Satellite dish

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A C band satellite dish. Satellite dish 1 C-Band.jpg
A C band satellite dish.
A mobile satellite dish used by TVNZ news reporters. One News satellite ute, Mt Allan fire, NZ 2010.jpg
A mobile satellite dish used by TVNZ news reporters.

A satellite dish is a dish-shaped type of parabolic antenna designed to receive or transmit information by radio waves to or from a communication satellite. The term most commonly means a dish used by consumers to receive direct-broadcast satellite television from a direct broadcast satellite in geostationary orbit.

Parabolic antenna type of antenna

A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or parabolic dish. The main advantage of a parabolic antenna is that it has high directivity. It functions similarly to a searchlight or flashlight reflector to direct the radio waves in a narrow beam, or receive radio waves from one particular direction only. Parabolic antennas have some of the highest gains, meaning that they can produce the narrowest beamwidths, of any antenna type. In order to achieve narrow beamwidths, the parabolic reflector must be much larger than the wavelength of the radio waves used, so parabolic antennas are used in the high frequency part of the radio spectrum, at UHF and microwave (SHF) frequencies, at which the wavelengths are small enough that conveniently-sized reflectors can be used.

Radio wave type of electromagnetic radiation

Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies as high as 300 gigahertz (GHz) to as low as 30 hertz (Hz). At 300 GHz, the corresponding wavelength is 1 mm, and at 30 Hz is 10,000 km. Like all other electromagnetic waves, radio waves travel at the speed of light. They are generated by electric charges undergoing acceleration, such as time varying electric currents. Naturally occurring radio waves are emitted by lightning and astronomical objects.

Geostationary orbit circular orbit above the Earths equator and following the direction of the Earths rotation

A geostationary orbit, often referred to as a geosynchronous equatorial orbit (GEO), is a circular geosynchronous orbit 35,786 km (22,236 mi) above Earth's equator and following the direction of Earth's rotation. An object in such an orbit appears motionless, at a fixed position in the sky, to ground observers. Communications satellites and weather satellites are often placed in geostationary orbits, so that the satellite antennae that communicate with them do not have to rotate to track them, but can be pointed permanently at the position in the sky where the satellites are located. Using this characteristic, ocean-color monitoring satellites with visible and near-infrared light sensors can also be operated in geostationary orbit in order to monitor sensitive changes of ocean environments.

Contents

Principle of operation

Schematics of reflection principles used in parabolic antennas. Parabolic antenna types2.svg
Schematics of reflection principles used in parabolic antennas.

The parabolic shape of a dish reflects the signal to the dish’s focal point. Mounted on brackets at the dish's focal point is a device called a feedhorn. This feedhorn is essentially the front-end of a waveguide that gathers the signals at or near the focal point and 'conducts' them to a low-noise block downconverter or LNB. The LNB converts the signals from electromagnetic or radio waves to electrical signals and shifts the signals from the downlinked C-band and/or Ku-band to the L-band range. Direct broadcast satellite dishes use an LNBF, which integrates the feedhorn with the LNB. (A new form of omnidirectional satellite antenna, which does not use a directed parabolic dish and can be used on a mobile platform such as a vehicle was announced by the University of Waterloo in 2004. [1]

Focus (optics) point where light rays originating from a point converge on an object

In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by aberrations of the imaging optics. In the absence of significant aberrations, the smallest possible blur circle is the Airy disc, which is caused by diffraction from the optical system's aperture. Aberrations tend to get worse as the aperture diameter increases, while the Airy circle is smallest for large apertures.

Waveguide structure that guides waves, typically electromagnetic waves

A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting expansion to one dimension or two. There is a similar effect in water waves constrained within a canal, or guns that have barrels which restrict hot gas expansion to maximize energy transfer to their bullets. Without the physical constraint of a waveguide, wave amplitudes decrease according to the inverse square law as they expand into three dimensional space.

C band (IEEE) 4-8GHz

The C band is a designation by the Institute of Electrical and Electronics Engineers (IEEE) for a portion of the electromagnetic spectrum in the microwave range of frequencies ranging from 4.0 to 8.0 gigahertz (GHz); however, this definition is the one used by radar manufacturers and users, not necessarily by microwave radio telecommunications users. The C band is used for many satellite communications transmissions, some Wi-Fi devices, some cordless telephones as well as some surveillance and weather radar systems.

The theoretical gain (directive gain) of a dish increases as the frequency increases. The actual gain depends on many factors including surface finish, accuracy of shape, feedhorn matching. A typical value for a consumer type 60 cm satellite dish at 11.75 GHz is 37.50 dB.

