High-power Direct Broadcast Satellites (DBS), for example DirecTV satellites, broadcast each channel with either left-hand or right-hand circular polarization (LHCP and RHCP), also referred to as horizontal and vertical polarization, respectively. Approximately half of the channels are broadcast with each kind of polarization.

An LNB (Low Noise Blocker) is a device put at the focal point of a Dish Antenna to capture the signals from a satellite. Every LNB is required to receive, amplify and downconvert the satellite signal. An LNB consists of:

• Feedhorn: the front receptive part of an LNB, in most cases the white plastic "cup" of the LNB.
• Polarizer: a buit-in device positioned in the wave guide between the feedhorn and LNB circuitry which allows the receiver to select signals of a specific polarity. Left or Right Circular Polarization is selected by the receiver by applying 13 or 18 volts to the LNB.
• Amplifier and Down-Converter: this circuit amplifies the satellite signal and also brings the signal down into the C band for input to the satellite receiver.

Most of the modern satellites operate in Low (10.70 GHz - 11.70 GHz) and High (11.70 - 12.75 GHz) Ku Bands. In order to receive the entire bandwidth from 10.70 to 12.75 GHz, a Universal LNB is required. A satellite receiver selects between the High and Low bands by sending a switching signal (22 kHz) to the LNB.

Performance of an LNB depends on the amount of noise the LNB itself adds to the signal. Modern Ku-band LNBs are characterise by 0.5 - 0.7 dB of noise.

A Satellite Receiver connected to a LNB actually commands the LNB to receive either left-hand or right-hand circular polarized signals. The Receiver has a table (also downloaded from the satellite) of which channels are on which polarization. When the user requests a channel on the receiver, the receiver looks the polarization up in its table for that channel. The receiver then signals the LNB to look for satellite signals of the required polarization. It does this by placing a variable voltage on the cable connecting the LNB to the receiver. The receiver places a nominal +13V to select RHCP (or vertical) polarized signals, or a nominal +18V to select LHCP (or horizontal) polarized signals.

Connecting Two Receivers to a Satellite Dish

To connect two receiversto a satellite dish, a Dual-LNB is required. A dual-LNB is simply two independent LNBs mounted side-by-side on the satellite dish, with one cable coming out of each. Each of the two cables can then be connected to a different satellite receiver. Each receiver will control one of the two LNBs, and will thus be able to receive left-hand or right-hand polarized signals irresepective of the other LNB.

Connecting Multiple Receivers to a Satellite Dish

A device called "Multiswitch" (also known as a "Magic Switch") is required to connect more than two receivers to a Dual-LNB. A Multiswitch works by permanently tuning one LNB to left-hand and the other LNB to righ-hand polarized signal reception, and connecting the proper LNB to each receiver as indicated by the voltage placed by that receiver.

A Multiswitch is also capable of taking in an outdoor antenna or CATV signal and, using built-in diplexers, combining this signal onto each of the outputs. If this feature is used, a device called a "Diplexer" is required at the other end to divide the combined signal into two different signals for the "Satellite In" and "Antenna In" connectors on the receiver.

Detailed information on Broadcast Satellite and Digital Television can be found at:

• The Digital Video Broadcasting Project