A feed has the important function of funneling signals reflected from a dish into the LNB. It is tuned to the frequency of the downlink signal and must have minimal losses while ignoring noise and other unwanted signals coming from off-axis directions. A poorly matched feed that does not properly illuminate a dish can add as much as 20°K of noise to a receiving system. Feeds must also select the required circular or linear polarity to properly detect a broadcast. Use of an inappropriate feed can result in poor cross-polarization discrimination, namely the reception of a signal with not only the desired polarity but also an unacceptable amount of the opposite polarity signal.ve general types of feeds have been developed for use in receiving satellite broadcasts: scalar, circular, dielectric, conical and pyramidal. Scalar, dielectric and conical feeds are the most commonly employed; the conical feed is employed with offset dishes. The scalar feed, the most common type in use today, consists of a section of circular waveguide with a set of concentric "scalar" rings. The circular waveguide is designed to detect both senses of linear polarity and the scalar rings to minimize signal reflections. Typically about 1.5% of the energy entering a scalar feed, about 0.08 dB, is lost due to reflections. A fixed ring scalar feed generally operates in the 0.33 to 0.45 f/D range. In some brands the position of the rings can be adjusted to extend this range from 0.28 to as high as 0.5 to match the f/D ratio of a dish. In general, the additional expense of an adjustable ring version is justifiable. An improvement of as much as 0.5 dB in carrier-to-noise ratio can be gained by matching a feed to a dish.