In electromagnetics, an antenna's power gain or simply gain is a key performance number which combines the antenna's directivity and electrical efficiency. In a transmitting antenna, the gain describes how well the antenna converts input power into radio waves headed in a specified direction. In a receiving antenna, the gain describes how well the antenna converts radio waves arriving from a specified direction into electrical power. When no direction is specified, "gain" is understood to refer to the peak value of the gain, the gain in the direction of the antenna's main lobe. A plot of the gain as a function of direction is called the radiation pattern.

With lower frequencies, C-band for example, dish designers have a wider choice of materials. The large size of dish required for lower frequencies led to the dishes being constructed from metal mesh on a metal framework. At higher frequencies, mesh type designs are rarer though some designs have used a solid dish with perforations.

A common misconception is that the LNBF (low-noise block/feedhorn), the device at the front of the dish, receives the signal directly from the atmosphere. For instance, one BBC News downlink shows a "red signal" being received by the LNBF directly instead of being beamed to the dish, which because of its parabolic shape will collect the signal into a smaller area and deliver it to the LNBF. [2]

BBC News (TV channel) British 24-hour television news channel

BBC News is a British free-to-air television news channel. It was launched as BBC News 24 on 9 November 1997 at 5:30 pm as part of the BBC's foray into digital domestic television channels, becoming the first competitor to Sky News, which had been running since 1989. For a time, looped news, sport and weather bulletins were available to view via BBC Red Button.

Modern dishes intended for home television use are generally 43 cm (18 in) to 80 cm (31 in) in diameter, and are fixed in one position, for Ku-band reception from one orbital position. Prior to the existence of direct broadcast satellite services, home users would generally have a motorised C-band dish of up to 3 m in diameter for reception of channels from different satellites. Overly small dishes can still cause problems, however, including rain fade and interference from adjacent satellites.

Diameter straight line segment that passes through the center of a circle

In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints lie on the circle. It can also be defined as the longest chord of the circle. Both definitions are also valid for the diameter of a sphere.

Rain fade refers primarily to the absorption of a microwave radio frequency (RF) signal by atmospheric rain, snow, or ice, and losses which are especially prevalent at frequencies above 11 GHz. It also refers to the degradation of a signal caused by the electromagnetic interference of the leading edge of a storm front. Rain fade can be caused by precipitation at the uplink or downlink location. However, it does not need to be raining at a location for it to be affected by rain fade, as the signal may pass through precipitation many miles away, especially if the satellite dish has a low look angle. From 5 to 20 percent of rain fade or satellite signal attenuation may also be caused by rain, snow, or ice on the uplink or downlink antenna reflector, radome or feed horn. Rain fade is not limited to satellite uplinks or downlinks, it also can affect terrestrial point to point microwave links.

Europe

In Europe, the frequencies used by DBS services are 10.7–12.75 GHz on two polarisations H (Horizontal) and V (Vertical). This range is divided into a "low band" with 10.7–11.7 GHz, and a "high band" with 11.7–12.75 GHz. This results in two frequency bands, each with a bandwidth of about 1 GHz, each with two possible polarizations. In the LNB they become down converted to 950–2150 MHz, which is the frequency range allocated for the satellite service on the coaxial cable between LNBF and receiver. Lower frequencies are allocated to cable and terrestrial TV, FM radio, etc. Only one of these frequency bands fits on the coaxial cable, so each of these bands needs a separate cable from the LNBF to a switching matrix or the receiver needs to select one of the 4 possibilities at a time.[ citation needed ]

Europe Continent in the Northern Hemisphere and mostly in the Eastern Hemisphere

Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west and the Mediterranean Sea to the south. It comprises the westernmost part of Eurasia.

Cable television system of delivering television programming to paying subscribers via radio frequency (RF) signals transmitted through coaxial cables, or in more recent systems, light pulses through fiber-optic cables

Cable television is a system of delivering television programming to consumers via radio frequency (RF) signals transmitted through coaxial cables, or in more recent systems, light pulses through fiber-optic cables. This contrasts with broadcast television, in which the television signal is transmitted over the air by radio waves and received by a television antenna attached to the television; or satellite television, in which the television signal is transmitted by a communications satellite orbiting the Earth and received by a satellite dish on the roof. FM radio programming, high-speed Internet, telephone services, and similar non-television services may also be provided through these cables. Analog television was standard in the 20th century, but since the 2000s, cable systems have been upgraded to digital cable operation.

