Skywave

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Radio waves (black) reflecting off the ionosphere (red) during skywave propagation. Line altitude in this image is significantly exaggerated and not to scale. Skywave.jpg
Radio waves (black) reflecting off the ionosphere (red) during skywave propagation. Line altitude in this image is significantly exaggerated and not to scale.

In radio communication, skywave or skip refers to the propagation of radio waves reflected or refracted back toward Earth from the ionosphere, an electrically charged layer of the upper atmosphere. Since it is not limited by the curvature of the Earth, skywave propagation can be used to communicate beyond the horizon, at intercontinental distances. It is mostly used in the shortwave frequency bands.

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As a result of skywave propagation, a signal from a distant AM broadcasting station, a shortwave station, or – during sporadic E propagation 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) radio communication – between 3 and 30 MHz – is a result of skywave propagation. Since the early 1920s amateur radio operators (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 line-of-sight propagation, in which radio waves travel in a straight line, and from non-line-of-sight propagation.

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

Example of Skywave Propagation taken from PSK Reporter. PSKReporter Skip Example.jpg
Example of Skywave Propagation taken from PSK Reporter.

The ionosphere is a region of the upper atmosphere, from about 80 km (50 miles) to 1000 km (600 miles) in altitude, where neutral air is ionized by solar photons, solar particles, and cosmic rays. When high-frequency signals enter the ionosphere at a low angle they are bent back towards the Earth by the ionized layer. [1] If the peak ionization 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 maximum usable frequency, 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 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 (2200 miles) may be reached. Longer transmissions can occur with two or more hops. [2]

Near-vertical skywaves

Skywaves directed almost vertically are referred to as near-vertical-incidence skywaves (NVIS). At some frequencies, generally in the lower shortwave 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. [3] 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

Antenna vertical angle required vs distance for skywave propagation Antenna Vertical Angle vs 1 Hop Distance.png
Antenna vertical angle required vs distance for skywave propagation

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 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 Digital Radio Mondiale 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 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 below approximately 10 MHz (wavelengths longer than 30 meters), including broadcasts in the mediumwave and shortwave bands (and to some extent longwave), 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 for skywave propagation is strongly influenced by sunspot number.

Skywave propagation is usually degraded – sometimes seriously – during geomagnetic storms. 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 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 are credited with the discovery of skywave propagation on the shortwave bands. Early long-distance services used ground wave propagation at very low frequencies, [4] 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 using waves shorter than those used by commercial services [5] in December 1921, operating in the 200 meter mediumwave 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 at the upper edge of the 150-200 meter band—the official wavelengths allocated to amateurs by the Second National Radio Conference [6] 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 [7] 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 [8] at 80 meters (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 [9] on 25 November 1927. The 15-meter band (21 MHz) was opened to amateurs in the United States on 1 May 1952.

Marconi

Guglielmo Marconi was the first to show that radios could communicate beyond line-of-sight, using the reflective properties of the ionosphere. On December 12, 1901, he sent a message around 2,200 miles (3,500 km) from his transmission station in Cornwall, England, to St. John's, Newfoundland (now part of Canada). However, Marconi believed the radio waves were following the curvature of the Earth – the reflective properties of the ionosphere that enables 'sky waves' were not yet understood. Skepticism from the scientific community and his wired telegraph competitors drove Marconi to continue experimenting with wireless transmissions and associated business ventures over the next few decades. [10]

In June and July 1923, Guglielmo Marconi's land-to-ship transmissions were completed during nights on 97 meters from Poldhu Wireless Station, Cornwall, to his yacht Ellette in the Cape Verde Islands. 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 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, [11] 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 Conference [12] 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.

See also

Related Research Articles

<span class="mw-page-title-main">Ionosphere</span> Ionized part of Earths upper atmosphere

The ionosphere is the ionized part of the upper atmosphere of Earth, from about 48 km (30 mi) to 965 km (600 mi) 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 important role in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propagation to distant places on Earth. It also affects GPS signals that travel through this layer.

Ground wave is a mode of radio propagation that consists of currents traveling through the earth. Ground waves propagate parallel to and adjacent to the surface of the Earth, and are capable covering long distances by diffracting around the Earth's curvature. This radiation is also known as the Norton surface wave, or more properly the Norton ground wave, because ground waves in radio propagation are not confined to the surface. Groundwave contrasts with line-of-sight propagation that requires no medium, and skywave via the ionosphere.

In radio transmission, maximum usable frequency (MUF) is the highest radio frequency that can be used for transmission between two points on Earth by reflection from the ionosphere at a specified time, independent of transmitter power. This index is especially useful for shortwave transmissions.

