Friday, March 23, 2007

Satellite television

Satellite television is television delivered by means of communications satellites in geostationary orbit 37,000 km (22,300 miles) above the earth’s equator. The first satellite television signal was relayed from Europe to the Telstar satellite over North America in 1962. The first domestic North American satellite to carry television was Canada’s Anik 1, which was launched in 1973.
Satellite television, like extra communications relayed by satellite, starts with a transmitting antenna located at an uplink facility. Uplink satellite dishes are directed toward the satellite that their signals will be transmitted to, and are very large, as much as 9 to 12 meters (30 to 40 feet) in diameter. The increased diameter results in more correct aiming and increased signal strength at the satellite. The signal is received by transponders aboard the satellite, which retransmit the signal back to Earth at a dissimilar frequency. The leg of the signal path from the satellite to the receiving Earth station is called the downlink.
The satellite signal, quite weak after travelling a great distance (see inverse-square law), is together by a parabolic receiving dish, which reflects the weak signal to the dish’s focal point and is received, down-converted to a lower frequency band and amplified by a device called a low-noise block converter (LNB). 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 lately announced by the University of Waterloo. The signal, now amplified, travels to a satellite receiver box through coaxial cable (RG-6 or RG-10; cannot be standard RG-59) and is converted by a local oscillator to the L-band range of frequencies (approximately). Special on-board electronics in the receiver box help tune the signal and after that convert it to a frequency that a standard television can use.


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