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At the focal point, the received satellite signals are gathered by, i.e. pass into, an apparatus called the 'feed', or feedhorn. The feedhorn is located exactly where the mathematics of the parabola, used in the dish design (dish design formula), determine the focal point to be located. The feedhorn is designed to accept incoming satellite signal while rejecting unwanted signal (such as signals bounced from nearby walls into the dish or signals from nearby telephone and/or television towers that might enter the dish) and it is designed to select signal polarity and to efficiently direct the gathered signal towards (into) the LNB (low noise, block downconverter, amplifier). Feedhorns have a scalar plate, composed of concentric rings, which surround the feed throat. Scalar rings are designed to accept desired signals and assist in rejecting undesired frequencies - notice the difference in scalars on a C-band feed and that of a Ku-band feed. The position of the scalar around the feed throat determines the feedhorn's field of view and, to some extent, its acceptance or rejection of unwanted signal (scalar settings for deep and shallow dish). The proper scalar location is determined by dish design mathematics and is the F/D setting. Inside the feed throat is a polarity probe which is the acual antenna that receives signals from a satellite. The feed throat and probe are designed for efficient reception of specific microwave frequencies and is why they should never be tampered with; they are designed to pass (channel) frequencies to the LNB with minimal signal loss or distortion.
A single LNB feedhorn is called a polarotor and if you have a C-band system only then you are using a C-band polarotor and if you have a Ku-band system only then you are using a Ku-band polarotor. A small motor is mounted atop the polarotor that moves the polarity probe, inside the throat of the feed, and this motor is called the 'servo motor' or 'servo' for short. Satellite signals are transmitted at two polarities and, on command from the satellite receiver, the servo moves the probe so as to accept one polarity and reject the other.
Satellites use a dual polarization transmission system to allow more efficient use of their equipment, this is termed frequency reuse (for further discussion on frequency reuse). Some satellite manufacturer's design their satellites to transmit signals in a linear format and some in a circular format and some satellites, such as the Soviet Gorizont satellites, are designed to be linear in one band (C-band) and circular in the other. In a linear format, signal polarization is either horizontal or vertical. In a circular format, signal polarization is either right hand or left hand circular polarization, abbreviated to RHCP/LHCP. To receive circular polarized signals, a circular feed is required - this is often called an 'international feed'. To further understand, for example, in other words, channels 1 and 2 on earth could be transponder 1-horizontal (or RHCP) and transponder 1-vertical (LHCP) in space on the satellite. It is the role of the probe inside the feed (whether linear feed or international feed) to pass one polarity and reject the other; this action by the feed is transparant to the user as it is automatically controlled by the receiver. To pass a polarity, the probe within the fee throat moves to be in line with the desired incoming signal polarization thereby being in-phase with the desired polarization and out of phase with the opposite polarization. The physical act of the probe to be out of phase with the undesired polarization has the effect of disrupting the coherency of that polarization therefore prohibiting it to pass into the LNB. When you change channels, while watching TV, the feedhorn's servo motor rotates the probe, which swings back and forth while switching between the polarized signals (horizontal/vertical channels or RHCP/LHCP channels as appropriate). The act of using one main signal transmission to host two polarizations within that signal is termed 'frequency reuse' and is a technique to double a satellite's channel capacity without adding additional transponders. It is common to use the term 'polarity' when referring to signal polarizations though polarization is the correct term.
When both polarities of signal are desired to be received at the same time (as in the case for a distribution system as used for a motel or apartment complex or to allow each TV in your home to independently receive all satellite channels) two LNBs are installed on the feed, one for each polarity, and this style feed is called a dual feed, not a dual band feed as band typically refers to either C or Ku signals and a dual feed receives only one band. A dual feed can be for any frequency band; a dual feedhorn does not have a servo motor. Note on a dual feed the LNBs are at right angles (orthogonal) to each other; technically, a dual feed is called an 'orthomode feedhorn'. Another popular type feed is one that accepts multiple frequencies, usually C-band and Ku-band signals, this style feed is called a corotor and is a dual band feedhorn (sample receiver wiring for corotor system). A corotor will have a servo motor to control which incoming signal polarity is passed on to the LNB and it uses both a C-band and a Ku-band LNB. The style feed combining the dual C-band and single Ku-band LNBs (three in total, the Ku uses a servo) is called a 'dual C corotor' (sample receiver wiring for dual C, single Ku system); and a dual C/dual Ku (four LNBs total) is called a 'bullseye' feed. When a feed is designed to receive circular polarity it is called an international polarotor, international corotor, etc.
