Antenna Noise: The Practical Picture
- The antenna collects noise as well as the wanted signal
- Sky, atmosphere, ground, and the Moon can change
- The antenna pattern and pointing determine how much enters
- Use a specified antenna temperature for the link calculation
- Keep external antenna noise separate from receiver noise figure
- Antenna noise need not be 290 K — a 30 K antenna with a 50 K receiver gives 80 K total, vs. 340 K with a 290 K source (which contribution matters)
The Pattern Weights the Whole Scene
- Sky brightness and ground brightness → the antenna pattern weights each direction, then sums the noise → antenna port delivers
- Sky (brightness) temperature describes radiation from a direction
- Antenna temperature describes the noise delivered at the port
Antenna Temperature (lossless antenna)
- = integrated antenna response (pattern weight) in each region of the scene
- Source: Condon & Ransom, Essential Radio Astronomy, Ch. 3
How Much Does Ground Matter?
Sky and Ground Pickup
- Sky: 10 K with 95% pattern weight; ground: 290 K with 5% pattern weight
- The 5% ground pickup contributes more noise than the 95% sky pickup
The Moon and Antenna Beam Coupling
- Off the lunar disk: assume
- On the lunar disk: assume
- (Assumed scene temperatures; beam outlines schematic)
- “Off the lunar disk” means the line of sight is outside the apparent Moon (clarified Oct. 19)
- Effect on the lunar link: rises by 200 K and drops 3.4 dB (pointing at the Moon)
Atmosphere and Weather
- An absorbing atmosphere attenuates the signal and adds its own emission to : Atmospheric Loss and Noise