A low-noise block downconverter (or LNB) is the receiving device mounted on satellite dishes used for satellite TV reception, which collects the radio waves from the dish. The LNB is a combination of low-noise amplifier, frequency mixer, local oscillator and IF amplifier. It receives the microwave signal from the satellite collected by the dish, amplifies it, and downconverts the block of frequencies to a lower block of intermediate frequencies (IF). This downconversion allows the signal to be carried to the indoor satellite TV receiver using relatively cheap coaxial cable; if the signal remained at its original microwave frequency it would require an expensive and impractical waveguide line.
The LNB is usually a small box suspended on one or more short booms, or feed arms, in front of the dish reflector, at its focus (although some dish designs have the LNB on or behind the reflector). The microwave signal from the dish is picked up by a feedhorn on the LNB and is fed to a section of waveguide. One or more metal pins, or probes, protrude into the waveguide at right angles to the axis and act as antennas, feeding the signal to a PCBinside the LNB's shielded box for processing. The lower frequency IF output signal emerges from a socket on the box to which the coaxial cable connects.