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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 which receives direct-broadcast satellite television from a direct broadcast satellite in geostationary orbit.
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. [1]
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. [2] The dishes were nearly 20 feet (6.1 m) in diameter. [3] The satellite dishes of the early 1980s were 10 to 16 feet (3.0 to 4.9 m) in diameter [4] and made of fiberglass with an embedded layer of wire mesh or aluminium foil, or solid aluminium or steel. [5]
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. [6] Larger dishes continued to be used, however. [6] In December 1988, Luxembourg's Astra 1A satellite began transmitting analog television signals on the Ku band for the European market. [7] This allowed small dishes (90 cm) to be used reliably for the first time. [7]
In the early 1990s, four large American cable companies founded PrimeStar, a direct broadcasting company using medium power satellites. [8] The relatively strong Ku band transmissions allowed the use of dishes as small as 90 cm for the first time. [8] On 4 March 1996, EchoStar introduced Digital Sky Highway (Dish Network). [9] This was the first widely used direct-broadcast satellite television system and allowed dishes as small as 20 inches (51 cm) to be used. This great decrease of dish size also allowed satellite dishes to be installed on vehicles. [10] 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. [11]
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. [12]
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.
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. [13]
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.
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 ]
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 vertical / horizontal antenna polarization, and an on/off pilot tone (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 'multiswitch' switching matrix, which allows the connection of multiple receivers to the multiswitch in a star topology using the same signalling method as in a single receiver installation.[ citation needed ]
A satellite finder (or sat finder) is a satellite field strength meter used to accurately point satellite dishes at communications satellites in geostationary orbit. [14] [15] [16] Professional satellite finder meters allow better dish alignment and provide received signal parameter values as well.
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. There are three competing standards: DiSEqC, USALS, and 36 V positioners. Many receivers support all of these standards.
Motor-driven dishes come in a variety of sizes, but a dish of at least 120 centimetres (47 in) is required to receive signals from distant satellites which are intended to serve other areas.[ citation needed ]
With DiSEqC and USALS, the satellite dish will automatically aim itself at one of sixteen satellites programmed in previously when pressing one of the channel buttons on the remote.[ citation needed ] Motor-driven satellite dishes using USALS can detect other satellites in a constellation after one has been found and aimed at.[ citation needed ]
Most receivers sold at present[ when? ] are compatible with USALS and DiSEqC 1.0 and 1.2.
Every standard-size dish enables simultaneous reception from multiple different satellite positions without re-positioning the dish, just by adding additional LNB or using special duo LNB, or triple- or four-feed monoblock LNB.
However, some designs much more effectively optimize simultaneous reception from multiple different satellite positions without re-positioning the dish. The vertical axis operates as an off-axis concave parabolic concave hyperbolic[ clarification needed ] Cassegrain reflector, while the horizontal axis operates as a concave convex[ clarification needed ] 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.
Switching between satellites is possible by using DiSEqC switches added to a satellite installation, or built-in Duo LNBs or Monoblock LNBs.
Most receivers sold presently[ when? ] are compatible with at least DiSEqC 1.0, which can switch automatically between 4 satellites (all of contemporary Monoblock LNBs) as the user changes channels using the remote control.
DiSEqC 1.1 allows for switching automatically between 16 satellite positions or more (through cascading switches).
Motor-driven dishes assure better optimal focusing for the given dish size; LNB is always in central alignment with the broadcasting satellite, but DiSEqC switches are faster than DiSEqC motors as no physical movement is required.[ citation needed ]
A common type of dish is the very small aperture terminal (VSAT). This provides two way satellite Internet communications for both individuals and private networks for organizations. At present,[ when? ] most VSATs operate in Ku band; C band is restricted to less populated regions of the world. In 2005, dish manufacturers began moving towards new Ka band satellites operating at higher frequencies, offering greater performance at lower cost.[ citation needed ] 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).
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.
A feed horn is a small horn antenna used to couple a waveguide to e.g. a parabolic dish antenna or offset dish antenna for reception or transmission of microwaves. A typical application is the use for satellite television reception with a satellite dish. In that case the feed horn can either be a separate part used together with e.g. a "low-noise block downconverter" (LNB), or more typically today is integrated into a "low-noise block feedhorn" (LNBF).
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 to 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, South 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 Satellite 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.
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 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.
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).
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 with RG-59/U cable.
DiSEqC is a special communication protocol for use between a satellite receiver and a device such as a multi-dish switch or a small dish antenna rotor. DiSEqC was developed by European satellite provider Eutelsat, which now acts as the standards agency for the protocol.
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.
Universal Satellites Automatic Location System (USALS), also known (unofficially) as DiSEqC 1.3, Go X or Go to XX is a satellite dish motor protocol that automatically creates a list of available satellite positions in a motorised satellite dish setup. It is used in conjunction with the DiSEqC 1.2 protocol. It was developed by STAB, an Italian motor manufacturer, who still make the majority of USALS compatible motors.
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.
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.
Low-noise block downconverters (LNBs) are electronic devices coupled to satellite dishes for TV reception or general telecommunication that convert electromagnetic waves into digital signals that can be used to transform information into human or machine interpretable data, e.g., optical images, video, code, communications, etc.
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.
A free-to-air or FTA Receiver is a satellite television receiver designed to receive unencrypted broadcasts. Modern decoders are typically compliant with the MPEG-4/DVB-S2 standard and formerly the MPEG-2/DVB-S standard, while older FTA receivers relied on analog satellite transmissions which have declined rapidly in recent years.
The Squarial was a satellite antenna used for reception of the now defunct British Satellite Broadcasting television service (BSB). 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.
Automatic Tracking Satellite Dishes are satellite dishes used while a vehicle, boat or ship is in motion. Automatic tracking satellite dishes utilize gyroscopes, GPS position sensors, and uses unique satellite identification data and an integrated DVB decoder to aid in identification of the satellite that it is pointing at.
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 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.
Saorsat is a free-to-air satellite service in Ireland. The service launched on 3 May 2012. It was designed to provide TV service to a final 1-2% of the Irish population unable to receive other signals.
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