<span class="mw-page-title-main">Transmission medium</span> Conduit for signal propagation

A transmission medium is a system or substance that can mediate the propagation of signals for the purposes of telecommunication. Signals are typically imposed on a wave of some kind suitable for the chosen medium. For example, data can modulate sound, and a transmission medium for sounds may be air, but solids and liquids may also act as the transmission medium. Vacuum or air constitutes a good transmission medium for electromagnetic waves such as light and radio waves. While a material substance is not required for electromagnetic waves to propagate, such waves are usually affected by the transmission media they pass through, for instance, by absorption or reflection or refraction at the interfaces between media. Technical devices can therefore be employed to transmit or guide waves. Thus, an optical fiber or a copper cable is used as transmission media.

<span class="mw-page-title-main">Shortwave radio</span> Radio transmissions using wavelengths between 10 m and 100 m

Shortwave radio is radio transmission using radio frequencies in the shortwave bands (SW). There is no official definition of the band range, but it always includes all of the high frequency band (HF), which extends from 3 to 30 MHz ; above the medium frequency band (MF), to the bottom of the VHF band.

<span class="mw-page-title-main">Very high frequency</span> Electromagnetic wave range of 30-300 MHz

Very high frequency (VHF) is the ITU designation for the range of radio frequency electromagnetic waves from 30 to 300 megahertz (MHz), with corresponding wavelengths of ten meters to one meter. Frequencies immediately below VHF are denoted high frequency (HF), and the next higher frequencies are known as ultra high frequency (UHF).

<span class="mw-page-title-main">Radio wave</span> Type of electromagnetic radiation

Radio waves are a type of electromagnetic radiation with the lowest frequencies and the longest wavelengths in the electromagnetic spectrum, typically with frequencies below 300 gigahertz (GHz) and wavelengths greater than 1 millimeter, about the diameter of a grain of rice. Radio waves with frequencies above about 1 GHz and wavelengths shorter than 30 centimeters are called microwaves. Like all electromagnetic waves, radio waves in a vacuum travel at the speed of light, and in the Earth's atmosphere at a slightly lower speed. Radio waves are generated by charged particles undergoing acceleration, such as time-varying electric currents. Naturally occurring radio waves are emitted by lightning and astronomical objects, and are part of the blackbody radiation emitted by all warm objects.

<span class="mw-page-title-main">Ultra high frequency</span> Electromagnetic spectrum 300–3000 MHz

Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter. Radio waves with frequencies above the UHF band fall into the super-high frequency (SHF) or microwave frequency range. Lower frequency signals fall into the VHF or lower bands. UHF radio waves propagate mainly by line of sight; they are blocked by hills and large buildings although the transmission through building walls is strong enough for indoor reception. They are used for television broadcasting, cell phones, satellite communication including GPS, personal radio services including Wi-Fi and Bluetooth, walkie-talkies, cordless phones, satellite phones, and numerous other applications.

<span class="mw-page-title-main">Very low frequency</span> The range 3–30 kHz of the electromagnetic spectrum

Very low frequency or VLF is the ITU designation for radio frequencies (RF) in the range of 3–30 kHz, corresponding to wavelengths from 100 to 10 km, respectively. The band is also known as the myriameter band or myriameter wave as the wavelengths range from one to ten myriameters. Due to its limited bandwidth, audio (voice) transmission is highly impractical in this band, and therefore only low data rate coded signals are used. The VLF band is used for a few radio navigation services, government time radio stations and for secure military communication. Since VLF waves can penetrate at least 40 meters (131 ft) into saltwater, they are used for military communication with submarines.

<span class="mw-page-title-main">Medium frequency</span> The range 300-3000 kHz of the electromagnetic spectrum

Medium frequency (MF) is the ITU designation for radio frequencies (RF) in the range of 300 kilohertz (kHz) to 3 megahertz (MHz). Part of this band is the medium wave (MW) AM broadcast band. The MF band is also known as the hectometer band as the wavelengths range from ten to one hectometers. Frequencies immediately below MF are denoted as low frequency (LF), while the first band of higher frequencies is known as high frequency (HF). MF is mostly used for AM radio broadcasting, navigational radio beacons, maritime ship-to-shore communication, and transoceanic air traffic control.

<span class="mw-page-title-main">Longwave</span> Radio transmission using wavelengths above 1000 m

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 early 20th century, when the radio spectrum was considered to consist of longwave (LW), medium-wave (MW), and short-wave (SW) radio bands. Most modern radio systems and devices use wavelengths which would then have been considered 'ultra-short'.

<span class="mw-page-title-main">High frequency</span> The range 3-30 MHz of the electromagnetic spectrum

High frequency (HF) is the ITU designation for the band of radio waves with frequency 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 decameters. Frequencies immediately below HF are denoted medium frequency (MF), while the next band of higher frequencies is known as the very high frequency (VHF) band. The HF band is a major part of the shortwave band of frequencies, so communication at these frequencies is often called shortwave radio. Because radio waves in this band can be reflected back to Earth by the ionosphere layer in the atmosphere – a method known as "skip" or "skywave" propagation – these frequencies can be used for long-distance communication across intercontinental distances and for mountainous terrains which prevent line-of-sight communications. The band is used by international shortwave broadcasting stations (3.95–25.82 MHz), aviation communication, government time stations, weather stations, amateur radio and citizens band services, among other uses.

