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In any measurement there will be a power contribution from noise. This will include background sky, spillover, manmade terrestrial sources and a portion of noise generated from within the receiver. The spillover noise is received power that has been reflected from that portion of the ground outside the radius of the dish. The background sky temperature at 1420.4 MHz is approximately 6K. A hot and cold source can be used to quantify the noise temperature of the receiver.

In a laboratory with no man-made noise or spillover the only contributions come from the hot and cold sources and the receiver itself.


Y is simply a ratio and can be determined from the voltage outputs from the receiver with the hot and cold sources in the beam.


TR = the temperature generated by the receiver.

TH = the temperature of a hot body, usually an absorber at 300 K.

TC = the temperature of a cold source, often liquid nitrogen at 77 K.

A similar analysis can be carried out with the receiver mounted upon the antenna. When this is the case the cold sky is used as the cold source and spillover needs to be taken into account.