Satellite dish
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A satellite dish is a type of parabolic antenna designed with the specific purpose of transmitting signals to and/or receiving from satellites. A satellite dish is a particular type of microwave antenna. Satellite dishes come in varying sizes and designs, and are most commonly used to receive satellite television. Many of the offset type of satellite dishes are sections of a larger parabolic dish.
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. [1])
Modern dishes intended for home television use are generally 43 cm (18") to 80 cm (31") 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 satellite dish of up to 3 metres 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.
Motorised satellite dishes are still popular with enthusiasts. There are three competing standards, which often are all supported by a set-top box, DiSEqC, USALS, and 36v Positioners. They can only supply one receiver.
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. See, for instance, this BBC News 24 [2] countdown that shows a "red data stream" 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.
In Europe the frequencies used by DBS services are 10.7 - 12.75 GHz on two polarizations H and V. 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 four frequency bands, each with a bandwidth of about 1 GHz. 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 bands at a time.
In a single receiver residential installation there is a single cable from receiver to LNB and the receiver uses different power supply voltages (14/18V) and pilot tones (22 kHz) to instruct the LNB to select one of the 4 bands. In larger installations each band 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.
The theoretical gain of a dish increases as the frequency increases. The actual gain depends on many factors including surface finish, accuracy of shape, feedhorn matching.
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.
The dish is a reflector antenna and almost anything that reflects radio frequencies can be used as a reflector antenna. This has led to dustbin lids, woks and other items being used as "dishes". Coupled with low noise LNBs and the higher transmission power of DTH satellites, it is easier to get a usable signal on some of these "dishes".
[edit] Types
- Individual dishes serving one dwelling: Direct To Home (DTH).
- Collective dishes, shared by several dwellings: Satellite Master Antenna Television (SMATV) or Communal Antenna Broadcast Distribution (CABD).
[edit] See also
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