (communication), anything which alters, modifies, or disrupts a message * Adjacent-channel interference, caused by extr ...
at the upper edge of the 150-200 meter band—the official wavelengths allocated to
amateurs An amateur () is generally considered a person who pursues an avocation independent from their source of income. Amateurs and their pursuits are also described as popular, informal, self-taught, user-generated, DIY, and hobbyist. History ...
by the Second National Radio Conference in 1923—forced amateurs to shift to shorter and shorter wavelengths; however, amateurs were limited by regulation to wavelengths longer than 150 meters (2 MHz). A few fortunate amateurs who obtained special permission for experimental communications below 150 meters completed hundreds of long-distance two-way contacts on 100 meters (3 MHz) in 1923 including the first transatlantic two-way contacts in November 1923, on 110 meters (2.72 MHz) By 1924 many additional specially licensed amateurs were routinely making transoceanic contacts at distances of 6000 miles (~9600 km) and more. On 21 September several amateurs in California completed two way contacts with an amateur in New Zealand. On 19 October amateurs in New Zealand and England completed a 90-minute two-way contact nearly halfway around the world. On October 10, the Third National Radio Conference made three shortwave bands available to U.S. amateurs at
80 meters 8 (eight) is the natural number following 7 and preceding 9. In mathematics 8 is: * a composite number, its proper divisors being , , and . It is twice 4 or four times 2. * a power of two, being 2 (two cubed), and is the first number o ...
(3.75 MHz), 40 meters (7 MHz) and 20 meters (14 MHz). These were allocated worldwide, while the 10-meter band (28 MHz) was created by the Washington International Radiotelegraph Conference on 25 November 1927. The
15-meter band The 15-meter band (also called the 21-MHz band or 15 meters) is an amateur radio frequency band spanning the shortwave spectrum from 21 to 21.45 MHz. The band is suitable for amateur long-distance communications, and such use is permitted ...
(21 MHz) was opened to amateurs in the United States on 1 May 1952.


Marconi

In June and July 1923,
Guglielmo Marconi Guglielmo Giovanni Maria Marconi, 1st Marquis of Marconi (; 25 April 187420 July 1937) was an Italians, Italian inventor and electrical engineering, electrical engineer, known for his creation of a practical radio wave-based Wireless telegrap ...
's transmissions were completed during nights on 97 meters from Poldhu Wireless Station,
Cornwall Cornwall (; kw, Kernow ) is a historic county and ceremonial county in South West England. It is recognised as one of the Celtic nations, and is the homeland of the Cornish people. Cornwall is bordered to the north and west by the Atlantic ...
, to his yacht Ellette in the
Cape Verde Islands , national_anthem = () , official_languages = Portuguese , national_languages = Cape Verdean Creole , capital = Praia , coordinates = , largest_city = capital , demonym ...
. In September 1924, Marconi transmitted during daytime and nighttime on 32 meters from Poldhu to his yacht in Beirut. Marconi, in July 1924, entered into contracts with the British General Post Office (GPO) to install high speed shortwave telegraphy circuits from London to Australia, India, South Africa and Canada as the main element of the
Imperial Wireless Chain The Imperial Wireless Chain was a strategic international communications network of powerful long range radiotelegraphy stations, created by the British government to link the countries of the British Empire. The stations exchanged commercial a ...
. The UK-to-Canada shortwave "Beam Wireless Service" went into commercial operation on 25 October 1926. Beam Wireless Services from the UK to Australia, South Africa and India went into service in 1927. Far more spectrum is available for long-distance communication in the shortwave bands than in the long wave bands; and shortwave transmitters, receivers and antennas were orders of magnitude less expensive than the multi-hundred kilowatt transmitters and monstrous antennas needed for long wave. Shortwave communications began to grow rapidly in the 1920s, similar to the internet in the late 20th century. By 1928, more than half of long-distance communications had moved from transoceanic cables and long-wave wireless services to shortwave "skip" transmission, and the overall volume of transoceanic shortwave communications had vastly increased. Shortwave also ended the need for multimillion-dollar investments in new transoceanic telegraph cables and massive long-wave wireless stations, although some existing transoceanic telegraph cables and commercial long-wave communications stations remained in use until the 1960s. The cable companies began to lose large sums of money in 1927, and a serious financial crisis threatened the viability of cable companies that were vital to strategic British interests. The British government convened the Imperial Wireless and Cable ConferenceCable and Wireless Pl c History
in 1928 "to examine the situation that had arisen as a result of the competition of Beam Wireless with the Cable Services". It recommended and received Government approval for all overseas cable and wireless resources of the Empire to be merged into one system controlled by a newly formed company in 1929, Imperial and International Communications Ltd. The name of the company was changed to Cable and Wireless Ltd. in 1934. The signal that earned Guglielmo Marconi his place in history was not a long one: just the Morse code dots of the letter "S". But those three short radio pips represented a giant leap for mankind. The signal, which had travelled some 2,000 miles over open water from a transmitter in Poldhu, Cornwall, to a shack on a windy Newfoundland hill, was proof that radio waves could "bend" around the curvature of the Earth and effectively jump the 100-mile-high wall of water that blocks the view of America from Britain. For Marconi, then a young Italian man (with Irish ancestry from his mother's side), and his faithful assistant, an ex-petty officer called George Kemp, straining their ears over a primitive wireless set, those faint radio signals proved triumphantly that Marconi had been right all along when he insisted that there was no insuperable barrier to sending radio waves from one corner of the planet to another. "It was about half past 12, when I heard three little clicks in the earphones. Several times they sounded, but I hardly dared believe t" Marconi wrote in his diary. "The electric waves that were being sent out from Poldhu had traversed the Atlantic, serenely following the curvature of the Earth, which so many doubters had told me would be a fateful obstacle." Those three short pips led Marconi to unimagined commercial success, spawning a company that still bears his name today (albeit suffering catastrophically of late from the deflation of the dot.com bubble). They would also lead to accusations of plagiarism, theft and dishonesty, with rivals claiming that Marconi had "imagined" what he had heard, fearful of what failure would mean to the heavily invested experiment. Marconi was never the academic scientist, interested only in the pursuit of knowledge, but a "doer", with a keen sense of the value of an invention. His patents, and the industrial protection they gave him, earned him a fortune, but were a source of tension with other eminent radio pioneers of the day, whose own contributions to the field had been overshadowed by the somewhat brash young man from Bologna.


