As well as collecting signals antennas also collect external noise from sources such as the ground, the atmosphere (especially if rain or cloud is present), and extraterrestrial sources.
Antenna noise temperature varies with elevation angle, dish size, frequency and weather conditions. Variation with dish size is because a major component of antenna noise is pickup of the ground temperature (say 290K) by the sidelobes of the antenna.
Weather conditions have a big bearing on antenna temperature. Under clear sky conditions a 12 GHz antenna might have Tant from 30 to 100 K but if heavy cloud or rain is present higher values of Tant will apply (see section 9).
Figure 10 (a): Antenna noise temperature vs elevation angle for a 4 GHz earth station
Figure 10 (b): Brightness temperature vs elevation angle for earth stations operating in different frequency bands
(Note: Antenna noise temperature is related to brightness temp). [Ref 5].
Figure 10c: Variation in antenna noise temperature with dish size and elevation angle for two 12 GHz antennas [Ref 6].
Antenna noise generally will not be a major component of total system noise unless very low noise LNCs are being used. But if you are using, say, a 1 dB noise figure low noise converter you need to pay attention to antenna noise.
Prime focus dishes will tend to have higher antenna noise temperatures than offset fed dishes and especially more so than dual reflector dishes because, in as much as there is any spill over of the feed's reception pattern over the edges of the dish, the feed will be looking at the relatively high temperature ground (typically 290K), not at a low temperature object (the sky, 4K).
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