increase the weight and cost. The most common feed is a modified TE11 circular waveguide, which is designed to have a low VSWR and good pattern symmetry. Sometimes operation in two frequency bands is needed, in which case the feed must combine two waveguides and operate at the two frequencies. The VSWR can be reduced by replacing the center portion of the paraboloid with a flat plate, called a vertex plate. This minimizes the VSWR contribution from the dish, although it also degrades the near-in sidelobes and reduces the gain. Earth Station Antennas Earth station antennas are at the Earth end of satellite links. High gain is needed to receive the weak signals from the satellite or to transmit strong signals to it. The antennas can be divided into the following three types. Large antennas required for transmit and receive on the INTELSAT-type global networks with gains of 60 to 65 dBi (15 to 30 meters in diameter) Chapter 9 Antennas Sidelobe Envelope as Defined by FCC Rules and Regulations Part 25.209 Peak Sidelobe Envelope Smoothed Sidelobe Envelope Degrees from Peak of Main Beam Fig. 9.15 FCC Earth station antenna pattern specification. Medium-sized antennas for cable head (TVRO) or data receive-only terminals (3 to 7 meters in diameter) Small antennas for direct broadcast reception (0.5 to 2 meters in diameter) Types 1 and 2 have to satisfy stringent specifications imposed by various regulatory bodies. Previously, when the recommended spacing of geostationary satellites was 3 degrees, the pattern envelope was specified as 32 to 3510g O. This requirement could be met with a symmetric-reflector antenna. With the new spacing of 2 degrees, the pattern specification has been improved to 29 to 25 log O (see Figure 9.15). This can best be met with low-sidelobe, offset-reflector designs. The minimum receivable signal level is set by inherent noise in the system. Earth sta- tions are required to detect small signals, SO control of noise parameters is important. The noise appearing at the output terminals of an Earth sta