<span class="mw-page-title-main">TV and FM DX</span> Long-distance reception of signals on the VHF frequency band

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."

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 affected by the phenomena of reflection, refraction, diffraction, absorption, polarization, and scattering. Understanding the effects of varying conditions on radio propagation has many practical applications, from choosing frequencies for amateur radio communications, international shortwave broadcasters, to designing reliable mobile telephone systems, to radio navigation, to operation of radar systems.

The 2-meter amateur radio band is a portion of the VHF radio spectrum that comprises frequencies stretching from 144 MHz to 148 MHz in International Telecommunication Union region (ITU) Regions 2 and 3 and from 144 MHz to 146 MHz in ITU Region 1 . The license privileges of amateur radio operators include the use of frequencies within this band for telecommunication, usually conducted locally with a line-of-sight range of about 100 miles (160 km).

The 80 meter or 3.5 MHz band is a span of radio frequencies allocated for amateur use, from 3.5–4.0 MHz in North and South America ; generally 3.5–3.8 MHz in Europe, Africa, and northern Asia (Region 1); and 3.5–3.9 MHz in south and east Asia and the eastern Pacific (Region 3). The upper portion of the band, which is usually used for phone (voice), is sometimes referred to as 75 meters; however, in Europe, "75 m" is used to name an overlapping shortwave broadcast band between 3.9–4.0 MHz used by a number of national radio services.

Shortwave bands are frequency allocations for use within the shortwave radio spectrum. Radio waves in these frequency ranges can be used for very long distance (transcontinental) communication because they can reflect off layers of charged particles in the ionosphere and return to Earth beyond the horizon, a mechanism called skywave or “skip” propagation. They are allocated by the ITU for radio services such as maritime communications, international shortwave broadcasting and worldwide amateur radio. The bands are conventionally named by their wavelength in metres, for example the ‘20 meter band’. Radio propagation and possible communication distances vary depending on the time of day, the season and the level of solar activity.

Non-line-of-sight (NLOS) radio propagation occurs outside of the typical line-of-sight (LOS) between the transmitter and receiver, such as in ground reflections. Near-line-of-sight conditions refer to partial obstruction by a physical object present in the innermost Fresnel zone.

Near vertical incidence skywave, or NVIS, is a skywave radio-wave propagation path that provides usable signals in the medium distances range — usually 0–650 km. It is used for military and paramilitary communications, broadcasting, especially in the tropics, and by radio amateurs for nearby contacts circumventing line-of-sight barriers. The radio waves travel near-vertically upwards into the ionosphere, where they are refracted back down and can be received within a circular region up to 650 km from the transmitter. If the frequency is too high, refraction is insufficient to return the signal to earth and if it is too low, absorption in the ionospheric D layer may reduce the signal strength.

Amateur radio frequency allocation is done by national telecommunication authorities. Globally, the International Telecommunication Union (ITU) oversees how much radio spectrum is set aside for amateur radio transmissions. Individual amateur stations are free to use any frequency within authorized frequency ranges; authorized bands may vary by the class of the station license.

References

  1. ↑ Wave Handbook. Sony Corporation. 1998. p. 14. OCLC   734041509.
  2. ↑ Rawer, K. (1993). Wave Propagation in the Ionosphere. Dordrecht: Kluwer Academic Publications. ISBN   0-7923-0775-5.
  3. ↑ Silver, H.L., ed. (2011). The ARRL Handbook for Radio Communications (88th ed.). Newington, CT: American Radio Relay League.
  4. ↑ Stormfax. Marconi Wireless on Cape Cod
  5. ↑ "1921 - Club Station 1BCG and the Transatlantic Tests". Radio Club of America. Retrieved 2009-09-05.
  6. ↑ "Radio Service Bulletin No. 72". Bureau of Navigation, Department of Commerce. 1923-04-02. pp. 9–13. Retrieved 2018-03-05.{{cite magazine}}: Cite magazine requires |magazine= (help)
  7. ↑ Archived November 30, 2009, at the Wayback Machine
  8. ↑ "Frequency or wave band allocations", Recommendations for Regulation of Radio Adopted by the Third National Radio Conference (October 6–10, 1924), page 15.
  9. ↑ "Report". twiar.org.
  10. ↑ Marconi Archived 2022-11-21 at the Wayback Machine
  11. ↑ Full text of "Beyond the ionosphere : fifty years of satellite communication". 1997. ISBN   9780160490545 . Retrieved 2012-08-31.
  12. ↑ Cable and Wireless Pl c History Archived 2015-03-20 at the Wayback Machine

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