Multi-switches for DBS systems

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[img]http://www.hdtvprimer.com/ANTENNAS/multiswitch.gif[/img]
Every DBS satellite system has a multi-switch. Often it is mounted within 3 feet of the dish. Other times it is packaged with the three LNBFs in a single large head assembly. Basically it connects each TV to one of four lines from the LNBFs. Some multi-switches will also amplify the signal. Multi-switches come in 1-way, 2-way, 4-way, 8-way, and other configurations.

When the user selects a channel, the satellite receiver must figure out which satellite and transponder the channel is on and whether the transponder has left or right circular polarization. (A transponder is like a large channel. Each transponder carries several stations. Consecutive transponders have alternating polarization, which makes it easier for the receiver to separate the transponders.) The receiver selects the satellite by placing (or not) a 22-kilohertz tone on the coaxial cable. It selects the polarization by placing either 13 volts DC or 18 volts DC on the coaxial cable. The multi-switch uses this DC both as a signal and as a source of power for itself and the LNBFs.

For DirecTV, the third satellite carries only a few transponders, and the combiner maps those into unused transponder frequencies of satellite B. Thus the receiver functions as if there are only two satellites.