tion used as a receiver has three components: noise received by the main beam of the reflector; spillover noise due to spill- over from the feed; and receiver noise. The first component can be due to natural sources or due to human-made interference. The natural noise emitters are Earth and sea absorption, galactic noise, isotropic background radiation, quantum noise, and absorption due to the oxygen and water vapor in the Earth's atmosphere. A minimum isotropic background radiation of about 3 K is always seen by any antenna. The value of the other factors depends on frequency. Spillover noise is the only component under the control of the antenna designer. Its value can be reduced by designing an antenna with very low sidelobes. Receiver noise is normally the dominant noise factor. It depends on the method of amplification and detection. Early Earth stations all used cooled receivers, which have low noise temperatures. Modern Earth stations use uncooled receivers, which are dependent on the noise perfor- mance of the front-end transistor. This was improved dramatically in recent years, especially for small DBS terminals where the economies of scale support considerable research to reduce the noise temperature. Reflective Antenna Types Vertex of Paraboloid Parabolic Axis Feed Phase Fig. 9.16 Beam waveguide feed Center system for large Earth stations. The ratio of gain to noise temperature, the G/T ratio, is a useful measure of the influ- ence of the noise components. Typical values are 40.7 dBK for an INTELSAT A, 30-meter- diameter antenna operating at 4/6 GHz (Pratt & Bostian, 1986). Large Earth station antennas are expensive to construct and maintain, SO there is a premium in obtaining maximum efficiency from the system. The axisymmetric Cassegrain antenna (see the Nulls section) is the favorite choice for a number of reasons: 1. The gain can be increased over the standard parabola-hyperbola combination by shaping the reflectors. Up to an extra 1 dB is possible. 2. Low antenna noise temperatures can be achieved