Systems design

In a single receiver residential installation there is a single coaxial cable running from the receiver set-top box in the building to the LNB on the dish. The DC electric power for the LNB is provided through the same coaxial cable conductors that carry the signal to the receiver. In addition, control signals are also transmitted from the receiver to the LNB through the cable. The receiver uses different power supply voltages (13 / 18 V) to select antenna polarization, and pilot tones (22 kHz) to instruct the LNB to select one of the two frequency bands. In larger installations each band and polarization is given its own cable, so there are 4 cables from the LNB to a switching matrix, which allows the connection of multiple receivers in a star topology using the same signalling method as in a single receiver installation.[ citation needed ]

A satellite finder may aid in aiming.

Types

Motor-driven dish

A dish that is mounted on a pole and driven by a stepper motor or a servo can be controlled and rotated to face any satellite position in the sky. Motor-driven dishes are popular with enthusiasts. There are three competing standards: DiSEqC, USALS, and 36 V positioners. Many receivers support all of these standards.

Multi-satellite

Special dish for up to 16 satellite positions (Ku-band). Antenne-toroidale.jpg
Special dish for up to 16 satellite positions (Ku-band).

Some designs enable simultaneous reception from multiple different satellite positions without re-positioning the dish. The vertical axis operates as an off-axis concave parabolic concave hyperbolic Cassegrain reflector, while the horizontal axis operates as a concave convex Cassegrain. The spot from the main dish wanders across the secondary, which corrects astigmatism by its varying curvature. The elliptic aperture of the primary is designed to fit the deformed illumination by the horns. Due to double spill-over, this makes more sense for a large dish.

VSAT

A common type of dish is the very small aperture terminal (VSAT). This provides two way satellite internet communications for both consumers and private networks for organizations. Today most VSATs operate in Ku band; C band is restricted to less populated regions of the world. There is a move which started in 2005 towards new Ka band satellites operating at higher frequencies, offering greater performance at lower cost. These antennas vary from 74 to 120 cm (29 to 47 in) in most applications though C-band VSATs may be as large as 4 m (13 ft).

Others

U.S. residential satellite TV receiver dishes SatelliteDishes-5375.jpg
U.S. residential satellite TV receiver dishes
Geometry of offset parabolic antenna. The dish is an asymmetric segment of a paraboloid; the vertex of the paraboloid is below the bottom edge of the dish. The beam axis, which is aimed at the satellite, passes through the vertex and the focus, so the feed antenna at the focus is outside the beam. Off-axis parabolic reflector.svg
Geometry of offset parabolic antenna. The dish is an asymmetric segment of a paraboloid; the vertex of the paraboloid is below the bottom edge of the dish. The beam axis, which is aimed at the satellite, passes through the vertex and the focus, so the feed antenna at the focus is outside the beam.

Homemade dishes

Any metal surface which concentrates a significant fraction of the reflected microwaves at a focus can be used as a dish antenna, at a lower gain. This has led to trash can lids, woks, and other items being used as "dishes". Only modern low noise LNBs and the higher transmission power of DTH satellites allows a usable signal to be received from such inefficient DIY antennas.

History

Parabolic antennas referred to as "dish" antennas had been in use long before satellite television. The term "satellite dish" was coined in 1978 during the beginning of the satellite television industry, and came to refer to dish antennas that send and/or receive signals from communications satellites. Taylor Howard of San Andreas, California adapted an ex-military dish in 1976 and became the first person to receive satellite television signals using it. [3]

The first satellite television dishes were built to receive signals on the C-band analog, and were very large. The front cover of the 1979 Neiman-Marcus Christmas catalog featured the first home satellite TV stations on sale. [4] The dishes were nearly 20 feet (6.1 m) in diameter. [5] The satellite dishes of the early 1980s were 10 to 16 feet (3.0 to 4.9 m) in diameter [6] and made of fiberglass with an embedded layer of wire mesh or aluminum foil, or solid aluminum or steel. [7]

Satellite dishes made of wire mesh first came out in the early 1980s, and were at first 10 feet (3.0 m) in diameter. As the front-end technology improved and the noise figure of the LNBs fell, the size shrank to 8 feet (2.4 m) a few years later, and continued to get smaller reducing to 6 feet (1.8 m) feet by the late 1980s and 4 feet (1.2 m) by the early 1990s. [8] Larger dishes continued to be used, however. [8] In December 1988 Luxembourg's Astra 1A satellite began transmitting analog television signals on the Ku band for the European market. [9] This allowed small dishes (90 cm) to be used reliably for the first time. [9]

