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With the development of low-noise amplifiers, the noise contribution of the antenna has become a limiting factor on system performance in many applications. This paper discusses the calculation of the equivalent noise temperature contribution of parabolic reflector antennas using their radiation characteristics and environment. As an example, the equivalent noise temperature of an 85-foot reflector at 2 kMc is estimated as a function of orientation for two different primary illuminations. The relative contributions of the mainlobe, sidelobes and backlobes to the total antenna noise temperature is determined in each case to show the effect of antenna radiation characteristics on noise temperature. A functional relationship between the primary illumination of a parabolic reflector antenna and the system SNR is then derived from which the edge illumination producing the maximum SNR can be specified for a given family of primary illuminations.