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.
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, and three competing standards, which are often all supported by a set-top box, DiSEqC, USALS, and 36v Positioners.
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 [link] 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 represents a total of 4.1 GHz of spectral bandwidth which is split into 4 polarization/frequency bands. On the coaxial cable between the LNBF and the receiver frequencies 950 - 2150 MHz are allocated for the satellite service. Lower frequencies are allocated to Cable and Terrestrial TV, FM radio, etc. There are 4 bands - Vertical High, Vertical Low, Horizontal High and Horizontal Low, each of these bands needs a separate cable from the LNBF to the receiver 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 and the receiver uses different power supply voltages and pilot tones to instruct the LNB to select one of the 4 bands. In a larger installation each band is given its own cable and 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 quality of a satellite dish is usually expressed as a G/T ratio. This is the "gain" (I.E.: signal amplification) of the dish divided by the amount of noise the LNB produces. The gain depends on many factors including surface finish, accuracy of shape, feedhorn masking, and size (the bigger the dish the better). The amount of noise an LNB produces depends on design, temperature, and losses in the cables.
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).
See also
External links
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