In the early 1990s, four large American cable companies founded PrimeStar, a direct broadcasting company using medium power satellites. [10] The relatively strong Ku band transmissions allowed the use of dishes as small as 90 cm for the first time. [10] On 4 March 1996 EchoStar introduced Digital Sky Highway (Dish Network). [11] This was the first widely used direct-broadcast satellite television system and allowed dishes as small as 20 cm to be used. This great decrease of dish size also allowed satellite dishes to be installed on vehicles. [12] Dishes this size are still in use today. Television stations, however, still prefer to transmit their signals on the C-band analog with large dishes due to the fact that C-band signals are less prone to rain fade than Ku band signals. [13]

See also

Related Research Articles

Feed horn small horn antenna used to convey radio waves between a transmitter and/or receiver and a parabolic reflector

In parabolic antennas such as satellite dishes, a feed horn is a small horn antenna used to convey radio waves between the transmitter and/or receiver and the parabolic reflector. In transmitting antennas, it is connected to the transmitter and converts the radio frequency alternating current from the transmitter to radio waves and feeds them to the rest of the antenna, which focuses them into a beam. In receiving antennas, incoming radio waves are gathered and focused by the antenna's reflector on the feed horn, which converts them to a tiny radio frequency voltage which is amplified by the receiver. Feed horns are used mainly at microwave (SHF) and higher frequencies.

The Ku band is the portion of the electromagnetic spectrum in the microwave range of frequencies from 12 to 18 gigahertz (GHz). The symbol is short for "K-under", because it is the lower part of the original NATO K band, which was split into three bands because of the presence of the atmospheric water vapor resonance peak at 22.24 GHz, (1.35 cm) which made the center unusable for long range transmission. In radar applications, it ranges from 12-18 GHz according to the formal definition of radar frequency band nomenclature in IEEE Standard 521-2002.

Television receive-only (TVRO) is a term used chiefly in North America to refer to the reception of satellite television from FSS-type satellites, generally on C-band analog; free-to-air and unconnected to a commercial DBS provider. TVRO was the main means of consumer satellite reception in the United States and Canada until the mid-1990s with the arrival of direct-broadcast satellite television services such as PrimeStar, USSB, Bell TV, DirecTV, Dish Network, Sky TV that transmit Ku signals. While these services are at least theoretically based on open standards, the majority of services are encrypted and require proprietary decoder hardware. TVRO systems relied on feeds being transmitted unencrypted and using open standards, which heavily contrasts to DBS systems in the region.

Very-small-aperture terminal

A very small aperture terminal (VSAT) is a two-way satellite ground station with a dish antenna that is smaller than 3.8 meters. The majority of VSAT antennas range from 75 cm to 1.2 m. Data rates, in most cases, range from 4 kbit/s up to 16 Mbit/s. VSATs access satellites in geosynchronous orbit or geostationary orbit to relay data from small remote Earth stations (terminals) to other terminals or master Earth station "hubs".

Low-noise block downconverter

A low-noise block downconverter (LNB) is the receiving device mounted on satellite dishes used for satellite TV reception, which collects the radio waves from the dish and converts them to a signal which is sent through a cable to the receiver inside the building. Also called a low-noise block, low-noise converter (LNC), or even low-noise downconverter (LND), the device is sometimes inaccurately called a low-noise amplifier (LNA).

F connector

The F connector is a coaxial RF connector commonly used for "over the air" terrestrial television, cable television and universally for satellite television and cable modems, usually with RG-6/U cable or, in older installations, with RG-59/U cable.

Satellite Internet access is Internet access provided through communications satellites. Modern consumer grade satellite Internet service is typically provided to individual users through geostationary satellites that can offer relatively high data speeds, with newer satellites using Ku band to achieve downstream data speeds up to 506 Mbit/s.

Free-to-air (FTA) are television (TV) and radio services broadcast in clear (unencrypted) form, allowing any person with the appropriate receiving equipment to receive the signal and view or listen to the content without requiring a subscription, other ongoing cost or one-off fee. In the traditional sense, this is carried on terrestrial radio signals and received with an antenna.

Diplexer

A diplexer is a passive device that implements frequency-domain multiplexing. Two ports are multiplexed onto a third port. The signals on ports L and H occupy disjoint frequency bands. Consequently, the signals on L and H can coexist on port S without interfering with each other.

Multiswitch

A multiswitch is a device used with a dual or quattro LNB to distribute satellite TV signals to multiple receivers from a single dish and LNB.

Minidish

The Minidish is the tradename used for the small-sized satellite dish used by Freesat and Sky. The term has entered the vocabulary in the UK and Ireland as a generic term for a satellite dish, particularly small ones.

