I. Introduction
Microwave low-noise amplifier (LNA) is widely used in radio-astronomy receivers and its noise temperature can be just a few kelvins [1], [2]. To carry out accurate measurements both at room temperature and cryogenically, the noise from the input coaxial cable, which serves as a thermal buffer between the LNA and the noise source, needs to be known accurately. Ideally, by treating the cable as a uniform, i.e., homogeneous, medium with constant thermal conductivity, temperature along the cable can be assumed linear [3]. However, since a coaxial cable comprises inner conductor, outer conductor and dielectric insulator in between, and, even worse, thermal conductivity is in general a function of temperature, this linear assumption seems susceptible and should be reexamined. Knowledge of this input thermal buffer's noise will be even critical in the more demanding areas, such as cryogenic on-wafer noise measurements [4], [5].