Citation
Abstract
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, T , and effective receiver noise temperature, op T , will always increase. It is also not generally known that, when the horn and e waveguide are at cryogenic rather than ambient temperature, as will be shown in two examples in this article, the value of T defined at the LNA reference port can ia be either larger or smaller than the value of T defined at the horn aperture. This ia 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. In this article, the exact expressions for T at different ports are derived. The op exact and approximate formulas are compared, and calculations are made on a DSN 8.4-GHz (X-band) and DSN 32-GHz (Ka-band) feed system showing the magnitudes of errors that result when using the approximate formula. The magnitudes of errors were about 0.1 K and 1 K for the X- and Ka-band systems, respectively.
Details
- Volume
- 42-150
- Published
- August 15, 2002
- Pages
- 1–18
- File Size
- 175.8 KB