Low-noise Block Downconverter

A low-noise block downconverter (or LNB) is the receiving device on an antenna, for example a satellite dish commonly used for satellite TV reception. Also called a low-noise block, LNC (for low-noise converter), or even LND (for low-noise downconverter), the device is sometimes wrongly called an LNA (low-noise amplifier).

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, downconverts the block of frequencies to a lower block of intermediate frequencies (IF), so the signal can be carried to the indoor satellite TV receiver using relatively cheap coaxial cable. Transferring the signal from the antenna indoors at its original microwave frequency would require an expensive and impractical waveguide.

The LNB is usually 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 and is fed to a section of waveguide. In this waveguide a metal pin, or probe, protrudes into the waveguide at right angles to the axis and this acts as an antenna, and feeds the signal to a printed circuit board inside the LNB's shielded box for procesing. The lower frequency IF output signal emerges from a socket on the box to which the coaxial cable connects.

The LNB gets its power from the receiver or set-top box inside the house. This phantom power is sent "up" the same coaxial cable that carries the received signals "down" to the receiver.

A corresponding component, called a block upconverter (BUC), is used at the satellite earth station (uplink) dish to convert the band of television channels to the microwave uplink frequency.