Thermal Noise from a Passive Linear Multiport

The thermal noise from passive multiports is discussed from fundamentals so that it can be understood, measured, or calculated by a microwave engineer. The multiports are assumed to have a uniform temperature, but with no restriction on reciprocity or mismatch. The noise temperature, T_N, contributed by such a multiport is T_N=A T where T is its physical temperature and A is its absorption coefficient. An approximate method of measuring A, and a method of measuring an A as small as 0.008 within 5 percent, are pointed out. Also, exact and approximate expressions for A in terms of scattering matrix elements and termination reflection coefficients are derived. Finally, the crosscorrelation of the noise from different ports is briefly considered.