sergio mariotti
Joined: 22 Dec 2003
Posts: 467
Helped: 42
Location: Italy
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 05 Aug 2004 20:16 Antenna Measurement |
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Have you hear sometime, somewhere, that the sky is "cold"?
Far from human RF emission, when you point your antenna to zenith (the vertical over the head), you'll see a cold sky.
The basic temperature is the cosmic background (2.7 K) and at some frequency it is moderately warmed by galactic noise or atmospheric absobition.
Practically:
Below 1 Ghz: 100K ... 10 K
1-15 GHz: 3K... 10K
22 GHz: 30 (clear sky, mountain)...200K (cloudy sky, land)
This low noise temperature is also little warmed by antenna spill-over looking the ground. Remember that a spill-over (back lobes) looking at the ground produce a 200K...300K noise temperature but usually the back lobes are many times smaller than main lobe.
suggestion for books? Kraus- Radioastronomy - pag 237.
Note: to convert Kelvins into Power apply :
P=KTB
K: Boltzmann's kostant
T: Operational noise temperature
B: bandwidth
all in SI units
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