What is an LNB (or LNBF) and what does it do?
An LNB (Low-Noise Block downconverter) is the device mounted at the focal point of a satellite dish that receives the weak, high-frequency microwave signals reflected by the dish, amplifies them with low added noise, and downconverts them to a lower L-band frequency (typically 950-2150 MHz) that a satellite receiver can process over coaxial cable. An LNBF is an LNB with the feedhorn integrated into the same unit, combining signal collection and downconversion in one assembly.
Satellite signals arrive at extremely high frequencies (4-40 GHz depending on band) and very low power. These frequencies cannot travel efficiently through coaxial cable, so the LNB must convert them down to the intermediate L-band range before sending them indoors. The "low-noise" part is critical: because the incoming signal is so weak, any noise the amplifier adds directly degrades reception quality, which is why noise figure is a key LNB specification.
The LNB also handles polarization selection (vertical/horizontal or left/right circular) and, in universal designs, band switching between low and high oscillator frequencies. Because it sits outdoors at the dish, an LNB is built to withstand sun, rain, and temperature extremes while maintaining stable performance.