Discussion
Why is a cold sky measurement useful for antenna noise temperature?
Answer: A
At microwave frequencies away from absorption lines the zenith sky is only a few kelvin, which is a convenient and repeatable cold reference against an ambient absorber at about two hundred and ninety. The difference between the two is large, which conditions the measurement well. The sky is a noise source rather than a calibrated signal, side lobes still see warm ground, and interference rejection is incidental. Nature supplying a stable reference is a convenience worth exploiting.
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