Calculation of Antenna System Noise Temperatures at Different Ports—Revisited

With the advent of cryogenically cooled front-end assemblies, it has become the practice to define antenna system noise temperature at the horn aperture rather than at the low-noise amplifier (LNA) input port. It is not generally known that when the reference port is moved towards the horn aperture, the values of the operating system noise temperature, Top, and effective receiver noise temperature, Te, will always increase. It is also not generally known that, when the horn and waveguide are at cryogenic rather than ambient temperature, as will be shown in two examples in this article, the value of Tia defined at the LNA reference port can be either larger or smaller than the value of Tia defined at the horn aperture. This fact is not obvious from studying approximate formulas that neglect products of the loss factors of the components between the horn aperture and the LNA.