Block upconverter device used in the transmission of satellite signals

A block upconverter (BUC) is used in the transmission (uplink) of satellite signals. It converts a band of frequencies from a lower frequency to a higher frequency. Modern BUCs convert from the L band to Ku band, C band and Ka band. Older BUCs convert from a 70 MHz intermediate frequency (IF) to Ku band or C band.

Single cable distribution

Single cable distribution is a satellite TV technology that enables the delivery of broadcast programming to multiple users over a single coaxial cable, and eliminates the numerous cables required to support consumer electronics devices such as twin-tuner Digital Video Recorders (DVRs) and high end receivers.

Satellite television is a service that delivers television programming to viewers by relaying it from a communications satellite orbiting the Earth directly to the viewer's location. The signals are received via an outdoor parabolic antenna commonly referred to as a satellite dish and a low-noise block downconverter.

WokFi homemade Wi-Fi antenna

WokFi is a slang term for a style of homemade Wi-Fi antenna consisting of a crude parabolic antenna made with a low-cost Asian kitchen wok, spider skimmer or similar household metallic dish. The dish forms a directional antenna which is pointed at the wireless access point antenna, allowing reception of the wireless signal at greater distances than standard omnidirectional Wi-Fi antennas.

Squarial

The Squarial is a satellite antenna used for reception of the now defunct British Satellite Broadcasting television service. The Squarial was a flat plate satellite antenna, built to be unobtrusive and unique. BSB were counting on the form factor of the antenna to clearly differentiate themselves from their competitors at the time. At the time of development, satellite installations usually required a 90 cm dish in order to receive a clear signal from the transmitting satellite. The smaller antenna was BSB's unique selling point and was heavily advertised in order to attract customers to their service.

Duo LNB

A Duo LNB is a double low-noise block downconverter (LNB) developed by SES for the simultaneous reception of satellite television signals from both the Astra 23.5°E and Astra 19.2°E satellite positions.

The term Bandstacked applies to an antenna or satellite feedhorn (LNBF) that is designed to operate on two or more bands of frequencies. Usually, a portion of the radio frequency spectrum that has been divided into a low band and a high band.

Fibre satellite distribution

Fibre satellite distribution is a technology that enables satellite TV signals from an antenna to be distributed using an optical fibre cable infrastructure and then converted to electrical signals for use with conventional set-top box receivers.

References

  1. "Team develops in-car satellite TV". University of Waterloo . 2006-05-06. Retrieved 2008-04-29.
  2. "News 24 STILL gets an F for physics" . Retrieved 2008-04-29.
  3. Feder, Barnaby J. (15 November 2002). "Taylor Howard, 70, Pioneer In Satellite TV for the Home". New York Times . Retrieved 19 July 2014.
  4. Browne, Ray (2001). The Guide to United States Popular Culture. Madison, Wisconsin: Popular Press. p. 706. ISBN   9780879728212 . Retrieved 1 July 2014.
  5. Giarrusso, Michael (28 July 1996). "Tiny Satellite Dishes Sprout in Rural Areas". Los Angeles Times . Los Angeles: Los Angeles Times. Retrieved 1 July 2014.
  6. Nye, Doug (14 January 1990). "SATELLITE DISHES SURVIVE GREAT SCRAMBLE OF 1980S". Deseret News . Salt Lake City: Deseret News. Retrieved 30 June 2014.
  7. Brooks, Andree (10 October 1993). "Old satellite dish restrictions under fire New laws urged for smaller models". The Baltimore Sun . Baltimore, MD: The Baltimore Sun. Retrieved 1 July 2014.
  8. 1 2 Somerfield, Harry (30 September 1990). "Satellite dishes getting smaller, sharper in future". The Tuscaloosa News. Tuscaloosa, Alabama . Retrieved 7 August 2014.
  9. 1 2 "ASTRA 1A Satellite details 1988-109B NORAD 19688". N2YO. 9 July 2014. Retrieved 12 July 2014.
  10. 1 2 Barber, Dave (18 August 1995). "500,000 families already made PRIMESTAR their choice in satellite TV". Bangor Daily News . Bangor, Maine . Retrieved 7 August 2014.
  11. Grant, August E. Communication Technology Update (10th ed.). Taylor & Francis. p. 87. ISBN   978-0-240-81475-9.
  12. Evangelista, Benny (10 November 2003). "Satellite TV in the car, on the move / New technology makes dish receivers small enough to fit atop an SUV". San Francisco Chronicle . San Francisco . Retrieved 7 August 2014.
  13. "Rain fade: satellite TV signal and adverse weather". Dish-cable.com. Dish-cable.com. 2010. Retrieved 16 July 2014.