View File - University of Engineering and Technology, Taxila

FIGURE 3.16Block diagram for estimating system noise temperature.In view of (3.8), the noise power at the output of the waveguide can be writtenasN ¼ kBL 1ðT a þ T e1 Þð3:50Þwhere T a and T e1 respectively correspond to the antenna noise temperature andwaveguide noise temperature. The equivalent noise temperature T ex for a lossytwo-port system whose power loss L is greater than unity can be related to theabsolute temperature T 0 ; using the expressionT ex ¼ T 0 ðL 1Þ ð3:51ÞHence, we can write the waveguide noise temperature asT e1 ¼ T 0 ðL 1 1Þ ð3:52ÞSubstituting (3.52) in (3.50), we have the noise power at the output of thewaveguideN ¼ kB T aþ T 0ðL 1 1Þð3:53ÞL 1L 1By comparing (3.53) with (3.8), the equivalent noise temperature at thewaveguide’s output isT w ¼ T aþ T 0ðL 1L 1L 11Þð3:54ÞThe earth station system noise temperature referred to the input of the LNAsimply isT ¼ T w þ T eð3:55ÞCopyright © 2002 by Marcel Dekker, Inc. All Rights Reserved.