See also

*
Radio propagation Radio propagation is the behavior of radio waves as they travel, or are propagated, from one point to another in vacuum, or into various parts of the atmosphere. As a form of electromagnetic radiation, like light waves, radio waves are affect ...
*
MW DX MW DX, short for mediumwave DXing, is the hobby of receiving distant mediumwave (also known as '' AM'') radio stations. MW DX is similar to TV and FM DX in that broadcast band (BCB) stations are the reception targets. However, the nature of the lo ...
*
TV-FM DX TV DX and FM DX is the active search for distant radio or television stations received during unusual atmospheric conditions. The term DX is an old telegraphic term meaning "long distance." VHF/UHF television and radio signals are normally limit ...
* Near-Vertical Incidence Skywave (NVIS) *
F-layer The F region of the ionosphere is home to the F layer of ionization, also called the Appleton–Barnett layer, after the English physicist Edward Appleton and New Zealand physicist and meteorologist Miles Barnett. As with other ionospheric sectors ...
*
Over-the-horizon radar Over-the-horizon radar (OTH), sometimes called beyond the horizon radar (BTH), is a type of radar system with the ability to detect targets at very long ranges, typically hundreds to thousands of kilometres, beyond the radar horizon, which is ...
*
Groundwave In physics, a surface wave is a mechanical wave that propagates along the interface between differing media. A common example is gravity waves along the surface of liquids, such as ocean waves. Gravity waves can also occur within liquids, at t ...
*
Schumann resonances The Schumann resonances (SR) are a set of spectrum peaks in the extremely low frequency (ELF) portion of the Earth's electromagnetic field spectrum. Schumann resonances are global electromagnetic resonances, generated and excited by lightning ...
* Kennelly–Heaviside layer *
Skip zone Skip or Skips may refer to: Acronyms * SKIP (Skeletal muscle and kidney enriched inositol phosphatase), a human gene * Simple Key-Management for Internet Protocol * SKIP of New York (Sick Kids need Involved People), a non-profit agency aidi ...
*
Project West Ford Project West Ford (also known as Westford Needles and Project Needles) was a test carried out by Massachusetts Institute of Technology's Lincoln Laboratory on behalf of the United States military in 1961 and 1963 to create an artificial ionospher ...
*
Radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the upp ...
* Clear-channel station *
Utility station The term utility station is used to describe fixed radio broadcasters disseminating signals that are not intended for reception by the general public (but such members are not actively prohibited from receiving). Utility stations, as the name sugge ...
*
Tropospheric ducting Tropospheric propagation describes electromagnetic propagation in relation to the troposphere. The service area from a VHF or UHF radio transmitter extends to just beyond the optical horizon, at which point signals start to rapidly reduce in str ...
*
Geomagnetic storm A geomagnetic storm, also known as a magnetic storm, is a temporary disturbance of the Earth's magnetosphere caused by a solar wind shock wave and/or cloud of magnetic field that interacts with the Earth's magnetic field. The disturbance that d ...
*
History of radio The early history of radio is the history of technology that produces and uses radio equipment, radio instruments that use radio waves. Within the timeline of radio, many people contributed theory and inventions in what became radio. Radio deve ...
*
Amateur radio history The history of amateur radio, dates from the dawn of radio communications, with published instructions for building simple wireless sets appearing at the beginning of the twentieth century. Throughout its history, amateur radio enthusiasts have m ...
*
List of electronics topics A ''list'' is any set of items in a row. List or lists may also refer to: People * List (surname) Organizations * List College, an undergraduate division of the Jewish Theological Seminary of America * SC Germania List, German rugby unio ...


References


Further reading

*


External links


Navy - Propagation of Waves



HFRadio Propagation forums

Rare gamma-ray flare disturbed ionosphere



Radio propagation overview
Details of many forms of radio propagation {{Telecommunications Broadcast engineering Ionosphere Radio frequency propagation Surface waves