https://en.wikipedia.org/wiki/Feed_horn
In parabolic antennas such as satellite dishes, a feed horn (or feedhorn) is a small horn antenna used to convey radio waves between the transmitter and/or receiver and the parabolic reflector. In transmitting antennas, it is connected to the transmitter and converts the radio frequency alternating current from the transmitter to radio waves and feeds them to the rest of the antenna, which focuses them into a beam. In receiving antennas, incoming radio waves are gathered and focused by the antenna's reflector on the feed horn, which converts them to a tiny radio frequency voltage which is amplified by the receiver. Feed horns are used mainly at microwave (SHF) and higher frequencies.
The feedhorn also selects the polarization of the waves to be received, which helps to attenuate unwanted signals from adjacent channels and transponders, and from other communications satellites at nearby orbital positions. This can be either horizontal or vertical if the polarization is linear, or clockwise or counterclockwise (also called left- and right-handed) if it is circular. Certain devices can also allow a feedhorn to accept both linear and circular, though these cause a slight insertion loss to all incoming signals.
When used with a parabolic reflector or lens antenna, the phase center of the horn is usually placed at the focal point of the reflector, with the 3 dB points of the horn's radiation pattern set to be at the edge of the reflector. On a satellite dish, the feedhorn is what is mounted at the end of a mast from the center of the dish, or on tripod legs mounted to the edge of the dish.
In satellite dishes, part of the receiver electronics, the RF front end, is usually mounted in a box just behind the feed horn. This converts the high satellite microwave downlink frequencies to a lower frequency so it can be more easily sent through the coaxial cable feedline to the receiver inside the building. This is called the low-noise block downconverter (LNB), and the unit consisting of the horn plus the converter is called the LNB with feedhorn (LNBF).