Antenna noise temperature

Expressions for calculating the noise temperature of an antenna are deduced. The influence of the atmosphere and the environment of the antenna are discussed. It appears that analytical expressions for the sky temperature T(¿, ¿) are available. So, if the antenna gain function G(¿, ¿) is also known analytically, it is possible to calculate the noise temperature of the antenna. If G(¿, ¿) is unknown, it is still possible to predict the noise temperature by finding the power lost through several causes, such as blocking and diffraction. This power does not contribute to the main lobe and is mostly scattered in all directions. In some cases, however, it is also possible to predict into what solid angles this lost power is radiated.