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A low-noise block downconverter (LNB) is the receiving device mounted on satellite dishes used for satellite TV reception, which collects the radio waves from the dish and converts them to a signal which is sent through a cable to the receiver inside the building. Also called a low-noise block, low-noise converter (LNC), or even low-noise downconverter (LND), the device is sometimes inaccurately called a low noise amplifier (LNA).
The LNB is a combination of low-noise amplifier, frequency mixer, local oscillator and intermediate frequency (IF) amplifier. It serves as the RF front end of the satellite receiver, receiving the microwave signal from the satellite collected by the dish, amplifying it, and downconverting the block of frequencies to a lower block of intermediate frequencies (IF). This down conversion 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 printed circuit board inside 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.
LNBF disassembled. The waveguide from the feedhorn, carrying the microwave radio signal collected by the dish passes through the hole in the center. The pins visible at the top and left side of the hole project into the waveguide and act as antennas to receive the signal, converting it to radio frequency, alternating currents which are processed by the circuit board. The lower frequency output signal is taken from the coaxial cable jacks, bottom.
Cross-section across a low-noise block downconverter.
Viewing of the pin and the horn antenna in a low-noise block downconverter.
The LNB gets its power from the receiver or set top box, using the same coaxial cable that carries signals from the LNB to the receiver. This phanton power travels to the LNB; opposite to the signals from the LNB.
A corresponding component, called a block upconvertor (BUC), is used at the satellite earth station (uplink) dish to convert the band of television channels to the microwave uplink frequency.